The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom
by
Charles Darwin
Part 1 out of 10
This etext was prepared by Sue Asscher asschers@dingoblue.net.au
THE EFFECTS OF CROSS & SELF-FERTILISATION IN THE VEGETABLE KINGDOM.
BY
CHARLES DARWIN, M.A., F.R.S., ETC.
CONTENTS.
CHAPTER I.
INTRODUCTORY REMARKS.
Various means which favour or determine the cross-fertilisation of
plants.--Benefits derived from cross-fertilisation.--Self-fertilisation
favourable to the propagation of the species.--Brief history of the
subject.--Object of the experiments, and the manner in which they were
tried.--Statistical value of the measurements.--The experiments carried
on during several successive generations.--Nature of the relationship of
the plants in the later generations.--Uniformity of the conditions to
which the plants were subjected.--Some apparent and some real causes of
error.--Amount of pollen employed.--Arrangement of the work.--Importance
of the conclusions.
CHAPTER II.
CONVOLVULACEAE.
Ipomoea purpurea, comparison of the height and fertility of the crossed
and self-fertilised plants during ten successive generations.--Greater
constitutional vigour of the crossed plants.--The effects on the
offspring of crossing different flowers on the same plant, instead of
crossing distinct individuals.--The effects of a cross with a fresh
stock.--The descendants of the self-fertilised plant named
Hero.--Summary on the growth, vigour, and fertility of the successive
crossed and self-fertilised generations.--Small amount of pollen in the
anthers of the self-fertilised plants of the later generations, and the
sterility of their first-produced flowers.--Uniform colour of the
flowers produced by the self-fertilised plants.--The advantage from a
cross between two distinct plants depends on their differing in
constitution.
CHAPTER III.
SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC.
Mimulus luteus; height, vigour, and fertility of the crossed and
self-fertilised plants of the first four generations.--Appearance of a
new, tall, and highly self-fertile variety.--Offspring from a cross
between self-fertilised plants.--Effects of a cross with a fresh
stock.--Effects of crossing flowers on the same plant.--Summary on
Mimulus luteus.--Digitalis purpurea, superiority of the crossed
plants.--Effects of crossing flowers on the same
plant.--Calceolaria.--Linaria vulgaris.--Verbascum thapsus.--Vandellia
nummularifolia.--Cleistogene flowers.--Gesneria pendulina.--Salvia
coccinea.--Origanum vulgare, great increase of the crossed plants by
stolons.--Thunbergia alata.
CHAPTER IV.
CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC.
Brassica oleracea, crossed and self-fertilised plants.--Great effect of
a cross with a fresh stock on the weight of the offspring.--Iberis
umbellata.--Papaver vagum.--Eschscholtzia californica, seedlings from a
cross with a fresh stock not more vigorous, but more fertile than the
self-fertilised seedlings.--Reseda lutea and odorata, many individuals
sterile with their own pollen.--Viola tricolor, wonderful effects of a
cross.--Adonis aestivalis.--Delphinium consolida.--Viscaria oculata,
crossed plants hardly taller, but more fertile than the
self-fertilised.--Dianthus caryophyllus, crossed and self-fertilised
plants compared for four generations.--Great effects of a cross with a
fresh stock.--Uniform colour of the flowers on the self-fertilised
plants.--Hibiscus africanus.
CHAPTER V.
GERANIACEAE, LEGUMINOSAE, ONAGRACEAE, ETC.
Pelargonium zonale, a cross between plants propagated by cuttings does
no good.--Tropaeolum minus.--Limnanthes douglasii.--Lupinus luteus and
pilosus.--Phaseolus multiflorus and vulgaris.--Lathyrus odoratus,
varieties of, never naturally intercross in England.--Pisum sativum,
varieties of, rarely intercross, but a cross between them highly
beneficial.--Sarothamnus scoparius, wonderful effects of a
cross.--Ononis minutissima, cleistogene flowers of.--Summary on the
Leguminosae.--Clarkia elegans.--Bartonia aurea.--Passiflora
gracilis.--Apium petroselinum.--Scabiosa atropurpurea.--Lactuca
sativa.--Specularia speculum.--Lobelia ramosa, advantages of a cross
during two generations.--Lobelia fulgens.--Nemophila insignis, great
advantages of a cross.--Borago officinalis.--Nolana prostrata.
CHAPTER VI.
SOLANACEAE, PRIMULACEAE, POLYGONEAE, ETC.
Petunia violacea, crossed and self-fertilised plants compared for four
generations.--Effects of a cross with a fresh stock.--Uniform colour of
the flowers on the self-fertilised plants of the fourth
generation.--Nicotiana tabacum, crossed and self-fertilised plants of
equal height.--Great effects of a cross with a distinct sub-variety on
the height, but not on the fertility, of the offspring.--Cyclamen
persicum, crossed seedlings greatly superior to the
self-fertilised.--Anagallis collina.--Primula veris.--Equal-styled
variety of Primula veris, fertility of, greatly increased by a cross
with a fresh stock.--Fagopyrum esculentum.--Beta vulgaris.--Canna
warscewiczi, crossed and self-fertilised plants of equal height.--Zea
mays.--Phalaris canariensis.
CHAPTER VII.
SUMMARY OF THE HEIGHTS AND WEIGHTS OF THE CROSSED AND SELF-FERTILISED
PLANTS.
Number of species and plants measured.--Tables given.--Preliminary
remarks on the offspring of plants crossed by a fresh stock.--Thirteen
cases specially considered.--The effects of crossing a self-fertilised
plant either by another self-fertilised plant or by an intercrossed
plant of the old stock.--Summary of the results.--Preliminary remarks on
the crossed and self-fertilised plants of the same stock.--The
twenty-six exceptional cases considered, in which the crossed plants did
not exceed greatly in height the self-fertilised.--Most of these cases
shown not to be real exceptions to the rule that cross-fertilisation is
beneficial.--Summary of results.--Relative weights of the crossed and
self-fertilised plants.
CHAPTER VIII.
DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL
VIGOUR AND IN OTHER RESPECTS.
Greater constitutional vigour of crossed plants.--The effects of great
crowding.--Competition with other kinds of plants.--Self-fertilised
plants more liable to premature death.--Crossed plants generally flower
before the self-fertilised.--Negative effects of intercrossing flowers
on the same plant.--Cases described.--Transmission of the good effects
of a cross to later generations.--Effects of crossing plants of closely
related parentage.--Uniform colour of the flowers on plants
self-fertilised during several generations and cultivated under similar
conditions.
CHAPTER IX.
THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE
PRODUCTION OF SEEDS.
Fertility of plants of crossed and self-fertilised parentage, both lots
being fertilised in the same manner.--Fertility of the parent-plants
when first crossed and self-fertilised, and of their crossed and
self-fertilised offspring when again crossed and
self-fertilised.--Comparison of the fertility of flowers fertilised with
their own pollen and with that from other flowers on the same
plant.--Self-sterile plants.--Causes of self-sterility.--The appearance
of highly self-fertile varieties.--Self-fertilisation apparently in some
respects beneficial, independently of the assured production of
seeds.--Relative weights and rates of germination of seeds from crossed
and self-fertilised flowers.
CHAPTER X.
MEANS OF FERTILISATION.
Sterility and fertility of plants when insects are excluded.--The means
by which flowers are cross-fertilised.--Structures favourable to
self-fertilisation.--Relation between the structure and conspicuousness
of flowers, the visits of insects, and the advantages of
cross-fertilisation.--The means by which flowers are fertilised with
pollen from a distinct plant.--Greater fertilising power of such
pollen.--Anemophilous species.--Conversion of anemophilous species into
entomophilous.--Origin of nectar.--Anemophilous plants generally have
their sexes separated.--Conversion of diclinous into hermaphrodite
flowers.--Trees often have their sexes separated.
CHAPTER XI.
THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.
Insects visit the flowers of the same species as long as they
can.--Cause of this habit.--Means by which bees recognise the flowers of
the same species.--Sudden secretion of nectar.--Nectar of certain
flowers unattractive to certain insects.--Industry of bees, and the
number of flowers visited within a short time.--Perforation of the
corolla by bees.--Skill shown in the operation.--Hive-bees profit by the
holes made by humble-bees.--Effects of habit.--The motive for
perforating flowers to save time.--Flowers growing in crowded masses
chiefly perforated.
CHAPTER XII.
GENERAL RESULTS.
Cross-fertilisation proved to be beneficial, and self-fertilisation
injurious.--Allied species differ greatly in the means by which
cross-fertilisation is favoured and self-fertilisation avoided.--The
benefits and evils of the two processes depend on the degree of
differentiation in the sexual elements.--The evil effects not due to the
combination of morbid tendencies in the parents.--Nature of the
conditions to which plants are subjected when growing near together in a
state of nature or under culture, and the effects of such
conditions.--Theoretical considerations with respect to the interaction
of differentiated sexual elements.--Practical lessons.--Genesis of the
two sexes.--Close correspondence between the effects of
cross-fertilisation and self-fertilisation, and of the legitimate and
illegitimate unions of heterostyled plants, in comparison with hybrid
unions.
INDEX.
...
THE EFFECTS OF CROSS AND SELF-FERTILISATION IN THE VEGETABLE KINGDOM.
CHAPTER I.
INTRODUCTORY REMARKS.
Various means which favour or determine the cross-fertilisation of plants.
Benefits derived from cross-fertilisation.
Self-fertilisation favourable to the propagation of the species.
Brief history of the subject.
Object of the experiments, and the manner in which they were tried.
Statistical value of the measurements.
The experiments carried on during several successive generations.
Nature of the relationship of the plants in the later generations.
Uniformity of the conditions to which the plants were subjected.
Some apparent and some real causes of error.
Amount of pollen employed.
Arrangement of the work.
Importance of the conclusions.
There is weighty and abundant evidence that the flowers of most kinds of
plants are constructed so as to be occasionally or habitually
cross-fertilised by pollen from another flower, produced either by the
same plant, or generally, as we shall hereafter see reason to believe,
by a distinct plant. Cross-fertilisation is sometimes ensured by the
sexes being separated, and in a large number of cases by the pollen and
stigma of the same flower being matured at different times. Such plants
are called dichogamous, and have been divided into two sub-classes:
proterandrous species, in which the pollen is mature before the stigma,
and proterogynous species, in which the reverse occurs; this latter form
of dichogamy not being nearly so common as the other.
Cross-fertilisation is also ensured, in many cases, by mechanical
contrivances of wonderful beauty, preventing the impregnation of the
flowers by their own pollen. There is a small class of plants, which I
have called dimorphic and trimorphic, but to which Hildebrand has given
the more appropriate name of heterostyled; this class consists of plants
presenting two or three distinct forms, adapted for reciprocal
fertilisation, so that, like plants with separate sexes, they can hardly
fail to be intercrossed in each generation. The male and female organs
of some flowers are irritable, and the insects which touch them get
dusted with pollen, which is thus transported to other flowers. Again,
there is a class, in which the ovules absolutely refuse to be fertilised
by pollen from the same plant, but can be fertilised by pollen from any
other individual of the same species. There are also very many species
which are partially sterile with their own pollen. Lastly, there is a
large class in which the flowers present no apparent obstacle of any
kind to self-fertilisation, nevertheless these plants are frequently
intercrossed, owing to the prepotency of pollen from another individual
or variety over the plant's own pollen.
As plants are adapted by such diversified and effective means for
cross-fertilisation, it might have been inferred from this fact alone
that they derived some great advantage from the process; and it is the
object of the present work to show the nature and importance of the
benefits thus derived. There are, however, some exceptions to the rule
of plants being constructed so as to allow of or to favour
cross-fertilisation, for some few plants seem to be invariably
self-fertilised; yet even these retain traces of having been formerly
adapted for cross-fertilisation. These exceptions need not make us doubt
the truth of the above rule, any more than the existence of some few
plants which produce flowers, and yet never set seed, should make us
doubt that flowers are adapted for the production of seed and the
propagation of the species.
We should always keep in mind the obvious fact that the production of
seed is the chief end of the act of fertilisation; and that this end can
be gained by hermaphrodite plants with incomparably greater certainty by
self-fertilisation, than by the union of the sexual elements belonging
to two distinct flowers or plants. Yet it is as unmistakably plain that
innumerable flowers are adapted for cross-fertilisation, as that the
teeth and talons of a carnivorous animal are adapted for catching prey;
or that the plumes, wings, and hooks of a seed are adapted for its
dissemination. Flowers, therefore, are constructed so as to gain two
objects which are, to a certain extent, antagonistic, and this explains
many apparent anomalies in their structure. The close proximity of the
anthers to the stigma in a multitude of species favours, and often
leads, to self-fertilisation; but this end could have been gained far
more safely if the flowers had been completely closed, for then the
pollen would not have been injured by the rain or devoured by insects,
as often happens. Moreover, in this case, a very small quantity of
pollen would have been sufficient for fertilisation, instead of millions
of grains being produced. But the openness of the flower and the
production of a great and apparently wasteful amount of pollen are
necessary for cross-fertilisation. These remarks are well illustrated by
the plants called cleistogene, which bear on the same stock two kinds of
flowers. The flowers of the one kind are minute and completely closed,
so that they cannot possibly be crossed; but they are abundantly
fertile, although producing an extremely small quantity of pollen. The
flowers of the other kind produce much pollen and are open; and these
can be, and often are, cross-fertilised. Hermann Muller has also made
the remarkable discovery that there are some plants which exist under
two forms; that is, produce on distinct stocks two kinds of
hermaphrodite flowers. The one form bears small flowers constructed for
self-fertilisation; whilst the other bears larger and much more
conspicuous flowers plainly constructed for cross-fertilisation by the
aid of insects; and without their aid these produce no seed.
The adaptation of flowers for cross-fertilisation is a subject which has
interested me for the last thirty-seven years, and I have collected a
large mass of observations, but these are now rendered superfluous by
the many excellent works which have been lately published. In the year
1857 I wrote a short paper on the fertilisation of the kidney bean (1/1.
'Gardeners' Chronicle' 1857 page 725 and 1858 pages 824 and 844. 'Annals
and Magazine of Natural History' 3rd series volume 2 1858 page 462.);
and in 1862 my work 'On the Contrivances by which British and Foreign
Orchids are Fertilised by Insects' appeared. It seemed to me a better
plan to work out one group of plants as carefully as I could, rather
than to publish many miscellaneous and imperfect observations. My
present work is the complement of that on Orchids, in which it was shown
how admirably these plants are constructed so as to permit of, or to
favour, or to necessitate cross-fertilisation. The adaptations for
cross-fertilisation are perhaps more obvious in the Orchideae than in
any other group of plants, but it is an error to speak of them, as some
authors have done, as an exceptional case. The lever-like action of the
stamens of Salvia (described by Hildebrand, Dr. W. Ogle, and others), by
which the anthers are depressed and rubbed on the backs of bees, shows
as perfect a structure as can be found in any orchid. Papilionaceous
flowers, as described by various authors--for instance, by Mr. T.H.
Farrer--offer innumerable curious adaptations for cross-fertilisation.
The case of Posoqueria fragrans (one of the Rubiaceae), is as wonderful
as that of the most wonderful orchid. The stamens, according to Fritz
Muller, are irritable, so that as soon as a moth visits a flower, the
anthers explode and cover the insect with pollen; one of the filaments
which is broader than the others then moves and closes the flower for
about twelve hours, after which time it resumes its original position.
(1/2. 'Botanische Zeitung' 1866 page 129.) Thus the stigma cannot be
fertilised by pollen from the same flower, but only by that brought by a
moth from some other flower. Endless other beautiful contrivances for
this same purpose could be specified.
Long before I had attended to the fertilisation of flowers, a remarkable
book appeared in 1793 in Germany, 'Das Entdeckte Geheimniss der Natur,'
by C.K. Sprengel, in which he clearly proved by innumerable
observations, how essential a part insects play in the fertilisation of
many plants. But he was in advance of his age, and his discoveries were
for a long time neglected. Since the appearance of my book on Orchids,
many excellent works on the fertilisation of flowers, such as those by
Hildebrand, Delpino, Axell and Hermann Muller, and numerous shorter
papers, have been published. (1/3. Sir John Lubbock has given an
interesting summary of the whole subject in his 'British Wild Flowers
considered in relation to Insects' 1875. Hermann Muller's work 'Die
Befruchtung der Blumen durch Insekten' 1873, contains an immense number
of original observations and generalisations. It is, moreover,
invaluable as a repertory with references to almost everything which has
been published on the subject. His work differs from that of all others
in specifying what kinds of insects, as far as known, visit the flowers
of each species. He likewise enters on new ground, by showing not only
that flowers are adapted for their own good to the visits of certain
insects; but that the insects themselves are excellently adapted for
procuring nectar or pollen from certain flowers. The value of H.
Muller's work can hardly be over-estimated, and it is much to be desired
that it should be translated into English. Severin Axell's work is
written in Swedish, so that I have not been able to read it.) A list
would occupy several pages, and this is not the proper place to give
their titles, as we are not here concerned with the means, but with the
results of cross-fertilisation. No one who feels interest in the
mechanism by which nature effects her ends, can read these books and
memoirs without the most lively interest.
From my own observations on plants, guided to a certain extent by the
experience of the breeders of animals, I became convinced many years ago
that it is a general law of nature that flowers are adapted to be
crossed, at least occasionally, by pollen from a distinct plant.
Sprengel at times foresaw this law, but only partially, for it does not
appear that he was aware that there was any difference in power between
pollen from the same plant and from a distinct plant. In the
introduction to his book (page 4) he says, as the sexes are separated in
so many flowers, and as so many other flowers are dichogamous, "it
appears that nature has not willed that any one flower should be
fertilised by its own pollen." Nevertheless, he was far from keeping
this conclusion always before his mind, or he did not see its full
importance, as may be perceived by anyone who will read his observations
carefully; and he consequently mistook the meaning of various
structures. But his discoveries are so numerous and his work so
excellent, that he can well afford to bear a small amount of blame. A
most capable judge, H. Muller, likewise says: "It is remarkable in how
very many cases Sprengel rightly perceived that pollen is necessarily
transported to the stigmas of other flowers of the same species by the
insects which visit them, and yet did not imagine that this
transportation was of any service to the plants themselves." (1/4. 'Die
Befruchtung der Blumen' 1873 page 4. His words are: "Es ist merkwurdig,
in wie zahlreichen Fallen Sprengel richtig erkannte, dass durch die
Besuchenden Insekten der Bluthenstaub mit Nothwendigkeit auf die Narben
anderer Bluthen derselben Art ubertragen wird, ohne auf die Vermuthung
zu kommen, dass in dieser Wirkung der Nutzen des Insektenbesuches fur
die Pflanzen selbst gesucht werden musse.")
Andrew Knight saw the truth much more clearly, for he remarks, "Nature
intended that a sexual intercourse should take place between
neighbouring plants of the same species." (1/5. 'Philosophical
Transactions' 1799 page 202.) After alluding to the various means by
which pollen is transported from flower to flower, as far as was then
imperfectly known, he adds, "Nature has something more in view than that
its own proper males would fecundate each blossom." In 1811 Kolreuter
plainly hinted at the same law, as did afterwards another famous
hybridiser of plants, Herbert. (1/6. Kolreuter 'Mem. de l'Acad. de St.
Petersbourg' tome 3 1809 published 1811 page 197. After showing how well
the Malvaceae are adapted for cross-fertilisation, he asks, "An id
aliquid in recessu habeat, quod hujuscemodi flores nunquam proprio suo
pulvere, sed semper eo aliarum suae speciei impregnentur, merito
quaeritur? Certe natura nil facit frustra." Herbert 'Amaryllidaceae,
with a Treatise on Cross-bred Vegetables' 1837.) But none of these
distinguished observers appear to have been sufficiently impressed with
the truth and generality of the law, so as to insist on it and impress
their beliefs on others.
In 1862 I summed up my observations on Orchids by saying that nature
"abhors perpetual self-fertilisation." If the word perpetual had been
omitted, the aphorism would have been false. As it stands, I believe
that it is true, though perhaps rather too strongly expressed; and I
should have added the self-evident proposition that the propagation of
the species, whether by self-fertilisation or by cross-fertilisation, or
asexually by buds, stolons, etc. is of paramount importance. Hermann
Muller has done excellent service by insisting repeatedly on this latter
point.
It often occurred to me that it would be advisable to try whether
seedlings from cross-fertilised flowers were in any way superior to
those from self-fertilised flowers. But as no instance was known with
animals of any evil appearing in a single generation from the closest
possible interbreeding, that is between brothers and sisters, I thought
that the same rule would hold good with plants; and that it would be
necessary at the sacrifice of too much time to self-fertilise and
intercross plants during several successive generations, in order to
arrive at any result. I ought to have reflected that such elaborate
provisions favouring cross-fertilisation, as we see in innumerable
plants, would not have been acquired for the sake of gaining a distant
and slight advantage, or of avoiding a distant and slight evil.
Moreover, the fertilisation of a flower by its own pollen corresponds to
a closer form of interbreeding than is possible with ordinary bi-sexual
animals; so that an earlier result might have been expected.
I was at last led to make the experiments recorded in the present volume
from the following circumstance. For the sake of determining certain
points with respect to inheritance, and without any thought of the
effects of close interbreeding, I raised close together two large beds
of self-fertilised and crossed seedlings from the same plant of Linaria
vulgaris. To my surprise, the crossed plants when fully grown were
plainly taller and more vigorous than the self-fertilised ones. Bees
incessantly visit the flowers of this Linaria and carry pollen from one
to the other; and if insects are excluded, the flowers produce extremely
few seeds; so that the wild plants from which my seedlings were raised
must have been intercrossed during all previous generations. It seemed
therefore quite incredible that the difference between the two beds of
seedlings could have been due to a single act of self-fertilisation; and
I attributed the result to the self-fertilised seeds not having been
well ripened, improbable as it was that all should have been in this
state, or to some other accidental and inexplicable cause. During the
next year, I raised for the same purpose as before two large beds close
together of self-fertilised and crossed seedlings from the carnation,
Dianthus caryophyllus. This plant, like the Linaria, is almost sterile
if insects are excluded; and we may draw the same inference as before,
namely, that the parent-plants must have been intercrossed during every
or almost every previous generation. Nevertheless, the self-fertilised
seedlings were plainly inferior in height and vigour to the crossed.
My attention was now thoroughly aroused, for I could hardly doubt that
the difference between the two beds was due to the one set being the
offspring of crossed, and the other of self-fertilised flowers.
Accordingly I selected almost by hazard two other plants, which happened
to be in flower in the greenhouse, namely, Mimulus luteus and Ipomoea
purpurea, both of which, unlike the Linaria and Dianthus, are highly
self-fertile if insects are excluded. Some flowers on a single plant of
both species were fertilised with their own pollen, and others were
crossed with pollen from a distinct individual; both plants being
protected by a net from insects. The crossed and self-fertilised seeds
thus produced were sown on opposite sides of the same pots, and treated
in all respects alike; and the plants when fully grown were measured and
compared. With both species, as in the cases of the Linaria and
Dianthus, the crossed seedlings were conspicuously superior in height
and in other ways to the self-fertilised. I therefore determined to
begin a long series of experiments with various plants, and these were
continued for the following eleven years; and we shall see that in a
large majority of cases the crossed beat the self-fertilised plants.
Several of the exceptional cases, moreover, in which the crossed plants
were not victorious, can be explained.
It should be observed that I have spoken for the sake of brevity, and
shall continue to do so, of crossed and self-fertilised seeds,
seedlings, or plants; these terms implying that they are the product of
crossed or self-fertilised flowers. Cross-fertilisation always means a
cross between distinct plants which were raised from seeds and not from
cuttings or buds. Self-fertilisation always implies that the flowers in
question were impregnated with their own pollen.
My experiments were tried in the following manner. A single plant, if it
produced a sufficiency of flowers, or two or three plants were placed
under a net stretched on a frame, and large enough to cover the plant
(together with the pot, when one was used) without touching it. This
latter point is important, for if the flowers touch the net they may be
cross-fertilised by bees, as I have known to happen; and when the net is
wet the pollen may be injured. I used at first "white cotton net," with
very fine meshes, but afterwards a kind of net with meshes one-tenth of
an inch in diameter; and this I found by experience effectually excluded
all insects excepting Thrips, which no net will exclude. On the plants
thus protected several flowers were marked, and were fertilised with
their own pollen; and an equal number on the same plants, marked in a
different manner, were at the same time crossed with pollen from a
distinct plant. The crossed flowers were never castrated, in order to
make the experiments as like as possible to what occurs under nature
with plants fertilised by the aid of insects. Therefore, some of the
flowers which were crossed may have failed to be thus fertilised, and
afterwards have been self-fertilised. But this and some other sources of
error will presently be discussed. In some few cases of spontaneously
self-fertile species, the flowers were allowed to fertilise themselves
under the net; and in still fewer cases uncovered plants were allowed to
be freely crossed by the insects which incessantly visited them. There
are some great advantages and some disadvantages in my having
occasionally varied my method of proceeding; but when there was any
difference in the treatment, it is always so stated under the head of
each species.
Care was taken that the seeds were thoroughly ripened before being
gathered. Afterwards the crossed and self-fertilised seeds were in most
cases placed on damp sand on opposite sides of a glass tumbler covered
by a glass plate, with a partition between the two lots; and the glass
was placed on the chimney-piece in a warm room. I could thus observe the
germination of the seeds. Sometimes a few would germinate on one side
before any on the other, and these were thrown away. But as often as a
pair germinated at the same time, they were planted on opposite sides of
a pot, with a superficial partition between the two; and I thus
proceeded until from half-a-dozen to a score or more seedlings of
exactly the same age were planted on the opposite sides of several pots.
If one of the young seedlings became sickly or was in any way injured,
it was pulled up and thrown away, as well as its antagonist on the
opposite side of the same pot.
As a large number of seeds were placed on the sand to germinate, many
remained after the pairs had been selected, some of which were in a
state of germination and others not so; and these were sown crowded
together on the opposite sides of one or two rather larger pots, or
sometimes in two long rows out of doors. In these cases there was the
most severe struggle for life among the crossed seedlings on one side of
the pot, and the self-fertilised seedlings on the other side, and
between the two lots which grew in competition in the same pot. A vast
number soon perished, and the tallest of the survivors on both sides
when fully grown were measured. Plants treated in this manner, were
subjected to nearly the same conditions as those growing in a state of
nature, which have to struggle to maturity in the midst of a host of
competitors.
On other occasions, from the want of time, the seeds, instead of being
allowed to germinate on damp sand, were sown on the opposite sides of
pots, and the fully grown plants measured. But this plan is less
accurate, as the seeds sometimes germinated more quickly on one side
than on the other. It was however necessary to act in this manner with
some few species, as certain kinds of seeds would not germinate well
when exposed to the light; though the glasses containing them were kept
on the chimney-piece on one side of a room, and some way from the two
windows which faced the north-east. (1/7. This occurred in the plainest
manner with the seeds of Papaver vagum and Delphinium consolida, and
less plainly with those of Adonis aestivalis and Ononis minutissima.
Rarely more than one or two of the seeds of these four species
germinated on the bare sand, though left there for some weeks; but when
these same seeds were placed on earth in pots, and covered with a thin
layer of sand, they germinated immediately in large numbers.)
The soil in the pots in which the seedlings were planted, or the seeds
sown, was well mixed, so as to be uniform in composition. The plants on
the two sides were always watered at the same time and as equally as
possible; and even if this had not been done, the water would have
spread almost equally to both sides, as the pots were not large. The
crossed and self-fertilised plants were separated by a superficial
partition, which was always kept directed towards the chief source of
the light, so that the plants on both sides were equally illuminated. I
do not believe it possible that two sets of plants could have been
subjected to more closely similar conditions, than were my crossed and
self-fertilised seedlings, as grown in the above described manner.
In comparing the two sets, the eye alone was never trusted. Generally
the height of every plant on both sides was carefully measured, often
more than once, namely, whilst young, sometimes again when older, and
finally when fully or almost fully grown. But in some cases, which are
always specified, owing to the want of time, only one or two of the
tallest plants on each side were measured. This plan, which is not a
good one, was never followed (except with the crowded plants raised from
the seeds remaining after the pairs had been planted) unless the tallest
plants on each side seemed fairly to represent the average difference
between those on both sides. It has, however, some great advantages, as
sickly or accidentally injured plants, or the offspring of ill-ripened
seeds, are thus eliminated. When the tallest plants alone on each side
were measured, their average height of course exceeds that of all the
plants on the same side taken together. But in the case of the much
crowded plants raised from the remaining seeds, the average height of
the tallest plants was less than that of the plants in pairs, owing to
the unfavourable conditions to which they were subjected from being
greatly crowded. For our purpose, however, of the comparison of the
crossed and self-fertilised plants, their absolute height signifies
little.
As the plants were measured by an ordinary English standard divided into
inches and eighths of an inch, I have not thought it worth while to
change the fractions into decimals. The average or mean heights were
calculated in the ordinary rough method by adding up the measurements of
all, and dividing the product by the number of plants measured; the
result being here given in inches and decimals. As the different species
grow to various heights, I have always for the sake of easy comparison
given in addition the average height of the crossed plants of each
species taken as 100, and have calculated the average height of the
self-fertilised plant in relation to this standard. With respect to the
crowded plants raised from the seeds remaining after the pairs had been
planted, and of which only some of the tallest on each side were
measured, I have not thought it worth while to complicate the results by
giving separate averages for them and for the pairs, but have added up
all their heights, and thus obtained a single average.
I long doubted whether it was worth while to give the measurements of
each separate plant, but have decided to do so, in order that it may be
seen that the superiority of the crossed plants over the
self-fertilised, does not commonly depend on the presence of two or
three extra fine plants on the one side, or of a few very poor plants on
the other side. Although several observers have insisted in general
terms on the offspring from intercrossed varieties being superior to
either parent-form, no precise measurements have been given (1/8. A
summary of these statements, with references, may be found in my
'Variation of Animals and Plants under Domestication' chapter 17 2nd
edition 1875 volume 2 page 109.); and I have met with no observations on
the effects of crossing and self-fertilising the individuals of the same
variety. Moreover, experiments of this kind require so much time--mine
having been continued during eleven years--that they are not likely soon
to be repeated.
As only a moderate number of crossed and self-fertilised plants were
measured, it was of great importance to me to learn how far the averages
were trustworthy. I therefore asked Mr. Galton, who has had much
experience in statistical researches, to examine some of my tables of
measurements, seven in number, namely, those of Ipomoea, Digitalis,
Reseda lutea, Viola, Limnanthes, Petunia, and Zea. I may premise that if
we took by chance a dozen or score of men belonging to two nations and
measured them, it would I presume be very rash to form any judgment from
such small numbers on their average heights. But the case is somewhat
different with my crossed and self-fertilised plants, as they were of
exactly the same age, were subjected from first to last to the same
conditions, and were descended from the same parents. When only from two
to six pairs of plants were measured, the results are manifestly of
little or no value, except in so far as they confirm and are confirmed
by experiments made on a larger scale with other species. I will now
give the report on the seven tables of measurements, which Mr. Galton
has had the great kindness to draw up for me.
["I have examined the measurements of the plants with care, and by many
statistical methods, to find out how far the means of the several sets
represent constant realities, such as would come out the same so long as
the general conditions of growth remained unaltered. The principal
methods that were adopted are easily explained by selecting one of the
shorter series of plants, say of Zea mays, for an example."
TABLE 1/1. Zea mays (young plants). (Mr. Galton.)
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed, as recorded by Mr. Darwin.
Column 3: Self-fertilised, as recorded by Mr. Darwin.
Column 4: Crossed, in Separate Pots, arranged in order of magnitude.
Column 5: Self-fertilised, in Separate Pots, arranged in order of magnitude.
Column 6: Crossed, in a Single Series, arranged in order of magnitude.
Column 7: Self-fertilised, in a Single Series, arranged in order of
magnitude.
Column 8: Difference, in a Single Series, arranged in order of magnitude.
Pot 1 : 23 4/8 : 17 3/8 :: 23 4/8 : 20 3/8 :: 23 4/8 : 20 3/8 : -3 1/8.
Pot 1 : 12 : 20 3/8 :: 21 : 20 :: 23 2/8 : 20 : -3 2/8.
Pot 1 : 21 : 20 :: 12 : 17 3/8 :: 23 : 20 : -3.
Pot 1 : - : - :: - : - :: 22 1/8 : 18 5/8 : -3 4/8.
Pot 1 : 22 : 20 :: 22 : 20 :: 22 1/8 : 18 5/8 : -3 4/8.
Pot 2 : 19 1/8 : 18 3/8 :: 21 4/8 : 18 5/8 :: 22 : 18 3/8 : -3 5/8.
Pot 2 : 21 4/8 : 18 5/8 :: 19 1/8 : 18 3/8 :: 21 5/8 : 18 : -3 5/8.
Pot 2 : - : - :: - : - :: 21 4/8 : 18 : -3 4/8.
Pot 2 : 22 1/8 : 18 5/8 :: 23 2/8 : 18 5/8 :: 21 : 18 : -3.
Pot 2 : 20 3/8 : 15 2/8 :: 22 1/8 : 18 :: 21 : 17 3/8 : -3 5/8.
Pot 3 : 18 2/8 : 16 4/8 :: 21 5/8 : 16 4/8 :: 20 3/8 : 16 4/8 : -3 7/8.
Pot 3 : 21 5/8 : 18 :: 20 3/8 : 16 2/8 :: 19 1/8 : 16 2/8 : -2 7/8.
Pot 3 : 23 2/8 : 16 2/8 :: 18 2/8 : 15 2/8 :: 18 2/8 : 15 4/8 : -2 6/8.
Pot 3 : - : - :: - : - :: 12 : 15 2/8 : +3 2/8.
Pot 3 : 21 : 18 :: 23 : 18 :: 12 : 12 6/8 : +0 6/8.
Pot 4 : 22 1/8 : 12 6/8 :: 22 1/8 : 18.
Pot 4 : 23 : 15 4/8 :: 21 : 15 4/8.
Pot 4 : 12 : 18 :: 12 : 12 6/8.
"The observations as I received them are shown in Table 1/1, Columns 2
and 3, where they certainly have no prima facie appearance of
regularity. But as soon as we arrange them the in order of their
magnitudes, as in columns 4 and 5, the case is materially altered. We
now see, with few exceptions, that the largest plant on the crossed side
in each pot exceeds the largest plant on the self-fertilised side, that
the second exceeds the second, the third the third, and so on. Out of
the fifteen cases in the table, there are only two exceptions to this
rule. We may therefore confidently affirm that a crossed series will
always be found to exceed a self-fertilised series, within the range of
the conditions under which the present experiment has been made."
TABLE 1/2.
Column 1: Number (Name) of Pot.
Column 2: Crossed.
Column 3: Self-fertilised.
Column 4: Difference.
Pot 1 : 18 7/8 : 19 2/8 : +0 3/8.
Pot 2 : 20 7/8 : 19 : -1 7/8.
Pot 3 : 21 1/8 : 16 7/8 : -4 2/8.
Pot 4 : 19 6/8 : 16 : -3 6/8.
"Next as regards the numerical estimate of this excess. The mean values
of the several groups are so discordant, as is shown in Table 1/2, that
a fairly precise numerical estimate seems impossible. But the
consideration arises, whether the difference between pot and pot may not
be of much the same order of importance as that of the other conditions
upon which the growth of the plants has been modified. If so, and only
on that condition, it would follow that when all the measurements,
either of the crossed or the self-fertilised plants, were combined into
a single series, that series would be statistically regular. The
experiment is tried in Table 1/1, columns 7 and 8, where the regularity
is abundantly clear, and justifies us in considering its mean as
perfectly reliable. I have protracted these measurements, and revised
them in the usual way, by drawing a curve through them with a free hand,
but the revision barely modifies the means derived from the original
observations. In the present, and in nearly all the other cases, the
difference between the original and revised means is under 2 per cent of
their value. It is a very remarkable coincidence that in the seven kinds
of plants, whose measurements I have examined, the ratio between the
heights of the crossed and of the self-fertilised ranges in five cases
within very narrow limits. In Zea mays it is as 100 to 84, and in the
others it ranges between 100 to 76 and 100 to 86."
"The determination of the variability (measured by what is technically
called the 'probable error') is a problem of more delicacy than that of
determining the means, and I doubt, after making many trials, whether it
is possible to derive useful conclusions from these few observations. We
ought to have measurements of at least fifty plants in each case, in
order to be in a position to deduce fair results. One fact, however,
bearing on variability, is very evident in most cases, though not in Zea
mays, namely, that the self-fertilised plants include the larger number
of exceptionally small specimens, while the crossed are more generally
full grown."
"Those groups of cases in which measurements have been made of a few of
the tallest plants that grew in rows, each of which contained a
multitude of plants, show very clearly that the crossed plants exceed
the self-fertilised in height, but they do not tell by inference
anything about their respective mean values. If it should happen that a
series is known to follow the law of error or any other law, and if the
number of individuals in the series is known, it would be always
possible to reconstruct the whole series when a fragment of it has been
given. But I find no such method to be applicable in the present case.
The doubt as to the number of plants in each row is of minor importance;
the real difficulty lies in our ignorance of the precise law followed by
the series. The experience of the plants in pots does not help us to
determine that law, because the observations of such plants are too few
to enable us to lay down more than the middle terms of the series to
which they belong with any sort of accuracy, whereas the cases we are
now considering refer to one of its extremities. There are other special
difficulties which need not be gone into, as the one already mentioned
is a complete bar."]
Mr. Galton sent me at the same time graphical representations which he
had made of the measurements, and they evidently form fairly regular
curves. He appends the words "very good" to those of Zea and Limnanthes.
He also calculated the average height of the crossed and self-fertilised
plants in the seven tables by a more correct method than that followed
by me, namely, by including the heights, as estimated in accordance with
statistical rules, of a few plants which died before they were measured;
whereas I merely added up the heights of the survivors, and divided the
sum by their number. The difference in our results is in one way highly
satisfactory, for the average heights of the self-fertilised plants, as
deduced by Mr. Galton, is less than mine in all the cases excepting one,
in which our averages are the same; and this shows that I have by no
means exaggerated the superiority of the crossed over the
self-fertilised plants.
After the heights of the crossed and self-fertilised plants had been
taken, they were sometimes cut down close to the ground, and an equal
number of both weighed. This method of comparison gives very striking
results, and I wish that it had been oftener followed. Finally a record
was often kept of any marked difference in the rate of germination of
the crossed and self-fertilised seeds,--of the relative periods of
flowering of the plants raised from them,--and of their productiveness,
that is, of the number of seed-capsules which they produced and of the
average number of seeds which each capsule contained.
When I began my experiments I did not intend to raise crossed and
self-fertilised plants for more than a single generation; but as soon as
the plants of the first generation were in flower I thought that I would
raise one more generation, and acted in the following manner. Several
flowers on one or more of the self-fertilised plants were again
self-fertilised; and several flowers on one or more of the crossed
plants were fertilised with pollen from another crossed plant of the
same lot. Having thus once begun, the same method was followed for as
many as ten successive generations with some of the species. The seeds
and seedlings were always treated in exactly the same manner as already
described. The self-fertilised plants, whether originally descended from
one or two mother-plants, were thus in each generation as closely
interbred as was possible; and I could not have improved on my plan. But
instead of crossing one of the crossed plants with another crossed
plant, I ought to have crossed the self-fertilised plants of each
generation with pollen taken from a non-related plant--that is, one
belonging to a distinct family or stock of the same species and variety.
This was done in several cases as an additional experiment, and gave
very striking results. But the plan usually followed was to put into
competition and compare intercrossed plants, which were almost always
the offspring of more or less closely related plants, with the
self-fertilised plants of each succeeding generation;--all having been
grown under closely similar conditions. I have, however, learnt more by
this method of proceeding, which was begun by an oversight and then
necessarily followed, than if I had always crossed the self-fertilised
plants of each succeeding generation with pollen from a fresh stock.
I have said that the crossed plants of the successive generations were
almost always inter-related. When the flowers on an hermaphrodite plant
are crossed with pollen taken from a distinct plant, the seedlings thus
raised may be considered as hermaphrodite brothers or sisters; those
raised from the same capsule being as close as twins or animals of the
same litter. But in one sense the flowers on the same plant are distinct
individuals, and as several flowers on the mother-plant were crossed by
pollen taken from several flowers on the father-plant, such seedlings
would be in one sense half-brothers or sisters, but more closely related
than are the half-brothers and sisters of ordinary animals. The flowers
on the mother-plant were, however, commonly crossed by pollen taken from
two or more distinct plants; and in these cases the seedlings might be
called with more truth half-brothers or sisters. When two or three
mother-plants were crossed, as often happened, by pollen taken from two
or three father-plants (the seeds being all intermingled), some of the
seedlings of the first generation would be in no way related, whilst
many others would be whole or half-brothers and sisters. In the second
generation a large number of the seedlings would be what may be called
whole or half first-cousins, mingled with whole and half-brothers and
sisters, and with some plants not at all related. So it would be in the
succeeding generations, but there would also be many cousins of the
second and more remote degrees. The relationship will thus have become
more and more inextricably complex in the later generations; with most
of the plants in some degree and many of them closely related.
I have only one other point to notice, but this is one of the highest
importance; namely, that the crossed and self-fertilised plants were
subjected in the same generation to as nearly similar and uniform
conditions as was possible. In the successive generations they were
exposed to slightly different conditions as the seasons varied, and they
were raised at different periods. But in other respects all were treated
alike, being grown in pots in the same artificially prepared soil, being
watered at the same time, and kept close together in the same greenhouse
or hothouse. They were therefore not exposed during successive years to
such great vicissitudes of climate as are plants growing out of doors.
ON SOME APPARENT AND REAL CAUSES OF ERROR IN MY EXPERIMENTS.
It has been objected to such experiments as mine, that covering plants
with a net, although only for a short time whilst in flower, may affect
their health and fertility. I have seen no such effect except in one
instance with a Myosotis, and the covering may not then have been the
real cause of injury. But even if the net were slightly injurious, and
certainly it was not so in any high degree, as I could judge by the
appearance of the plants and by comparing their fertility with that of
neighbouring uncovered plants, it would not have vitiated my
experiments; for in all the more important cases the flowers were
crossed as well as self-fertilised under a net, so that they were
treated in this respect exactly alike.
As it is impossible to exclude such minute pollen-carrying insects as
Thrips, flowers which it was intended to fertilise with their own pollen
may sometimes have been afterwards crossed with pollen brought by these
insects from another flower on the same plant; but as we shall hereafter
see, a cross of this kind does not produce any effect, or at most only a
slight one. When two or more plants were placed near one another under
the same net, as was often done, there is some real though not great
danger of the flowers which were believed to be self-fertilised being
afterwards crossed with pollen brought by Thrips from a distinct plant.
I have said that the danger is not great because I have often found that
plants which are self-sterile, unless aided by insects, remained sterile
when several plants of the same species were placed under the same net.
If, however, the flowers which had been presumably self-fertilised by me
were in any case afterwards crossed by Thrips with pollen brought from a
distinct plant, crossed seedlings would have been included amongst the
self-fertilised; but it should be especially observed that this
occurrence would tend to diminish and not to increase any superiority in
average height, fertility, etc., of the crossed over the self-fertilised
plants.
As the flowers which were crossed were never castrated, it is probable
or even almost certain that I sometimes failed to cross-fertilise them
effectually, and that they were afterwards spontaneously
self-fertilised. This would have been most likely to occur with
dichogamous species, for without much care it is not easy to perceive
whether their stigmas are ready to be fertilised when the anthers open.
But in all cases, as the flowers were protected from wind, rain, and the
access of insects, any pollen placed by me on the stigmatic surface
whilst it was immature, would generally have remained there until the
stigma was mature; and the flowers would then have been crossed as was
intended. Nevertheless, it is highly probable that self-fertilised
seedlings have sometimes by this means got included amongst the crossed
seedlings. The effect would be, as in the former case, not to exaggerate
but to diminish any average superiority of the crossed over the
self-fertilised plants.
Errors arising from the two causes just named, and from others,--such as
some of the seeds not having been thoroughly ripened, though care was
taken to avoid this error--the sickness or unperceived injury of any of
the plants,--will have been to a large extent eliminated, in those cases
in which many crossed and self-fertilised plants were measured and an
average struck. Some of these causes of error will also have been
eliminated by the seeds having been allowed to germinate on bare damp
sand, and being planted in pairs; for it is not likely that ill-matured
and well-matured, or diseased and healthy seeds, would germinate at
exactly the same time. The same result will have been gained in the
several cases in which only a few of the tallest, finest, and healthiest
plants on each side of the pots were measured.
Kolreuter and Gartner have proved that with some plants several, even as
many as from fifty to sixty, pollen-grains are necessary for the
fertilisation of all the ovules in the ovarium. (1/9. 'Kentniss der
Befruchtung' 1844 page 345. Naudin 'Nouvelles Archives du Museum' tome 1
page 27.) Naudin also found in the case of Mirabilis that if only one or
two of its very large pollen-grains were placed on the stigma, the
plants raised from such seeds were dwarfed. I was therefore careful to
give an amply sufficient supply of pollen, and generally covered the
stigma with it; but I did not take any special pains to place exactly
the same amount on the stigmas of the self-fertilised and crossed
flowers. After having acted in this manner during two seasons, I
remembered that Gartner thought, though without any direct evidence,
that an excess of pollen was perhaps injurious; and it has been proved
by Spallanzani, Quatrefages, and Newport, that with various animals an
excess of the seminal fluid entirely prevents fertilisation. (1/10.
'Transactions of the Philosophical Society' 1853 pages 253-258.) It was
therefore necessary to ascertain whether the fertility of the flowers
was affected by applying a rather small and an extremely large quantity
of pollen to the stigma. Accordingly a very small mass of pollen-grains
was placed on one side of the large stigma in sixty-four flowers of
Ipomoea purpurea, and a great mass of pollen over the whole surface of
the stigma in sixty-four other flowers. In order to vary the experiment,
half the flowers of both lots were on plants produced from
self-fertilised seeds, and the other half on plants from crossed seeds.
The sixty-four flowers with an excess of pollen yielded sixty-one
capsules; and excluding four capsules, each of which contained only a
single poor seed, the remainder contained on an average 5.07 seeds per
capsule. The sixty-four flowers with only a little pollen placed on one
side of the stigma yielded sixty-three capsules, and excluding one from
the same cause as before, the remainder contained on an average 5.129
seeds. So that the flowers fertilised with little pollen yielded rather
more capsules and seeds than did those fertilised with an excess; but
the difference is too slight to be of any significance. On the other
hand, the seeds produced by the flowers with an excess of pollen were a
little heavier of the two; for 170 of them weighed 79.67 grains, whilst
170 seeds from the flowers with very little pollen weighed 79.20 grains.
Both lots of seeds having been placed on damp sand presented no
difference in their rate of germination. We may therefore conclude that
my experiments were not affected by any slight difference in the amount
of pollen used; a sufficiency having been employed in all cases.
The order in which our subject will be treated in the present volume is
as follows. A long series of experiments will first be given in Chapters
2 to 6. Tables will afterwards be appended, showing in a condensed form
the relative heights, weights, and fertility of the offspring of the
various crossed and self-fertilised species. Another table exhibits the
striking results from fertilising plants, which during several
generations had either been self-fertilised or had been crossed with
plants kept all the time under closely similar conditions, with pollen
taken from plants of a distinct stock and which had been exposed to
different conditions. In the concluding chapters various related points
and questions of general interest will be discussed.
Anyone not specially interested in the subject need not attempt to read
all the details (marked []); though they possess, I think, some value,
and cannot be all summarised. But I would suggest to the reader to take
as an example the experiments on Ipomoea in Chapter 2; to which may be
added those on Digitalis, Origanum, Viola, or the common cabbage, as in
all these cases the crossed plants are superior to the self-fertilised
in a marked degree, but not in quite the same manner. As instances of
self-fertilised plants being equal or superior to the crossed, the
experiments on Bartonia, Canna, and the common pea ought to be read; but
in the last case, and probably in that of Canna, the want of any
superiority in the crossed plants can be explained.
Species were selected for experiment belonging to widely distinct
families, inhabiting various countries. In some few cases several genera
belonging to the same family were tried, and these are grouped together;
but the families themselves have been arranged not in any natural order,
but in that which was the most convenient for my purpose. The
experiments have been fully given, as the results appear to me of
sufficient value to justify the details. Plants bearing hermaphrodite
flowers can be interbred more closely than is possible with bisexual
animals, and are therefore well-fitted to throw light on the nature and
extent of the good effects of crossing, and on the evil effects of close
interbreeding or self-fertilisation. The most important conclusion at
which I have arrived is that the mere act of crossing by itself does no
good. The good depends on the individuals which are crossed differing
slightly in constitution, owing to their progenitors having been
subjected during several generations to slightly different conditions,
or to what we call in our ignorance spontaneous variation. This
conclusion, as we shall hereafter see, is closely connected with various
important physiological problems, such as the benefit derived from
slight changes in the conditions of life, and this stands in the closest
connection with life itself. It throws light on the origin of the two
sexes and on their separation or union in the same individual, and
lastly on the whole subject of hybridism, which is one of the greatest
obstacles to the general acceptance and progress of the great principle
of evolution.
In order to avoid misapprehension, I beg leave to repeat that throughout
this volume a crossed plant, seedling, or seed, means one of crossed
PARENTAGE, that is, one derived from a flower fertilised with pollen
from a distinct plant of the same species. And that a self-fertilised
plant, seedling, or seed, means one of self-fertilised PARENTAGE, that
is, one derived from a flower fertilised with pollen from the same
flower, or sometimes, when thus stated, from another flower on the same
plant.
CHAPTER II.
CONVOLVULACEAE.
Ipomoea purpurea, comparison of the height and fertility of the crossed
and self-fertilised plants during ten successive generations.
Greater constitutional vigour of the crossed plants.
The effects on the offspring of crossing different flowers on the same
plant, instead of crossing distinct individuals.
The effects of a cross with a fresh stock.
The descendants of the self-fertilised plant named Hero.
Summary on the growth, vigour, and fertility of the successive crossed
and self-fertilised generations.
Small amount of pollen in the anthers of the self-fertilised plants of
the later generations, and the sterility of their first-produced
flowers.
Uniform colour of the flowers produced by the self-fertilised plants.
The advantage from a cross between two distinct plants depends on their
differing in constitution.
A plant of Ipomoea purpurea, or as it is often called in England the
convolvulus major, a native of South America, grew in my greenhouse. Ten
flowers on this plant were fertilised with pollen from the same flower;
and ten other flowers on the same plant were crossed with pollen from a
distinct plant. The fertilisation of the flowers with their own pollen
was superfluous, as this convolvulus is highly self-fertile; but I acted
in this manner to make the experiments correspond in all respects.
Whilst the flowers are young the stigma projects beyond the anthers; and
it might have been thought that it could not be fertilised without the
aid of humble-bees, which often visit the flowers; but as the flower
grows older the stamens increase in length, and their anthers brush
against the stigma, which thus receives some pollen. The number of seeds
produced by the crossed and self-fertilised flowers differed very
little.
[Crossed and self-fertilised seeds obtained in the above manner were
allowed to germinate on damp sand, and as often as pairs germinated at
the same time they were planted in the manner described in the
Introduction (Chapter 1), on the opposite sides of two pots. Five pairs
were thus planted; and all the remaining seeds, whether or not in a
state of germination, were planted on the opposite sides of a third pot,
so that the young plants on both sides were here greatly crowded and
exposed to very severe competition. Rods of iron or wood of equal
diameter were given to all the plants to twine up; and as soon as one of
each pair reached the summit both were measured. A single rod was placed
on each side of the crowded pot, Number 3, and only the tallest plant on
each side was measured.
TABLE 2/1. Ipomoea purpurea (First Generation.).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Seedlings from Crossed Plants.
Column 3: Seedlings from Self-fertilised Plants.
Pot 1 : 87 4/8 : 69.
Pot 1 : 87 4/8 : 66.
Pot 1 : 89 : 73.
Pot 2 : 88 : 68 4/8.
Pot 2 : 87 : 60 4/8.
Pot 3 : 77 : 57.
Plants crowded; the tallest one measured on each side.
Total : 516 : 394.
The average height of the six crossed plants is here 86 inches, whilst
that of the six self-fertilised plants is only 65.66 inches, so that the
crossed plants are to the self-fertilised in height as 100 to 76. It
should be observed that this difference is not due to a few of the
crossed plants being extremely tall, or to a few of the self-fertilised
being extremely short, but to all the crossed plants attaining a greater
height than their antagonists. The three pairs in Pot 1 were measured at
two earlier periods, and the difference was sometimes greater and
sometimes less than that at the final measuring. But it is an
interesting fact, of which I have seen several other instances, that one
of the self-fertilised plants, when nearly a foot in height, was half an
inch taller than the crossed plant; and again, when two feet high, it
was 1 3/8 of an inch taller, but during the ten subsequent days the
crossed plant began to gain on its antagonist, and ever afterward
asserted its supremacy, until it exceeded its self-fertilised opponent
by 16 inches.
The five crossed plants in Pots 1 and 2 were covered with a net, and
produced 121 capsules; the five self-fertilised plants produced
eighty-four capsules, so that the numbers of capsules were as 100 to 69.
Of the 121 capsules on the crossed plants sixty-five were the product of
flowers crossed with pollen from a distinct plant, and these contained
on an average 5.23 seeds per capsule; the remaining fifty-six capsules
were spontaneously self-fertilised. Of the eighty-four capsules on the
self-fertilised plants, all the product of renewed self-fertilisation,
fifty-five (which were alone examined) contained on an average 4.85
seeds per capsule. Therefore the cross-fertilised capsules, compared
with the self-fertilised capsules, yielded seeds in the proportion of
100 to 93. The crossed seeds were relatively heavier than the
self-fertilised seeds. Combining the above data (i.e., number of
capsules and average number of contained seeds), the crossed plants,
compared with the self-fertilised, yielded seeds in the ratio of 100 to
64.
These crossed plants produced, as already stated, fifty-six
spontaneously self-fertilised capsules, and the self-fertilised plants
produced twenty-nine such capsules. The former contained on an average,
in comparison with the latter, seeds in the proportion of 100 to 99.
In Pot 3, on the opposite sides of which a large number of crossed and
self-fertilised seeds had been sown and the seedlings allowed to
struggle together, the crossed plants had at first no great advantage.
At one time the tallest crossed was 25 1/8 inches high, and the tallest
self-fertilised plants 21 3/8. But the difference afterwards became much
greater. The plants on both sides, from being so crowded, were poor
specimens. The flowers were allowed to fertilise themselves
spontaneously under a net; the crossed plants produced thirty-seven
capsules, the self-fertilised plants only eighteen, or as 100 to 47. The
former contained on an average 3.62 seeds per capsule; and the latter
3.38 seeds, or as 100 to 93. Combining these data (i.e., number of
capsules and average number of seeds), the crowded crossed plants
produced seeds compared with the self-fertilised as 100 to 45. These
latter seeds, however, were decidedly heavier, a hundred weighing 41.64
grains, than those from the capsules on the crossed plants, of which a
hundred weighed 36.79 grains; and this probably was due to the fewer
capsules borne by the self-fertilised plants having been better
nourished. We thus see that the crossed plants in this the first
generation, when grown under favourable conditions, and when grown under
unfavourable conditions from being much crowded, greatly exceeded in
height, and in the number of capsules produced, and slightly in the
number of seeds per capsule, the self-fertilised plants.
CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.
Flowers on the crossed plants of the last generation (Table 2/1) were
crossed by pollen from distinct plants of the same generation; and
flowers on the self-fertilised plants were fertilised by pollen from the
same flower. The seeds thus produced were treated in every respect as
before, and we have in Table 2/2 the result.
TABLE 2/2. Ipomoea purpurea (Second Generation.).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 87 : 67 4/8.
Pot 1 : 83 : 68 4/8.
Pot 1 : 83 : 80 4/8.
Pot 2 : 85 4/8 : 61 4/8.
Pot 2 : 89 : 79.
Pot 2 : 77 4/8 : 41.
Total : 505 : 398.
Here again every single crossed plant is taller than its antagonist. The
self-fertilised plant in Pot 1, which ultimately reached the unusual
height of 80 4/8 inches, was for a long time taller than the opposed
crossed plant, though at last beaten by it. The average height of the
six crossed plants is 84.16 inches, whilst that of the six
self-fertilised plants is 66.33 inches, or as 100 to 79.
CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.
Seeds from the crossed plants of the last generation (Table 2/2) again
crossed, and from the self-fertilised plants again self-fertilised, were
treated in all respects exactly as before, with the following result:--
TABLE 2/3. Ipomoea purpurea (Third Generation.).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 74 : 56 4/8.
Pot 1 : 72 : 51 4/8.
Pot 1 : 73 4/8 : 54.
Pot 2 : 82 : 59.
Pot 2 : 81 : 30.
Pot 2 : 82 : 66.
Total : 464.5 : 317.
Again all the crossed plants are higher than their antagonists: their
average height is 77.41 inches, whereas that of the self-fertilised is
52.83 inches, or as 100 to 68.
I attended closely to the fertility of the plants of this third
generation. Thirty flowers on the crossed plants were crossed with
pollen from other crossed plants of the same generation, and the
twenty-six capsules thus produced contained, on an average, 4.73 seeds;
whilst thirty flowers on the self-fertilised plants, fertilised with the
pollen from the same flower, produced twenty-three capsules, each
containing 4.43 seeds. Thus the average number of seeds in the crossed
capsules was to that in the self-fertilised capsules as 100 to 94. A
hundred of the crossed seeds weighed 43.27 grains, whilst a hundred of
the self-fertilised seeds weighed only 37.63 grains. Many of these
lighter self-fertilised seeds placed on damp sand germinated before the
crossed; thus thirty-six of the former germinated whilst only thirteen
of the latter or crossed seeds germinated. In Pot 1 the three crossed
plants produced spontaneously under the net (besides the twenty-six
artificially cross-fertilised capsules) seventy-seven self-fertilised
capsules containing on an average 4.41 seeds; whilst the three
self-fertilised plants produced spontaneously (besides the twenty-three
artificially self-fertilised capsules) only twenty-nine self-fertilised
capsules, containing on an average 4.14 seeds. Therefore the average
number of seeds in the two lots of spontaneously self-fertilised
capsules was as 100 to 94. Taking into consideration the number of
capsules together with the average number of seeds, the crossed plants
(spontaneously self-fertilised) produced seeds in comparison with the
self-fertilised plants (spontaneously self-fertilised) in the proportion
of 100 to 35. By whatever method the fertility of these plants is
compared, the crossed are more fertile than the self-fertilised plants.
I tried in several ways the comparative vigour and powers of growth of
the crossed and self-fertilised plants of this third generation. Thus,
four self-fertilised seeds which had just germinated were planted on one
side of a pot, and after an interval of forty-eight hours, four crossed
seeds in the same state of germination were planted on the opposite
side; and the pot was kept in the hothouse. I thought that the advantage
thus given to the self-fertilised seedlings would have been so great
that they would never have been beaten by the crossed ones. They were
not beaten until all had grown to a height of 18 inches; and the degree
to which they were finally beaten is shown in Table 2/4. We here see
that the average height of the four crossed plants is 76.62, and of the
four self-fertilised plants 65.87 inches, or as 100 to 86; therefore
less than when both sides started fair.
TABLE 2/4. Ipomoea purpurea (Third Generation, the self-fertilised
plants having had a start of forty-eight hours).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 3 : 78 4/8 : 73 4/8.
Pot 3 : 77 4/8 : 53.
Pot 3 : 73 : 61 4/8.
Pot 3 : 77 4/8 : 75 4/8.
Total : 306.5 : 263.5.
Crossed and self-fertilised seeds of the third generation were also sown
out of doors late in the summer, and therefore under unfavourable
conditions, and a single stick was given to each lot of plants to twine
up. The two lots were sufficiently separate so as not to interfere with
each other's growth, and the ground was clear of weeds. As soon as they
were killed by the first frost (and there was no difference in their
hardiness), the two tallest crossed plants were found to be 24.5 and
22.5 inches, whilst the two tallest self-fertilised plants were only 15
and 12.5 inches in height, or as 100 to 59.
I likewise sowed at the same time two lots of the same seeds in a part
of the garden which was shady and covered with weeds. The crossed
seedlings from the first looked the most healthy, but they twined up a
stick only to a height of 7 1/4 inches; whilst the self-fertilised were
not able to twine at all; and the tallest of them was only 3 1/2 inches
in height.
Lastly, two lots of the same seeds were sown in the midst of a bed of
candy-tuft (Iberis) growing vigorously. The seedlings came up, but all
the self-fertilised ones soon died excepting one, which never twined and
grew to a height of only 4 inches. Many of the crossed seedlings, on the
other hand, survived; and some twined up the stems of the Iberis to the
height of 11 inches. These cases prove that the crossed seedlings have
an immense advantage over the self-fertilised, both when growing
isolated under very unfavourable conditions, and when put into
competition with each other or with other plants, as would happen in a
state of nature.
CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION.
Seedlings raised as before from the crossed and self-fertilised plants
of the third generation in Table 2/3, gave results as follows:--
TABLE 2/5. Ipomoea purpurea (Fourth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 84 : 80.
Pot 1 : 47 : 44 1/2.
Pot 2 : 83 : 73 1/2.
Pot 2 : 59 : 51 1/2.
Pot 3 : 82 : 56 1/2.
Pot 3 : 65 1/2 : 63.
Pot 3 : 68 : 52.
Total : 488.5 : 421.0.
Here the average height of the seven crossed plants is 69.78 inches, and
that of the seven self-fertilised plants 60.14; or as 100 to 86. This
smaller difference relatively to that in the former generations, may be
attributed to the plants having been raised during the depth of winter,
and consequently to their not having grown vigorously, as was shown by
their general appearance and from several of them never reaching the
summits of the rods. In Pot 2, one of the self-fertilised plants was for
a long time taller by two inches than its opponent, but was ultimately
beaten by it, so that all the crossed plants exceeded their opponents in
height. Of twenty-eight capsules produced by the crossed plants
fertilised by pollen from a distinct plant, each contained on an average
4.75 seeds; of twenty-seven self-fertilised capsules on the
self-fertilised plants, each contained on an average 4.47 seeds; so that
the proportion of seeds in the crossed and self-fertilised capsules was
as 100 to 94.
Some of the same seeds, from which the plants in Table 2/5 had been
raised, were planted, after they had germinated on damp sand, in a
square tub, in which a large Brugmansia had long been growing. The soil
was extremely poor and full of roots; six crossed seeds were planted in
one corner, and six self-fertilised seeds in the opposite corner. All
the seedlings from the latter soon died excepting one, and this grew to
the height of only 1 1/2 inches. Of the crossed plants three survived,
and they grew to the height of 2 1/2 inches, but were not able to twine
round a stick; nevertheless, to my surprise, they produced some small
miserable flowers. The crossed plants thus had a decided advantage over
the self-fertilised plants under this extremity of bad conditions.
CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION.
These were raised in the same manner as before, and when measured gave
the following results:--
TABLE 2/6. Ipomoea purpurea (Fifth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 96 : 73.
Pot 1 : 86 : 78.
Pot 1 : 69 : 29.
Pot 2 : 84 : 51.
Pot 2 : 84 : 84.
Pot 2 : 76 1/4 : 59.
Total : 495.25 : 374.00.
The average height of the six crossed plants is 82.54 inches, and that
of the six self-fertilised plants 62.33 inches, or as 100 to 75. Every
crossed plant exceeded its antagonist in height. In Pot 1 the middle
plant on the crossed side was slightly injured whilst young by a blow,
and was for a time beaten by its opponent, but ultimately recovered the
usual superiority. The crossed plants produced spontaneously a vast
number more capsules than did the self-fertilised plants; and the
capsules of the former contained on an average 3.37 seeds, whilst those
of the latter contained only 3.0 per capsule, or as 100 to 89. But
looking only to the artificially fertilised capsules, those on the
crossed plants again crossed contained on an average 4.46 seeds, whilst
those on the self-fertilised plants again self-fertilised contained 4.77
seeds; so that the self-fertilised capsules were the more fertile of the
two, and of this unusual fact I can offer no explanation.
CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION.
These were raised in the usual manner, with the following result. I
should state that there were originally eight plants on each side; but
as two of the self-fertilised became extremely unhealthy and never grew
to near their full height, these as well as their opponents have been
struck out of the list. If they had been retained, they would have made
the average height of the crossed plants unfairly greater than that of
the self-fertilised. I have acted in the same manner in a few other
instances, when one of a pair plainly became very unhealthy.
TABLE 2/7. Ipomoea purpurea (Sixth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 93 : 50 1/2.
Pot 1 : 91 : 65.
Pot 2 : 79 : 50.
Pot 2 : 86 1/2 : 87.
Pot 2 : 88 : 62.
Pot 3 : 87 1/2 : 64 1/2.
Total : 525 : 379.
The average height of the six crossed plants is here 87.5, and of the
six self-fertilised plants 63.16, or as 100 to 72. This large difference
was chiefly due to most of the plants, especially the self-fertilised
ones, having become unhealthy towards the close of their growth, and
they were severely attacked by aphides. From this cause nothing can be
inferred with respect to their relative fertility. In this generation we
have the first instance of a self-fertilised plant in Pot 2 exceeding
(though only by half an inch) its crossed opponent. This victory was
fairly won after a long struggle. At first the self-fertilised plant was
several inches taller than its opponent, but when the latter was 4 1/2
feet high it had grown equal; it then grew a little taller than the
self-fertilised plant, but was ultimately beaten by it to the extent of
half an inch, as shown in Table 2/7. I was so much surprised at this
case that I saved the self-fertilised seeds of this plant, which I will
call the "Hero," and experimented on its descendants, as will hereafter
be described.
Besides the plants included in Table 2/7, nine crossed and nine
self-fertilised plants of the same lot were raised in two other pots, 4
and 5. These pots had been kept in the hothouse, but from want of room
were, whilst the plants were young, suddenly moved during very cold
weather into the coldest part of the greenhouse. They all suffered
greatly, and never quite recovered. After a fortnight only two of the
nine self-fertilised seedlings were alive, whilst seven of the crossed
survived. The tallest of these latter plants when measured was 47 inches
in height, whilst the tallest of the two surviving self-fertilised
plants was only 32 inches. Here again we see how much more vigorous the
crossed plants are than the self-fertilised.
CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION.
These were raised as heretofore with the following result:--
TABLE 2/8. Ipomoea purpurea (Seventh Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 84 4/8 : 74 6/8.
Pot 1 : 84 6/8 : 84.
Pot 1 : 76 2/8 : 55 4/8.
Pot 2 : 84 4/8 : 65.
Pot 2 : 90 : 51 2/8.
Pot 2 : 82 2/8 : 80 4/8.
Pot 3 : 83 : 67 6/8.
Pot 3 : 86 : 60 2/8.
Pot 4 : 84 2/8 : 75 2/8.
Total : 755.50 : 614.25.
Each of these nine crossed plants is higher than its opponent, though in
one case only by three-quarters of an inch. Their average height is
83.94 inches, and that of the self-fertilised plants 68.25, or as 100 to
81. These plants, after growing to their full height, became very
unhealthy and infested with aphides, just when the seeds were setting,
so that many of the capsules failed, and nothing can be said on their
relative fertility.
CROSSED AND SELF-FERTILISED PLANTS OF THE EIGHTH GENERATION.
As just stated, the plants of the last generation, from which the
present ones were raised, were very unhealthy and their seeds of
unusually small size; and this probably accounts for the two lots
behaving differently to what they did in any of the previous or
succeeding generations. Many of the self-fertilised seeds germinated
before the crossed ones, and these were of course rejected. When the
crossed seedlings in Table 2/9 had grown to a height of between 1 and 2
feet, they were all, or almost all, shorter than their self-fertilised
opponents, but were not then measured. When they had acquired an average
height of 32.28 inches, that of the self-fertilised plants was 40.68, or
as 100 to 122. Moreover, every one of the self-fertilised plants, with a
single exception, exceeded its crossed opponent. When, however, the
crossed plants had grown to an average height of 77.56 inches, they just
exceeded (namely, by .7 of an inch) the average height of the
self-fertilised plants; but two of the latter were still taller than
their crossed opponents. I was so much astonished at this whole case,
that I tied string to the summits of the rods; the plants being thus
allowed to continue climbing upwards. When their growth was complete
they were untwined, stretched straight, and measured. The crossed plants
had now almost regained their accustomed superiority, as may be seen in
Table 2/9.
The average height of the eight crossed plants is here 113.25 inches,
and that of the self-fertilised plants 96.65, or as 100 to 85.
Nevertheless two of the self-fertilised plants, as may be seen in Table
2/9, were still higher than their crossed opponents. The latter
manifestly had much thicker stems and many more lateral branches, and
looked altogether more vigorous than the self-fertilised plants, and
generally flowered before them. The earlier flowers produced by these
self-fertilised plants did not set any capsules, and their anthers
contained only a small amount of pollen; but to this subject I shall
return. Nevertheless capsules produced by two other self-fertilised
plants of the same lot, not included in Table 2/9, which had been highly
favoured by being grown in separate pots, contained the large average
number of 5.1 seeds per capsule.
TABLE 2/9. Ipomoea purpurea (Eighth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 111 6/8 : 96.
Pot 1 : 127 : 54.
Pot 1 : 130 6/8 : 93 4/8.
Pot 2 : 97 2/8 : 94.
Pot 2 : 89 4/8 : 125 6/8.
Pot 3 : 103 6/8 : 115 4/8.
Pot 3 : 100 6/8 : 84 6/8.
Pot 3 : 147 4/8 : 109 6/8.
Total : 908.25 : 773.25.
CROSSED AND SELF-FERTILISED PLANTS OF THE NINTH GENERATION.
The plants of this generation were raised in the same manner as before,
with the result shown in Table 2/10.
The fourteen crossed plants average in height 81.39 inches and the
fourteen self-fertilised plants 64.07, or as 100 to 79. One
self-fertilised plant in Pot 3 exceeded, and one in Pot 4 equalled in
height, its opponent. The self-fertilised plants showed no sign of
inheriting the precocious growth of their parents; this having been due,
as it would appear, to the abnormal state of the seeds from the
unhealthiness of their parents. The fourteen self-fertilised plants
yielded only forty spontaneously self-fertilised capsules, to which must
be added seven, the product of ten flowers artificially self-fertilised.
On the other hand, the fourteen crossed plants yielded 152 spontaneously
self-fertilised capsules; but thirty-six flowers on these plants were
crossed (yielding thirty-three capsules), and these flowers would
probably have produced about thirty spontaneously self-fertilised
capsules. Therefore an equal number of the crossed and self-fertilised
plants would have produced capsules in the proportion of about 182 to
47, or as 100 to 26. Another phenomenon was well pronounced in this
generation, but I believe had occurred previously to a slight extent;
namely, that most of the flowers on the self-fertilised plants were
somewhat monstrous. The monstrosity consisted in the corolla being
irregularly split so that it did not open properly, with one or two of
the stamens slightly foliaceous, coloured, and firmly coherent to the
corolla. I observed this monstrosity in only one flower on the crossed
plants. The self-fertilised plants, if well nourished, would almost
certainly, in a few more generations, have produced double flowers, for
they had already become in some degree sterile. (2/1. See on this
subject 'Variation of Animals and Plants under Domestication' chapter 18
2nd edition volume 2 page 152.)
TABLE 2/10. Ipomoea purpurea (Ninth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 83 4/8 : 57.
Pot 1 : 85 4/8 : 71.
Pot 1 : 83 4/8 : 48 3/8.
Pot 2 : 83 2/8 : 45.
Pot 2 : 64 2/8 : 43 6/8.
Pot 2 : 64 3/8 : 38 4/8.
Pot 3 : 79 : 63.
Pot 3 : 88 1/8 : 71.
Pot 3 : 61 : 89 4/8.
Pot 4 : 82 4/8 : 82 4/8.
Pot 4 : 90 : 76 1/8.
Pot 5 : 89 4/8 : 67.
Pot 5 : 92 4/8 : 74 2/8.
Pot 5 : 92 4/8 : 70.
Crowded plants.
Total : 1139.5 : 897.0.
CROSSED AND SELF-FERTILISED PLANTS OF THE TENTH GENERATION.
Six plants were raised in the usual manner from the crossed plants of
the last generation (Table 2/10) again intercrossed, and from the
self-fertilised again self-fertilised. As one of the crossed plants in
Pot 1 in Table 2/11 became much diseased, having crumpled leaves, and
producing hardly any capsules, it and its opponent have been struck out
of the table.
TABLE 2/11. Ipomoea purpurea (Tenth Generation).
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 92 3/8 : 47 2/8.
Pot 1 : 94 4/8 : 34 6/8.
Pot 2 : 87 : 54 4/8.
Pot 2 : 89 5/8 : 49 2/8.
Pot 2 : 105 : 66 2/8.
Total : 468.5 : 252.0.
The five crossed plants average 93.7 inches, and the five
self-fertilised only 50.4, or as 100 to 54. This difference, however, is
so great that it must be looked at as in part accidental. The six
crossed plants (the diseased one here included) yielded spontaneously
101 capsules, and the six self-fertilised plants 88, the latter being
chiefly produced by one of the plants. But as the diseased plant, which
yielded hardly any seed, is here included, the ratio of 101 to 88 does
not fairly give the relative fertility of the two lots. The stems of the
six crossed plants looked so much finer than those of the six
self-fertilised plants, that after the capsules had been gathered and
most of the leaves had fallen off, they were weighed. Those of the
crossed plants weighed 2,693 grains, whilst those of the self-fertilised
plants weighed only 1,173 grains, or as 100 to 44; but as the diseased
and dwarfed crossed plant is here included, the superiority of the
former in weight was really greater.]
THE EFFECTS ON THE OFFSPRING OF CROSSING DIFFERENT FLOWERS ON THE SAME
PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS.
In all the foregoing experiments, seedlings from flowers crossed by
pollen from a distinct plant (though in the later generations more or
less closely related) were put into competition with, and almost
invariably proved markedly superior in height to the offspring from
self-fertilised flowers. I wished, therefore, to ascertain whether a
cross between two flowers on the same plant would give to the offspring
any superiority over the offspring from flowers fertilised with their
own pollen. I procured some fresh seed and raised two plants, which were
covered with a net; and several of their flowers were crossed with
pollen from a distinct flower on the same plant. Twenty-nine capsules
thus produced contained on an average 4.86 seeds per capsule; and 100 of
these seeds weighed 36.77 grains. Several other flowers were fertilised
with their own pollen, and twenty-six capsules thus produced contained
on an average 4.42 seeds per capsule; 100 of which weighed 42.61 grains.
So that a cross of this kind appears to have increased slightly the
number of seeds per capsule, in the ratio of 100 to 91; but these
crossed seeds were lighter than the self-fertilised in the ratio of 86
to 100. I doubt, however, from other observations, whether these results
are fully trustworthy. The two lots of seeds, after germinating on sand,
were planted in pairs on the opposite sides of nine pots, and were
treated in every respect like the plants in the previous experiments.
The remaining seeds, some in a state of germination and some not so,
were sown on the opposite sides of a large pot (Number 10); and the four
tallest plants on each side of this pot were measured. The result is
shown in Table 2/12.
TABLE 2/12. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 82 : 77 4/8.
Pot 1 : 75 : 87.
Pot 1 : 65 : 64.
Pot 1 : 76 : 87 2/8.
Pot 2 : 78 4/8 : 84.
Pot 2 : 43 : 86 4/8.
Pot 2 : 65 4/8 : 90 4/8.
Pot 3 : 61 2/8 : 86.
Pot 3 : 85 : 69 4/8.
Pot 3 : 89 : 87 4/8.
Pot 4 : 83 : 80 4/8.
Pot 4 : 73 4/8 : 88 4/8.
Pot 4 : 67 : 84 4/8.
Pot 5 : 78 : 66 4/8.
Pot 5 : 76 6/8 : 77 4/8.
Pot 5 : 57 : 81 4/8.
Pot 6 : 70 4/8 : 80.
Pot 6 : 79 : 82 4/8.
Pot 6 : 79 6/8 : 55 4/8.
Pot 7 : 76 : 77.
Pot 7 : 84 4/8 : 83 4/8.
Pot 7 : 79 : 73 4/8.
Pot 8 : 73 : 76 4/8.
Pot 8 : 67 : 82.
Pot 8 : 83 : 80 4/8.
Pot 9 : 73 2/8 : 78 4/8.
Pot 9 : 78 : 67 4/8.
Pot 10 : 34 : 82 4/8.
Pot 10 : 82 : 36 6/8.
Pot 10 : 84 6/8 : 69 4/8.
Pot 10 : 71 : 75 2/8.
Crowded plants.
Total : 2270.25 : 2399.75.
The average height of the thirty-one crossed plants is 73.23 inches, and
that of the thirty-one self-fertilised plants 77.41 inches; or as 100 to
106. Looking to each pair, it may be seen that only thirteen of the
crossed plants, whilst eighteen of the self-fertilised plants exceed
their opponents. A record was kept with respect to the plant which
flowered first in each pot; and only two of the crossed flowered before
one of the self-fertilised in the same pot; whilst eight of the
self-fertilised flowered first. It thus appears that the crossed plants
are slightly inferior in height and in earliness of flowering to the
self-fertilised. But the inferiority in height is so small, namely as
100 to 106, that I should have felt very doubtful on this head, had I
not cut down all the plants (except those in the crowded pot Number 10)
close to the ground and weighed them. The twenty-seven crossed plants
weighed 16 1/2 ounces, and the twenty-seven self-fertilised plants 20
1/2 ounces; and this gives a ratio of 100 to 124.
A self-fertilised plant of the same parentage as those in Table 2/12 had
been raised in a separate pot for a distinct purpose; and it proved
partially sterile, the anthers containing very little pollen. Several
flowers on this plant were crossed with the little pollen which could be
obtained from the other flowers on the same plant; and other flowers
were self-fertilised. From the seeds thus produced four crossed and four
self-fertilised plants were raised, which were planted in the usual
manner on the opposite sides of two pots. All these four crossed plants
were inferior in height to their opponents; they averaged 78.18 inches,
whilst the four self-fertilised plants averaged 84.8 inches; or as 100
to 108. (2/2. From one of these self-fertilised plants, spontaneously
self-fertilised, I gathered twenty-four capsules, and they contained on
an average only 3.2 seeds per capsule; so that this plant had apparently
inherited some of the sterility of its parent.) This case, therefore,
confirms the last. Taking all the evidence together, we must conclude
that these strictly self-fertilised plants grew a little taller, were
heavier, and generally flowered before those derived from a cross
between two flowers on the same plant. These latter plants thus present
a wonderful contrast with those derived from a cross between two
distinct individuals.
THE EFFECTS ON THE OFFSPRING OF A CROSS WITH A DISTINCT OR FRESH STOCK
BELONGING TO THE SAME VARIETY.
From the two foregoing series of experiments we see, firstly, the good
effects during several successive generations of a cross between
distinct plants, although these were in some degree inter-related and
had been grown under nearly the same conditions; and, secondly, the
absence of all such good effects from a cross between flowers on the
same plant; the comparison in both cases being made with the offspring
of flowers fertilised with their own pollen. The experiments now to be
given show how powerfully and beneficially plants, which have been
intercrossed during many successive generations, having been kept all
the time under nearly uniform conditions, are affected by a cross with
another plant belonging to the same variety, but to a distinct family or
stock, which had grown under different conditions.
[Several flowers on the crossed plants of the ninth generation in Table
2/10, were crossed with pollen from another crossed plant of the same
lot. The seedlings thus raised formed the tenth intercrossed generation,
and I will call them the "INTERCROSSED PLANTS." Several other flowers on
the same crossed plants of the ninth generation were fertilised (not
having been castrated) with pollen taken from plants of the same
variety, but belonging to a distinct family, which had been grown in a
distant garden at Colchester, and therefore under somewhat different
conditions. The capsules produced by this cross contained, to my
surprise, fewer and lighter seeds than did the capsules of the
intercrossed plants; but this, I think, must have been accidental. The
seedlings raised from them I will call the "COLCHESTER-CROSSED." The two
lots of seeds, after germinating on sand, were planted in the usual
manner on the opposite sides of five pots, and the remaining seeds,
whether or not in a state of germination, were thickly sown on the
opposite sides of a very large pot, Number 6 in Table 2/13. In three of
the six pots, after the young plants had twined a short way up their
sticks, one of the Colchester-crossed plants was much taller than any
one of the intercrossed plants on the opposite side of the same pot; and
in the three other pots somewhat taller. I should state that two of the
Colchester-crossed plants in Pot 4, when about two-thirds grown, became
much diseased, and were, together with their intercrossed opponents,
rejected. The remaining nineteen plants, when almost fully grown, were
measured, with the following result:
TABLE 2/13. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Colchester-Crossed Plants.
Column 3: Intercrossed Plants of the Tenth Generation.
Pot 1 : 87 : 78.
Pot 1 : 87 4/8 : 68 4/8.
Pot 1 : 85 1/8 : 94 4/8.
Pot 2 : 93 6/8 : 60.
Pot 2 : 85 4/8 : 87 2/8.
Pot 2 : 90 5/8 : 45 4/8.
Pot 3 : 84 2/8 : 70 1/8.
Pot 3 : 92 4/8 : 81 6/8.
Pot 3 : 85 : 86 2/8.
Pot 4 : 95 6/8 : 65 1/8.
Pot 5 : 90 4/8 : 85 6/8.
Pot 5 : 86 6/8 : 63.
Pot 5 : 84 : 62 6/8.
Pot 6 : 90 4/8 : 43 4/8.
Pot 6 : 75 : 39 6/8.
Pot 6 : 71 : 30 2/8.
Pot 6 : 83 6/8 : 86.
Pot 6 : 63 : 53.
Pot 6 : 65 : 48 6/8.
Crowded plants in a very large pot.
Total : 1596.50 : 1249.75.
In sixteen out of these nineteen pairs, the Colchester-crossed plant
exceeded in height its intercrossed opponent. The average height of the
Colchester-crossed is 84.03 inches, and that of the intercrossed 65.78
inches; or as 100 to 78. With respect to the fertility of the two lots,
it was too troublesome to collect and count the capsules on all the
plants; so I selected two of the best pots, 5 and 6, and in these the
Colchester-crossed produced 269 mature and half-mature capsules, whilst
an equal number of the intercrossed plants produced only 154 capsules;
or as 100 to 57. By weight the capsules from the Colchester-crossed
plants were to those from the intercrossed plants as 100 to 51; so that
the former probably contained a somewhat larger average number of
seeds.]
We learn from this important experiment that plants in some degree
related, which had been intercrossed during the nine previous
generations, when they were fertilised with pollen from a fresh stock,
yielded seedlings as superior to the seedlings of the tenth intercrossed
generation, as these latter were to the self-fertilised plants of the
corresponding generation. For if we look to the plants of the ninth
generation in Table 2/10 (and these offer in most respects the fairest
standard of comparison) we find that the intercrossed plants were in
height to the self-fertilised as 100 to 79, and in fertility as 100 to
26; whilst the Colchester-crossed plants are in height to the
intercrossed as 100 to 78, and in fertility as 100 to 51.
[THE DESCENDANTS OF THE SELF-FERTILISED PLANT, NAMED HERO, WHICH
APPEARED IN THE SIXTH SELF-FERTILISED GENERATION.
In the five generations before the sixth, the crossed plant of each pair
was taller than its self-fertilised opponent; but in the sixth
generation (Table 2/7, Pot 2) the Hero appeared, which after a long and
dubious struggle conquered its crossed opponent, though by only half an
inch. I was so much surprised at this fact, that I resolved to ascertain
whether this plant would transmit its powers of growth to its seedlings.
Several flowers on Hero were therefore fertilised with their own pollen,
and the seedlings thus raised were put into competition with
self-fertilised and intercrossed plants of the corresponding generation.
The three lots of seedlings thus all belong to the seventh generation.
Their relative heights are shown in Tables 2/14 and 2/15.
TABLE 2/14. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Plants of the Seventh Generation, Children of
Hero.
Column 3: Self-fertilised Plants of the Seventh Generation.
Pot 1 : 74 : 89 4/8.
Pot 1 : 60 : 61.
Pot 1 : 55 2/8 : 49.
Pot 2 : 92 : 82.
Pot 2 : 91 6/8 : 56.
Pot 2 : 74 2/8 : 38.
Total : 447.25 : 375.50.
The average height of the six self-fertilised children of Hero is 74.54
inches, whilst that of the ordinary self-fertilised plants of the
corresponding generation is only 62.58 inches, or as 100 to 84.
TABLE 2/15. Ipomoea purpurea.
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Self-fertilised Plants of the Seventh Generation, Children of
Hero.
Column 3: Intercrossed Plants of the Seventh Generation.
Pot 3 : 92 : 76 6/8.
Pot 4 : 87 : 89.
Pot 4 : 87 6/8 : 86 6/8.
Total : 266.75 : 252.50.
Here the average height of the three self-fertilised children of Hero is
88.91 inches, whilst that of the intercrossed plants is 84.16; or as 100
to 95. We thus see that the self-fertilised children of Hero certainly
inherit the powers of growth of their parents; for they greatly exceed
in height the self-fertilised offspring of the other self-fertilised
plants, and even exceed by a trifle the intercrossed plants,--all of the
corresponding generation.
Several flowers on the self-fertilised children of Hero in Table 2/14
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