The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom
by
Charles Darwin
Part 6 out of 10
61.59 inches in height; or as 100 to 93.
A large number of the same crossed and self-fertilised seeds were sown
in the middle of the summer in the open ground in two long rows. Very
much fewer of the self-fertilised than of the crossed plants produced
flowers; but those that did flower, flowered almost simultaneously. When
fully grown the ten tallest plants in each row were selected and
measured to the tips of their highest leaves, as well as to the summits
of their male flowers. The crossed averaged to the tips of their leaves
54 inches in height, and the self-fertilised 44.65, or as 100 to 83; and
to the summits of their male flowers, 53.96 and 43.45 inches; or as 100
to 80.
Phalaris canariensis.
Hildebrand has shown in the paper referred to under the last species,
that this hermaphrodite grass is better adapted for cross-fertilisation
than for self-fertilisation. Several plants were raised in the
greenhouse close together, and their flowers were mutually intercrossed.
Pollen from a single plant growing quite separately was collected and
placed on the stigmas of the same plant. The seeds thus produced were
self-fertilised, for they were fertilised with pollen from the same
plant, but it will have been a mere chance whether with pollen from the
same flowers. Both lots of seeds, after germinating on sand, were
planted in pairs on the opposite sides of four pots, which were kept in
the greenhouse. When the plants were a little over a foot in height they
were measured, and the crossed plants averaged 13.38, and the
self-fertilised 12.29 inches in height; or as 100 to 92.
When in full flower they were again measured to the extremities of their
culms, as shown in Table 6/98.
TABLE 6/98. Phalaris canariensis.
Heights of plants measured in inches.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 42 2/8 : 41 2/8.
Pot 1 : 39 6/8 : 45 4/8.
Pot 2 : 37 : 31 6/8.
Pot 2 : 49 4/8 : 37 2/8.
Pot 4 : 29 : 42 3/8.
Pot 2 : 37 : 34 7/8.
Pot 3 : 37 6/8 : 28.
Pot 3 : 35 4/8 : 28.
Pot 3 : 43 : 34.
Pot 4 : 40 2/8 : 35 1/8.
Pot 4 : 37 : 34 4/8.
Total : 428.00 : 392.63.
The eleven crossed plants now averaged 38.9, and the eleven
self-fertilised plants 35.69 inches in height; or as 100 to 92, which is
the same ratio as before. Differently to what occurred with the maize,
the crossed plants did not flower before the self-fertilised; and though
both lots flowered very poorly from having been kept in pots in the
greenhouse, yet the self-fertilised plants produced twenty-eight
flower-heads, whilst the crossed produced only twenty!
Two long rows of the same seeds were sown out of doors, and care was
taken that they were sown in nearly equal number; but a far greater
number of the crossed than of the self-fertilised seeds yielded plants.
The self-fertilised plants were in consequence not so much crowded as
the crossed, and thus had an advantage over them. When in full flower,
the twelve tallest plants were carefully selected from both rows and
measured, as shown in Table 6/99.
TABLE 6/99. Phalaris canariensis (growing in the open ground).
Heights of plants measured in inches.
Column 1: Crossed Plants, twelve tallest.
Column 2: Self-fertilised Plants, twelve tallest.
34 1/8 : 35 2/8.
35 7/8 : 31 1/8.
36 : 33.
35 5/8 : 32.
35 5/8 : 31 5/8.
36 1/8 : 36.
36 6/8 : 33.
38 6/8 : 32.
36 2/8 : 35 1/8.
35 5/8 : 33 5/8.
34 1/8 : 34 2/8.
34 5/8 : 35.
Total : 429.5 : 402.0.
The twelve crossed plants here average 35.78, and the twelve
self-fertilised 33.5 inches in height; or as 100 to 93. In this case the
crossed plants flowered rather before the self-fertilised, and thus
differed from those growing in the pots.]
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.
The details which have been given under the head of each species are so
numerous and so intricate, that it is necessary to tabulate the results.
In Table 7/A, the number of plants of each kind which were raised from a
cross between two individuals of the same stock and from self-fertilised
seeds, together with their mean or average heights, are given. In the
right hand column, the mean height of the crossed to that of the
self-fertilised plants, the former being taken as 100, is shown. To make
this clear, it may be advisable to give an example. In the first
generation of Ipomoea, six plants derived from a cross between two
plants were measured, and their mean height is 86.00 inches; six plants
derived from flowers on the same parent-plant fertilised with their own
pollen were measured, and their mean height is 65.66 inches. From this
it follows, as shown in the right hand column, that if the mean height
of the crossed plants be taken as 100, that of the self-fertilised
plants is 76. The same plan is followed with all the other species.
The crossed and self-fertilised plants were generally grown in pots in
competition with one another, and always under as closely similar
conditions as could be attained. They were, however, sometimes grown in
separate rows in the open ground. With several of the species, the
crossed plants were again crossed, and the self-fertilised plants again
self-fertilised, and thus successive generations were raised and
measured, as may be seen in Table 7/A. Owing to this manner of
proceeding, the crossed plants became in the later generations more or
less closely inter-related.
In Table 7/B the relative weights of the crossed and self-fertilised
plants, after they had flowered and had been cut down, are given in the
few cases in which they were ascertained. The results are, I think, more
striking and of greater value as evidence of constitutional vigour than
those deduced from the relative heights of the plants.
The most important table is Table 7/C, as it includes the relative
heights, weights, and fertility of plants raised from parents crossed by
a fresh stock (that is, by non-related plants grown under different
conditions), or by a distinct sub-variety, in comparison with
self-fertilised plants, or in a few cases with plants of the same old
stock intercrossed during several generations. The relative fertility of
the plants in this and the other tables will be more fully considered in
a future chapter.
TABLE 7/A. Relative heights of plants from parents crossed with pollen
from other plants of the same stock, and self-fertilised.
Heights of plants measured in inches.
Column 1: Name of Plant.
Column 2: Number of Crossed Plants measured.
Column 3: Average Height of Crossed Plants.
Column 4: Number of Self-fertilised Plants measured.
Column 5: Average Height of Self-fertilised Plants.
Column 6: x, where the ratio of the Average Height of the Crossed to the
Self-fertilised Plants is expressed as 100 to x.
Ipomoea purpurea--first generation:
6 : 86.00 : 6 : 65.66 : 76.
Ipomoea purpurea--second generation:
6 : 84.16 : 6 : 66.33 : 79.
Ipomoea purpurea--third generation:
6 : 77.41 : 6 : 52.83 : 68.
Ipomoea purpurea--fourth generation:
7 : 69.78 : 7 : 60.14 : 86.
Ipomoea purpurea--fifth generation:
6 : 82.54 : 6 : 62.33 : 75.
Ipomoea purpurea--sixth generation:
6 : 87.50 : 6 : 63.16 : 72.
Ipomoea purpurea--seventh generation:
9 : 83.94 : 9 : 68.25 : 81.
Ipomoea purpurea--eighth generation:
8 : 113.25 : 8 : 96.65 : 85.
Ipomoea purpurea--ninth generation:
14 : 81.39 : 14 : 64.07 : 79.
Ipomoea purpurea--tenth generation:
5 : 93.70 : 5 : 50.40 : 54.
Ipomoea purpurea--Number and average height of all the plants of the ten
generations:
73 : 85.84 : 73 : 66.02 : 77.
Mimulus luteus--three first generations, before the new and taller
self-fertilised variety appeared:
10 : 8.19 : 10 : 5.29 : 65.
Digitalis purpurea:
16 : 51.33 : 8 : 35.87 : 70.
Calceolaria--(common greenhouse variety):
1 : 19.50 : 1 : 15.00 : 77.
Linaria vulgaris:
3 : 7.08 : 3 : 5.75 : 81.
Verbascum thapsus:
6 : 65.34 : 6 : 56.50 : 86.
Vandellia nummularifolia--crossed and self-fertilised plants, raised
from perfect flowers:
20 : 4.30 : 20 : 4.27 : 99.
Vandellia nummularifolia--crossed and self-fertilised plants, raised
from perfect flowers: second trial, plants crowded:
24 : 3.60 : 24 : 3.38 : 94.
Vandellia nummularifolia--crossed plants raised from perfect flowers,
and self-fertilised plants from cleistogene flowers:
20 : 4.30 : 20 : 4.06 : 94.
Gesneria pendulina:
8 : 32.06 : 8 : 29.14 : 90.
Salvia coccinea:
6 : 27.85 : 6 : 21.16 : 76.
Origanum vulgare:
4 : 20.00 : 4 : 17.12 : 86.
Thunbergia alata:
6 : 60.00 : 6 : 65.00 : 108.
Brassica oleracea:
9 : 41.08 : 9 : 39.00 : 95.
Iberis umbellata--the self-fertilised plants of the third generation:
7 : 19.12 : 7 : 16.39 : 86.
Papaver vagum:
15 : 21.91 : 15 : 19.54 : 89.
Eschscholtzia californica--English stock, first generation:
4 : 29.68 : 4 : 25.56 : 86.
Eschscholtzia californica--English stock, second generation:
11 : 32.47 : 11 : 32.81 : 101.
Eschscholtzia californica--Brazilian stock, first generation:
14 : 44.64 : 14 : 45.12 : 101.
Eschscholtzia californica--Brazilian stock, second generation:
18 : 43.38 : 19 : 50.30 : 116.
Eschscholtzia californica--average height and number of all the plants
of Eschscholtzia:
47 : 40.03 : 48 : 42.72 : 107.
Reseda lutea--grown in pots:
24 : 17.17 : 24 : 14.61 : 85.
Reseda lutea--grown in open ground :
8 : 28.09 : 8 : 23.14 : 82.
Reseda odorata--self-fertilised seeds from a highly self-fertile plant,
grown in pots:
19 : 27.48 : 19 : 22.55 : 82.
Reseda odorata--self-fertilised seeds from a highly self-fertile plant,
grown in open ground:
8 : 25.76 : 8 : 27.09 : 105.
Reseda odorata--self-fertilised seeds from a semi-self-fertile plant,
grown in pots:
20 : 29.98 : 20 : 27.71 : 92.
Reseda odorata--self-fertilised seeds from a semi-self-fertile plant,
grown in open ground:
8 : 25.92 : 8 : 23.54 : 90.
Viola tricolor:
14 : 5.58 : 14 : 2.37 : 42.
Adonis aestivalis:
4 : 14.25 : 4 : 14.31 : 100.
Delphinium consolida:
6 : 14.95 : 6 : 12.50 : 84.
Viscaria oculata:
15 : 34.50 : 15 : 33.55 : 97.
Dianthus caryophyllus--open ground, about :
6?: 28? : 6?: 24? : 86.
Dianthus caryophyllus--second generation, in pots, crowded:
2 : 16.75 : 2 : 9.75 : 58.
Dianthus caryophyllus--third generation, in pots:
8 : 28.39 : 8 : 28.21 : 99.
Dianthus caryophyllus--offspring from plants of the third
self-fertilised generation crossed by intercrossed plants of the third
generation, compared with plants of fourth self-fertilised generation:
15 : 28.00 : 10 : 26.55 : 95.
Dianthus caryophyllus--number and average height of all the plants of
Dianthus:
31 : 27.37 : 26 : 25.18 : 92.
Hibiscus africanus:
4 : 13.25 : 4 : 14.43 : 109.
Pelargonium zonale:
7 : 22.35 : 7 : 16.62 : 74.
Tropaeolum minus:
8 : 58.43 : 8 : 46.00 : 79.
Limnanthes douglasii:
16 : 17.46 : 16 : 13.85 : 79.
Lupinus luteus--second generation:
8 : 30.78 : 8 : 25.21 : 82.
Lupinus pilosus--plants of two generations:
2 : 35.50 : 3 : 30.50 : 86.
Phaseolus multiflorus:
5 : 86.00 : 5 : 82.35 : 96.
Pisum sativum:
4 : 34.62 : 4 : 39.68 : 115.
Sarothamnus scoparius--small seedlings:
6 : 2.91 : 6 : 1.33 : 46.
Sarothamnus scoparius--the three survivors on each side after three
years' growth:
: 18.91 : : 11.83 : 63.
Ononis minutissima:
2 : 19.81 : 2 : 17.37 : 88.
Clarkia elegans:
4 : 33.50 : 4 : 27.62 : 82.
Bartonia aurea:
8 : 24.62 : 8 : 26.31 : 107.
Passiflora gracilis:
2 : 49.00 : 2 : 51.00 : 104.
Apium petroselinum:
* : : * : : 100.
*not measured.
Scabiosa atro-purpurea:
4 : 17.12 : 4 : 15.37 : 90.
Lactuca sativa--plants of two generations:
7 : 19.43 : 6 : 16.00 : 82.
Specularia speculum:
4 : 19.28 : 4 : 18.93 : 98.
Lobelia ramosa--first generation:
4 : 22.25 : 4 : 18.37 : 82.
Lobelia ramosa--second generation:
3 : 23.33 : 3 : 19.00 : 81.
Lobelia fulgens--first generation:
2 : 34.75 : 2 : 44.25 : 127.
Lobelia fulgens--second generation:
23 : 29.82 : 23 : 27.10 : 91.
Nemophila insignis--half-grown:
12 : 11.10 : 12 : 5.45 : 49.
Nemophila insignis--the same fully-grown:
: 33.28 : : 19.90 : 60.
Borago officinalis:
4 : 20.68 : 4 : 21.18 : 102.
Nolana prostrata:
5 : 12.75 : 5 : 13.40 : 105.
Petunia violacea--first generation:
5 : 30.80 : 5 : 26.00 : 84.
Petunia violacea--second generation:
4 : 40.50 : 6 : 26.25 : 65.
Petunia violacea--third generation:
8 : 40.96 : 8 : 53.87 : 131.
Petunia violacea--fourth generation:
15 : 46.79 : 14 : 32.39 : 69.
Petunia violacea--fourth generation, from a distinct parent:
13 : 44.74 : 13 : 26.87 : 60.
Petunia violacea--fifth generation:
22 : 54.11 : 21 : 33.23 : 61.
Petunia violacea--fifth generation, in open ground:
10 : 38.27 : 10 : 23.31 : 61.
Petunia violacea--Number and average height of all the plants in pots of
Petunia:
67 : 46.53 : 67 : 33.12 : 71.
Nicotiana tabacum--first generation:
4 : 18.50 : 4 : 32.75 : 178.
Nicotiana tabacum--second generation:
9 : 53.84 : 7 : 51.78 : 96.
Nicotiana tabacum--third generation:
7 : 95.25 : 7 : 79.60 : 83.
Nicotiana tabacum--third generation but raised from a distinct plant:
7 : 70.78 : 9 : 71.30 : 101.
Nicotiana tabacum--Number and average height of all the plants of
Nicotiana:
27 : 63.73 : 27 : 61.31 : 96.
Cyclamen persicum:
8 : 9.49 : 8?: 7.50 : 79.
Anagallis collina:
6 : 42.20 : 6 : 33.35 : 69.
Primula sinensis--a dimorphic species:
8 : 9.01 : 8 : 9.03 : 100.
Fagopyrum esculentum--a dimorphic species:
15 : 38.06 : 15 : 26.13 : 69.
Beta vulgaris--in pots:
8 : 34.09 : 8 : 29.81 : 87.
Beta vulgaris--in open ground:
8 : 30.92 : 8 : 30.70 : 99.
Canna warscewiczi--plants of three generations:
34 : 35.98 : 34 : 36.39 : 101.
Zea mays--in pots, whilst young, measured to tips of leaves:
15 : 20.19 : 15 : 17.57 : 87.
Zea mays--when full-grown, after the death of some, measured to tips of
leaves:
: 68.10 : : 62.34 : 91.
Zea mays--when full-grown, after the death of some, measured to tips of
flowers:
: 66.51 : : 61.59 : 93.
Zea mays--grown in open ground, measured to tips of leaves:
10 : 54.00 : 10 : 44.55 : 83.
Zea mays--grown in open ground, measured to tips of flowers:
: 53.96 : : 43.45 : 80.
Phalaris canariensis--in pots.
11 : 38.90 : 11 : 35.69 : 92.
Phalaris canariensis--in open ground:
12 : 35.78 : 12 : 33.50 : 93.
TABLE 7/B.--Relative weights of plants from parents crossed with pollen
from distinct plants of the same stock, and self-fertilised.
Column 1: Names of plants.
Column 2: Number of crossed plants.
Column 3: Number of self-fertilised plants.
Column 4: x, where the ratio of the Weight of the Crossed to the
Self-fertilised Plants is expressed as 100 to x.
Ipomoea purpurea--plants of the tenth generation:
6 : 6 : 44.
Vandellia nummularifolia--first generation:
41 : 41 : 97.
Brassica oleracea--first generation:
9 : 9 : 37.
Eschscholtzia californica--plants of the second generation:
19 : 19 : 118.
Reseda lutea--first generation, grown in pots:
24 : 24 : 21.
Reseda lutea--first generation, grown in open ground:
8 : 8 : 40.
Reseda odorata--first generation, descended from a highly self-fertile
plant, grown in pots:
19 : 19 : 67.
Reseda odorata--first generation, descended from a semi-self-fertile
plant, grown in pots:
20 : 20 : 99.
Dianthus caryophyllus--plants of the third generation:
8 : 8 : 49.
Petunia violacea--plants of the fifth generation, in pots:
22 : 21 : 22.
Petunia violacea--plants of the fifth generation, in open ground:
10 : 10 : 36.
TABLE 7/C.--Relative heights, weights, and fertility of plants from
parents crossed by a fresh stock, and from parents either
self-fertilised or intercrossed with plants of the same stock.
Column 1: Names of the plants and nature of the experiments.
Column 2: Number of plants from a cross with a fresh stock.
Column 3: Average height in inches and weight.
Column 4: Number of the plants from self-fertilised or intercrossed
parents of the same stock.
Column 5: Average height in inches and weight.
Column 4: x, where the ratio of the Height, Weight and Fertility of the
plants from the Cross with a fresh stock is expressed as 100 to x.
Ipomoea purpurea--offspring of plants intercrossed for nine generations
and then crossed by a fresh stock, compared with plants of the tenth
intercrossed generation:
19 : 84.03 : 19 : 65.78 : 78.
Ipomoea purpurea--offspring of plants intercrossed for nine generations
and then crossed by a fresh stock, compared with plants of the tenth
intercrossed generation, in fertility:
.. : .. : .. : .. : 51.
Mimulus luteus--offspring of plants self-fertilised for eight
generations and then crossed by a fresh stock, compared with plants of
the ninth self-fertilised generation:
28 : 21.62 : 19 : 10.44 : 52.
Mimulus luteus--offspring of plants self-fertilised for eight
generations and then crossed by a fresh stock, compared with plants of
the ninth self-fertilised generation, in fertility:
.. : .. : .. : .. : 3.
Mimulus luteus--offspring of plants self-fertilised for eight
generations and then crossed by a fresh stock, compared with the
offspring of a plant self-fertilised for eight generations, and then
intercrossed with another self-fertilised plant of the same generation:
28 : 21.62 : 27 : 12.20 : 56.
Mimulus luteus--offspring of plants self-fertilised for eight
generations and then crossed by a fresh stock, compared with the
offspring of a plant self-fertilised for eight generations, and then
intercrossed with another self-fertilised plant of the same generation,
in fertility:
.. : .. : .. : .. : 4.
Brassica oleracea--offspring of plants self-fertilised for two
generations and then crossed by a fresh stock, compared with plants of
the third self-fertilised generation, by weight:
6 : : 6 : : 22.
Iberis umbellata--offspring from English variety crossed by slightly
different Algerine variety, compared with the self-fertilised offspring
of the English variety:
30 : 17.34 : 29 : 15.51 : 89.
Iberis umbellata--offspring from English variety crossed by slightly
different Algerine variety, compared with the self-fertilised offspring
of the English variety, in fertility:
.. : .. : .. : .. : 75.
Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
self-fertilised generation:
19 : 45.92 : 19 : 50.30 : 109.
Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
self-fertilised generation, in weight:
.. : .. : .. : .. : 118.
Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
self-fertilised generation, in fertility:
.. : .. : .. : .. : 40.
Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
intercrossed generation, in height:
19 : 45.92 : 18 : 43.38 : 94.
Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
intercrossed generation, in weight:
.. : .. : .. : .. : 100.
Eschscholtzia californica--offspring of a Brazilian stock crossed by an
English stock, compared with plants of the Brazilian stock of the second
intercrossed generation, in fertility:
.. : .. : .. : .. : 45.
Dianthus caryophyllus--offspring of plants self-fertilised for three
generations and then crossed by a fresh stock, compared with plants of
the fourth self-fertilised generation:
16 : 32.82 : 10 : 26.55 : 81.
Dianthus caryophyllus--offspring of plants self-fertilised for three
generations and then crossed by a fresh stock, compared with plants of
the fourth self-fertilised generation, in fertility:
.. : .. : .. : .. : 33.
Dianthus caryophyllus--offspring of plants self-fertilised for three
generations and then crossed by a fresh stock, compared with the
offspring of plants self-fertilised for three generations and then
crossed by plants of the third intercrossed generation:
16 : 32.82 : 15 : 28.00 : 85.
Dianthus caryophyllus--offspring of plants self-fertilised for three
generations and then crossed by a fresh stock, compared with the
offspring of plants self-fertilised for three generations and then
crossed by plants of the third intercrossed generation, in fertility:
.. : .. : .. : .. : 45.
Pisum sativum--offspring from a cross between two closely allied
varieties, compared with the self-fertilised offspring of one of the
varieties, or with intercrossed plants of the same stock:
? : : ? : : 60 to 75.
Lathyrus odoratus--offspring from two varieties, differing only in
colour of their flowers, compared with the self-fertilised offspring of
one of the varieties: in first generation:
2 : 79.25 : 2 : 63.75 : 80.
Lathyrus odoratus--offspring from two varieties, differing only in
colour of their flowers, compared with the self-fertilised offspring of
one of the varieties: in second generation:
6 : 62.91 : 6 : 55.31 : 88.
Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth self-fertilised generation, in height:
21 : 50.05 : 21 : 33.23 : 66.
Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth self-fertilised generation, in weight:
.. : .. : .. : .. : 23.
Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth self-fertilised generation, grown in open ground, in height:
10 : 36.67 : 10 : 23.31 : 63.
Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth self-fertilised generation, grown in open ground, in weight:
.. : .. : .. : .. : 53.
Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth self-fertilised generation, grown in open ground, in
fertility:
.. : .. : .. : .. : 46.
Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth intercrossed generation, in height:
21 : 50.05 : 22 : 54.11 : 108.
Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth intercrossed generation, in weight:
.. : .. : .. : .. : 101.
Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth intercrossed generation, grown in open ground, in height:
10 : 36.67 : 10 : 38.27 : 104.
Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth intercrossed generation, grown in open ground, in weight:
.. : .. : .. : .. : 146.
Petunia violacea--offspring of plants self-fertilised for four
generations and then crossed by a fresh stock, compared with plants of
the fifth intercrossed generation, grown in open ground, in fertility:
.. : .. : .. : .. : 54.
Nicotiana tabacum--offspring of plants self-fertilised for three
generations and then crossed by a slightly different variety, compared
with plants of the fourth self-fertilised generation, grown not much
crowded in pots, in height:
26 : 63.29 : 26 : 41.67 : 66.
Nicotiana tabacum--offspring of plants self-fertilised for three
generations and then crossed by a slightly different variety, compared
with plants of the fourth self-fertilised generation, grown much crowded
in pots, in height:
12 : 31.53 : 12 : 17.21 : 54.
Nicotiana tabacum--offspring of plants self-fertilised for three
generations and then crossed by a slightly different variety, compared
with plants of the fourth self-fertilised generation, grown much crowded
in pots, in weight:
.. : .. : .. : .. : 37.
Nicotiana tabacum--offspring of plants self-fertilised for three
generations and then crossed by a slightly different variety, compared
with plants of the fourth self-fertilised generation, grown in open
ground, in height:
20 : 48.74 : 20 : 35.20 : 72.
Nicotiana tabacum--offspring of plants self-fertilised for three
generations and then crossed by a slightly different variety, compared
with plants of the fourth self-fertilised generation, grown in open
ground, in weight:
.. : .. : .. : .. : 63.
Anagallis collina--offspring from a red variety crossed by a blue
variety, compared with the self-fertilised offspring of the red variety:
3 : 27.62 : 3 : 18.21 : 66.
Anagallis collina--offspring from a red variety crossed by a blue
variety, compared with the self-fertilised offspring of the red variety,
in fertility:
.. : .. : .. : .. : 6.
Primula veris--offspring from long-styled plants of the third
illegitimate generation, crossed by a fresh stock, compared with plants
of the fourth illegitimate and self-fertilised generation:
8 : 7.03 : 8 : 3.21 : 46.
Primula veris--offspring from long-styled plants of the third
illegitimate generation, crossed by a fresh stock, compared with plants
of the fourth illegitimate and self-fertilised generation, in fertility:
.. : .. : .. : .. : 5.
Primula veris--offspring from long-styled plants of the third
illegitimate generation, crossed by a fresh stock, compared with plants
of the fourth illegitimate and self-fertilised generation, in fertility
in following year:
.. : .. : .. : .. : 3.5.
Primula veris--(equal-styled, red-flowered variety)--offspring from
plants self-fertilised for two generations and then crossed by a
different variety, compared with plants of the third self-fertilised
generation:
3 : 8.66 : 3 : 7.33 : 85.
Primula veris--(equal-styled, red-flowered variety)--offspring from
plants self-fertilised for two generations and then crossed by a
different variety, compared with plants of the third self-fertilised
generation, in fertility:
.. : .. : .. : .. : 11.
In these three tables the measurements of fifty-seven species, belonging
to fifty-two genera and to thirty great natural families, are given. The
species are natives of various parts of the world. The number of crossed
plants, including those derived from a cross between plants of the same
stock and of two different stocks, amounts to 1,101; and the number of
self-fertilised plants (including a few in Table 7/C derived from a
cross between plants of the same old stock) is 1,076. Their growth was
observed from the germination of the seeds to maturity; and most of them
were measured twice and some thrice. The various precautions taken to
prevent either lot being unduly favoured, have been described in the
introductory chapter. Bearing all these circumstances in mind, it may be
admitted that we have a fair basis for judging of the comparative
effects of cross-fertilisation and of self-fertilisation on the growth
of the offspring.
It will be the most convenient plan first to consider the results given
in Table 7/C, as an opportunity will thus be afforded of incidentally
discussing some important points. If the reader will look down the right
hand column of this table, he will see at a glance what an extraordinary
advantage in height, weight, and fertility the plants derived from a
cross with a fresh stock or with another sub-variety have over the
self-fertilised plants, as well as over the intercrossed plants of the
same old stock. There are only two exceptions to this rule, and these
are hardly real ones. In the case of Eschscholtzia, the advantage is
confined to fertility. In that of Petunia, though the plants derived
from a cross with a fresh stock had an immense superiority in height,
weight, and fertility over the self-fertilised plants, they were
conquered by the intercrossed plants of the same old stock in height and
weight, but not in fertility. It has, however, been shown that the
superiority of these intercrossed plants in height and weight was in all
probability not real; for if the two sets had been allowed to grow for
another month, it is almost certain that those from a cross with the
fresh stock would have been victorious in every way over the
intercrossed plants.
Before we consider in detail the several cases given in Table 7/C, some
preliminary remarks must be made. There is the clearest evidence, as we
shall presently see, that the advantage of a cross depends wholly on the
plants differing somewhat in constitution; and that the disadvantages of
self-fertilisation depend on the two parents, which are combined in the
same hermaphrodite flower, having a closely similar constitution. A
certain amount of differentiation in the sexual elements seems
indispensable for the full fertility of the parents, and for the full
vigour of the offspring. All the individuals of the same species, even
those produced in a state of nature, differ somewhat, though often very
slightly, from one another in external characters and probably in
constitution. This obviously holds good between the varieties of the
same species, as far as external characters are concerned; and much
evidence could be advanced with respect to their generally differing
somewhat in constitution. There can hardly be a doubt that the
differences of all kinds between the individuals and varieties of the
same species depend largely, and as I believe exclusively, on their
progenitors having been subjected to different conditions; though the
conditions to which the individuals of the same species are exposed in a
state of nature often falsely appear to us the same. For instance, the
individuals growing together are necessarily exposed to the same
climate, and they seem to us at first sight to be subjected to
identically the same conditions; but this can hardly be the case, except
under the unusual contingency of each individual being surrounded by
other kinds of plants in exactly the same proportional numbers. For the
surrounding plants absorb different amounts of various substances from
the soil, and thus greatly affect the nourishment and even the life of
the individuals of any particular species. These will also be shaded and
otherwise affected by the nature of the surrounding plants. Moreover,
seeds often lie dormant in the ground, and those which germinate during
any one year will often have been matured during very different seasons.
Seeds are widely dispersed by various means, and some will occasionally
be brought from distant stations, where their parents have grown under
somewhat different conditions, and the plants produced from such seeds
will intercross with the old residents, thus mingling their
constitutional peculiarities in all sorts of proportions.
Plants when first subjected to culture, even in their native country,
cannot fail to be exposed to greatly changed conditions of life, more
especially from growing in cleared ground, and from not having to
compete with many or any surrounding plants. They are thus enabled to
absorb whatever they require which the soil may contain. Fresh seeds are
often brought from distant gardens, where the parent-plants have been
subjected to different conditions. Cultivated plants like those in a
state of nature frequently intercross, and will thus mingle their
constitutional peculiarities. On the other hand, as long as the
individuals of any species are cultivated in the same garden, they will
apparently be subjected to more uniform conditions than plants in a
state of nature, as the individuals have not to compete with various
surrounding species. The seeds sown at the same time in a garden have
generally been matured during the same season and in the same place; and
in this respect they differ much from the seeds sown by the hand of
nature. Some exotic plants are not frequented by the native insects in
their new home, and therefore are not intercrossed; and this appears to
be a highly important factor in the individuals acquiring uniformity of
constitution.
In my experiments the greatest care was taken that in each generation
all the crossed and self-fertilised plants should be subjected to the
same conditions. Not that the conditions were absolutely the same, for
the more vigorous individuals will have robbed the weaker ones of
nutriment, and likewise of water when the soil in the pots was becoming
dry; and both lots at one end of the pot will have received a little
more light than those at the other end. In the successive generations,
the plants were subjected to somewhat different conditions, for the
seasons necessarily varied, and they were sometimes raised at different
periods of the year. But as they were all kept under glass, they were
exposed to far less abrupt and great changes of temperature and moisture
than are plants growing out of doors. With respect to the intercrossed
plants, their first parents, which were not related, would almost
certainly have differed somewhat in constitution; and such
constitutional peculiarities would be variously mingled in each
succeeding intercrossed generation, being sometimes augmented, but more
commonly neutralised in a greater or less degree, and sometimes revived
through reversion; just as we know to be the case with the external
characters of crossed species and varieties. With the plants which were
self-fertilised during the successive generations, this latter important
source of some diversity of constitution will have been wholly
eliminated; and the sexual elements produced by the same flower must
have been developed under as nearly the same conditions as it is
possible to conceive.
In Table 7/C the crossed plants are the offspring of a cross with a
fresh stock, or with a distinct variety; and they were put into
competition either with self-fertilised plants, or with intercrossed
plants of the same old stock. By the term fresh stock I mean a
non-related plant, the progenitors of which have been raised during some
generations in another garden, and have consequently been exposed to
somewhat different conditions. In the case of Nicotiana, Iberis, the red
variety of Primula, the common Pea, and perhaps Anagallis, the plants
which were crossed may be ranked as distinct varieties or sub-varieties
of the same species; but with Ipomoea, Mimulus, Dianthus, and Petunia,
the plants which were crossed differed exclusively in the tint of their
flowers; and as a large proportion of the plants raised from the same
lot of purchased seeds thus varied, the differences may be estimated as
merely individual. Having made these preliminary remarks, we will now
consider in detail the several cases given in Table 7/C, and they are
well worthy of full consideration.
1. Ipomoea purpurea.
Plants growing in the same pots, and subjected in each generation to the
same conditions, were intercrossed for nine consecutive generations.
These intercrossed plants thus became in the later generations more or
less closely inter-related. Flowers on the plants of the ninth
intercrossed generation were fertilised with pollen taken from a fresh
stock, and seedlings thus raised. Other flowers on the same intercrossed
plants were fertilised with pollen from another intercrossed plant,
producing seedlings of the tenth intercrossed generation. These two sets
of seedlings were grown in competition with one another, and differed
greatly in height and fertility. For the offspring from the cross with a
fresh stock exceeded in height the intercrossed plants in the ratio of
100 to 78; and this is nearly the same excess which the intercrossed had
over the self-fertilised plants in all ten generations taken together,
namely, as 100 to 77. The plants raised from the cross with a fresh
stock were also greatly superior in fertility to the intercrossed,
namely, in the ratio of 100 to 51, as judged by the relative weight of
the seed-capsules produced by an equal number of plants of the two sets,
both having been left to be naturally fertilised. It should be
especially observed that none of the plants of either lot were the
product of self-fertilisation. On the contrary, the intercrossed plants
had certainly been crossed for the last ten generations, and probably,
during all previous generations, as we may infer from the structure of
the flowers and from the frequency of the visits of humble-bees. And so
it will have been with the parent-plants of the fresh stock. The whole
great difference in height and fertility between the two lots must be
attributed to the one being the product of a cross with pollen from a
fresh stock, and the other of a cross between plants of the same old
stock.
This species offers another interesting case. In the five first
generations in which intercrossed and self-fertilised plants were put
into competition with one another, every single intercrossed plant beat
its self-fertilised antagonist, except in one instance, in which they
were equal in height. But in the sixth generation a plant appeared,
named by me the Hero, remarkable for its tallness and increased
self-fertility, and which transmitted its characters to the next three
generations. The children of Hero were again self-fertilised, forming
the eighth self-fertilised generation, and were likewise intercrossed
one with another; but this cross between plants which had been subjected
to the same conditions and had been self-fertilised during the seven
previous generations, did not effect the least good; for the
intercrossed grandchildren were actually shorter than the
self-fertilised grandchildren, in the ratio of 100 to 107. We here see
that the mere act of crossing two distinct plants does not by itself
benefit the offspring. This case is almost the converse of that in the
last paragraph, on which the offspring profited so greatly by a cross
with a fresh stock. A similar trial was made with the descendants of
Hero in the following generation, and with the same result. But the
trial cannot be fully trusted, owing to the extremely unhealthy
condition of the plants. Subject to this same serious cause of doubt,
even a cross with a fresh stock did not benefit the great-grandchildren
of Hero; and if this were really the case, it is the greatest anomaly
observed by me in all my experiments.
2. Mimulus luteus.
During the three first generations the intercrossed plants taken
together exceeded in height the self-fertilised taken together, in the
ratio of 100 to 65, and in fertility in a still higher degree. In the
fourth generation a new variety, which grew taller and had whiter and
larger flowers than the old varieties, began to prevail, especially
amongst the self-fertilised plants. This variety transmitted its
characters with remarkable fidelity, so that all the plants in the later
self-fertilised generations belonged to it. These consequently exceeded
the intercrossed plants considerably in height. Thus in the seventh
generation the intercrossed plants were to the self-fertilised in height
as 100 to 137. It is a more remarkable fact that the self-fertilised
plants of the sixth generation had become much more fertile than the
intercrossed plants, judging by the number of capsules spontaneously
produced, in the ratio of 147 to 100. This variety, which as we have
seen appeared amongst the plants of the fourth self-fertilised
generation, resembles in almost all its constitutional peculiarities the
variety called Hero which appeared in the sixth self-fertilised
generation of Ipomoea. No other such case, with the partial exception of
that of Nicotiana, occurred in my experiments, carried on during eleven
years.
Two plants of this variety of Mimulus, belonging to the sixth
self-fertilised generation, and growing in separate pots, were
intercrossed; and some flowers on the same plants were again
self-fertilised. From the seeds thus obtained, plants derived from a
cross between the self-fertilised plants, and others of the seventh
self-fertilised generation, were raised. But this cross did not do the
least good, the intercrossed plants being inferior in height to the
self-fertilised, in the ratio of 100 to 110. This case is exactly
parallel with that given under Ipomoea, of the grandchildren of Hero,
and apparently of its great-grandchildren; for the seedlings raised by
intercrossing these plants were not in any way superior to those of the
corresponding generation raised from the self-fertilised flowers.
Therefore in these several cases the crossing of plants, which had been
self-fertilised for several generations and which had been cultivated
all the time under as nearly as possible the same conditions, was not in
the least beneficial.
Another experiment was now tried. Firstly, plants of the eighth
self-fertilised generation were again self-fertilised, producing plants
of the ninth self-fertilised generation. Secondly, two of the plants of
the eighth self-fertilised generation were intercrossed one with
another, as in the experiment above referred to; but this was now
effected on plants which had been subjected to two additional
generations of self-fertilisation. Thirdly, the same plants of the
eighth self-fertilised generation were crossed with pollen from plants
of a fresh stock brought from a distant garden. Numerous plants were
raised from these three sets of seeds, and grown in competition with one
another. The plants derived from a cross between the self-fertilised
plants exceeded in height by a little the self-fertilised, namely, as
100 to 92; and in fertility in a greater degree, namely, as 100 to 73. I
do not know whether this difference in the result, compared with that in
the previous case, can be accounted for by the increased deterioration
of the self-fertilised plants from two additional generations of
self-fertilisation, and the consequent advantage of any cross whatever,
along merely between the self-fertilised plants. But however this may
be, the effects of crossing the self-fertilised plants of the eighth
generation with a fresh stock were extremely striking; for the seedlings
thus raised were to the self-fertilised of the ninth generation as 100
to 52 in height, and as 100 to 3 in fertility! They were also to the
intercrossed plants (derived from crossing two of the self-fertilised
plants of the eighth generation) in height as 100 to 56, and in
fertility as 100 to 4. Better evidence could hardly be desired of the
potent influence of a cross with a fresh stock on plants which had been
self-fertilised for eight generations, and had been cultivated all the
time under nearly uniform conditions, in comparison with plants
self-fertilised for nine generations continuously, or then once
intercrossed, namely in the last generation.
3. Brassica oleracea.
Some flowers on cabbage plants of the second self-fertilised generation
were crossed with pollen from a plant of the same variety brought from a
distant garden, and other flowers were again self-fertilised. Plants
derived from a cross with a fresh stock and plants of the third
self-fertilised generation were thus raised. The former were to the
self-fertilised in weight as 100 to 22; and this enormous difference
must be attributed in part to the beneficial effects of a cross with a
fresh stock, and in part to the deteriorating effects of
self-fertilisation continued during three generations.
4. Iberis umbellata.
Seedlings from a crimson English variety crossed by a pale-coloured
variety which had been grown for some generations in Algiers, were to
the self-fertilised seedlings from the crimson variety in height as 100
to 89, and as 100 to 75 in fertility. I am surprised that this cross
with another variety did not produce a still more strongly marked
beneficial effect; for some intercrossed plants of the crimson English
variety, put into competition with plants of the same variety
self-fertilised during three generations, were in height as 100 to 86,
and in fertility as 100 to 75. The slightly greater difference in height
in this latter case, may possibly be attributed to the deteriorating
effects of self-fertilisation carried on for two additional generations.
5. Eschscholtzia californica.
This plant offers an almost unique case, inasmuch as the good effects of
a cross are confined to the reproductive system. Intercrossed and
self-fertilised plants of the English stock did not differ in height
(nor in weight, as far as was ascertained) in any constant manner; the
self-fertilised plants usually having the advantage. So it was with the
offspring of plants of the Brazilian stock, tried in the same manner.
The parent-plants, however, of the English stock produced many more
seeds when fertilised with pollen from another plant than when
self-fertilised; and in Brazil the parent-plants were absolutely sterile
unless they were fertilised with pollen from another plant. Intercrossed
seedlings, raised in England from the Brazilian stock, compared with
self-fertilised seedlings of the corresponding second generation,
yielded seeds in number as 100 to 89; both lots of plants being left
freely exposed to the visits of insects. If we now turn to the effects
of crossing plants of the Brazilian stock with pollen from the English
stock,--so that plants which had been long exposed to very different
conditions were intercrossed,--we find that the offspring were, as
before, inferior in height and weight to the plants of the Brazilian
stock after two generations of self-fertilisation, but were superior to
them in the most marked manner in the number of seeds produced, namely,
as 100 to 40; both lots of plants being left freely exposed to the
visits of insects.
In the case of Ipomoea, we have seen that the plants derived from a
cross with a fresh stock were superior in height as 100 to 78, and in
fertility as 100 to 51, to the plants of the old stock, although these
had been intercrossed during the last ten generations. With
Eschscholtzia we have a nearly parallel case, but only as far as
fertility is concerned, for the plants derived from a cross with a fresh
stock were superior in fertility in the ratio of 100 to 45 to the
Brazilian plants, which had been artificially intercrossed in England
for the two last generations, and which must have been naturally
intercrossed by insects during all previous generations in Brazil, where
otherwise they are quite sterile.
6. Dianthus caryophyllus.
Plants self-fertilised for three generations were crossed with pollen
from a fresh stock, and their offspring were grown in competition with
plants of the fourth self-fertilised generation. The crossed plants thus
obtained were to the self-fertilised in height as 100 to 81, and in
fertility (both lots being left to be naturally fertilised by insects)
as 100 to 33.
These same crossed plants were also to the offspring from the plants of
the third generation crossed by the intercrossed plants of the
corresponding generation, in height as 100 to 85, and in fertility as
100 to 45.
We thus see what a great advantage the offspring from a cross with a
fresh stock had, not only over the self-fertilised plants of the fourth
generation, but over the offspring from the self-fertilised plants of
the third generation, when crossed by the intercrossed plants of the old
stock.
7. Pisum sativum.
It has been shown under the head of this species, that the several
varieties in this country almost invariably fertilise themselves, owing
to insects rarely visiting the flowers; and as the plants have been long
cultivated under nearly similar conditions, we can understand why a
cross between two individuals of the same variety does not do the least
good to the offspring either in height or fertility. This case is almost
exactly parallel with that of Mimulus, or that of the Ipomoea named
Hero; for in these two instances, crossing plants which had been
self-fertilised for seven generations did not at all benefit the
offspring. On the other hand, a cross between two varieties of the pea
causes a marked superiority in the growth and vigour of the offspring,
over the self-fertilised plants of the same varieties, as shown by two
excellent observers. From my own observations (not made with great care)
the offspring from crossed varieties were to self-fertilised plants in
height, in one case as 100 to about 75, and in a second case as 100 to
60.
8. Lathyrus odoratus.
The sweet-pea is in the same state in regard to self-fertilisation as
the common pea; and we have seen that seedlings from a cross between two
varieties, which differed in no respect except in the colour of their
flowers, were to the self-fertilised seedlings from the same
mother-plant in height as 100 to 80; and in the second generation as 100
to 88. Unfortunately I did not ascertain whether crossing two plants of
the same variety failed to produce any beneficial effect, but I venture
to predict such would be the result.
9. Petunia violacea.
The intercrossed plants of the same stock in four out of the five
successive generations plainly exceeded in height the self-fertilised
plants. The latter in the fourth generation were crossed by a fresh
stock, and the seedlings thus obtained were put into competition with
the self-fertilised plants of the fifth generation. The crossed plants
exceeded the self-fertilised in height in the ratio of 100 to 66, and in
weight as 100 to 23; but this difference, though so great, is not much
greater than that between the intercrossed plants of the same stock in
comparison with the self-fertilised plants of the corresponding
generation. This case, therefore, seems at first sight opposed to the
rule that a cross with a fresh stock is much more beneficial than a
cross between individuals of the same stock. But as with Eschscholtzia,
the reproductive system was here chiefly benefited; for the plants
raised from the cross with the fresh stock were to the self-fertilised
plants in fertility, both lots being naturally fertilised, as 100 to 46,
whereas the intercrossed plants of the same stock were to the
self-fertilised plants of the corresponding fifth generation in
fertility only as 100 to 86.
Although at the time of measurement the plants raised from the cross
with the fresh stock did not exceed in height or weight the intercrossed
plants of the old stock (owing to the growth of the former not having
been completed, as explained under the head of this species), yet they
exceeded the intercrossed plants in fertility in the ratio of 100 to 54.
This fact is interesting, as it shows that plants self-fertilised for
four generations and then crossed by a fresh stock, yielded seedlings
which were nearly twice as fertile as those from plants of the same
stock which had been intercrossed for the five previous generations. We
here see, as with Eschscholtzia and Dianthus, that the mere act of
crossing, independently of the state of the crossed plants, has little
efficacy in giving increased fertility to the offspring. The same
conclusion holds good, as we have already seen, in the analogous cases
of Ipomoea, Mimulus, and Dianthus, with respect to height.
10. Nicotiana tabacum.
My plants were remarkably self-fertile, and the capsules from the
self-fertilised flowers apparently yielded more seeds than those which
were cross-fertilised. No insects were seen to visit the flowers in the
hothouse, and I suspect that the stock on which I experimented had been
raised under glass, and had been self-fertilised during several previous
generations; if so, we can understand why, in the course of three
generations, the crossed seedlings of the same stock did not uniformly
exceed in height the self-fertilised seedlings. But the case is
complicated by individual plants having different constitutions, so that
some of the crossed and self-fertilised seedlings raised at the same
time from the same parents behaved differently. However this may be,
plants raised from self-fertilised plants of the third generation
crossed by a slightly different sub-variety, exceeded greatly in height
and weight the self-fertilised plants of the fourth generation; and the
trial was made on a large scale. They exceeded them in height when grown
in pots, and not much crowded, in the ratio of 100 to 66; and when much
crowded, as 100 to 54. These crossed plants, when thus subjected to
severe competition, also exceeded the self-fertilised in weight in the
ratio of 100 to 37. So it was, but in a less degree (as may be seen in
Table 7/C), when the two lots were grown out of doors and not subjected
to any mutual competition. Nevertheless, strange as is the fact, the
flowers on the mother-plants of the third self-fertilised generation did
not yield more seed when they were crossed with pollen from plants of
the fresh stock than when they were self-fertilised.
11. Anagallis collina.
Plants raised from a red variety crossed by another plant of the same
variety were in height to the self-fertilised plants from the red
variety as 100 to 73. When the flowers on the red variety were
fertilised with pollen from a closely similar blue-flowered variety,
they yielded double the number of seeds to what they did when crossed by
pollen from another individual of the same red variety, and the seeds
were much finer. The plants raised from this cross between the two
varieties were to the self-fertilised seedlings from the red variety, in
height as 100 to 66, and in fertility as 100 to 6.
12. Primula veris.
Some flowers on long-styled plants of the third illegitimate generation
were legitimately crossed with pollen from a fresh stock, and others
were fertilised with their own pollen. From the seeds thus produced
crossed plants, and self-fertilised plants of the fourth illegitimate
generation, were raised. The former were to the latter in height as 100
to 46, and in fertility during one year as 100 to 5, and as 100 to 3.5
during the next year. In this case, however, we have no means of
distinguishing between the evil effects of illegitimate fertilisation
continued during four generations (that is, by pollen of the same form,
but taken from a distinct plant) and strict self-fertilisation. But it
is probable that these two processes do not differ so essentially as at
first appears to be the case. In the following experiment any doubt
arising from illegitimate fertilisation was completely eliminated.
13. Primula veris. (Equal-styled, red-flowered variety.)
Flowers on plants of the second self-fertilised generation were crossed
with pollen from a distinct variety or fresh stock, and others were
again self-fertilised. Crossed plants and plants of the third
self-fertilised generation, all of legitimate origin, were thus raised;
and the former was to the latter in height as 100 to 85, and in
fertility (as judged by the number of capsules produced, together with
the average number of seeds) as 100 to 11.
SUMMARY OF THE MEASUREMENTS IN TABLE 7/C.
This table includes the heights and often the weights of 292 plants
derived from a cross with a fresh stock, and of 305 plants, either of
self-fertilised origin, or derived from an intercross between plants of
the same stock. These 597 plants belong to thirteen species and twelve
genera. The various precautions which were taken to ensure a fair
comparison have already been stated. If we now look down the right hand
column, in which the mean height, weight, and fertility of the plants
derived from a cross with a fresh stock are represented by 100, we shall
see by the other figures how wonderfully superior they are both to the
self-fertilised and to the intercrossed plants of the same stock. With
respect to height and weight, there are only two exceptions to the rule,
namely, with Eschscholtzia and Petunia, and the latter is probably no
real exception. Nor do these two species offer an exception in regard to
fertility, for the plants derived from the cross with a fresh stock were
much more fertile than the self-fertilised plants. The difference
between the two sets of plants in the table is generally much greater in
fertility than in height or weight. On the other hand, with some of the
species, as with Nicotiana, there was no difference in fertility between
the two sets, although a great difference in height and weight.
Considering all the cases in this table, there can be no doubt that
plants profit immensely, though in different ways, by a cross with a
fresh stock or with a distinct sub-variety. It cannot be maintained that
the benefit thus derived is due merely to the plants of the fresh stock
being perfectly healthy, whilst those which had been long intercrossed
or self-fertilised had become unhealthy; for in most cases there was no
appearance of such unhealthiness, and we shall see under Table 7/A that
the intercrossed plants of the same stock are generally superior to a
certain extent to the self-fertilised,--both lots having been subjected
to exactly the same conditions and being equally healthy or unhealthy.
We further learn from Table 7/C, that a cross between plants that have
been self-fertilised during several successive generations and kept all
the time under nearly uniform conditions, does not benefit the offspring
in the least or only in a very slight degree. Mimulus and the
descendants of Ipomoea named Hero offer instances of this rule. Again,
plants self-fertilised during several generations profit only to a small
extent by a cross with intercrossed plants of the same stock (as in the
case of Dianthus), in comparison with the effects of a cross by a fresh
stock. Plants of the same stock intercrossed during several generations
(as with Petunia) were inferior in a marked manner in fertility to those
derived from the corresponding self-fertilised plants crossed by a fresh
stock. Lastly, certain plants which are regularly intercrossed by
insects in a state of nature, and which were artificially crossed in
each succeeding generation in the course of my experiments, so that they
can never or most rarely have suffered any evil from self-fertilisation
(as with Eschscholtzia and Ipomoea), nevertheless profited greatly by a
cross with a fresh stock. These several cases taken together show us in
the clearest manner that it is not the mere crossing of any two
individuals which is beneficial to the offspring. The benefit thus
derived depends on the plants which are united differing in some manner,
and there can hardly be a doubt that it is in the constitution or nature
of the sexual elements. Anyhow, it is certain that the differences are
not of an external nature, for two plants which resemble each other as
closely as the individuals of the same species ever do, profit in the
plainest manner when intercrossed, if their progenitors have been
exposed during several generations to different conditions. But to this
latter subject I shall have to recur in a future chapter.
TABLE 7/A.
We will now turn to our first table, which relates to crossed and
self-fertilised plants of the same stock. These consist of fifty-four
species belonging to thirty natural orders. The total number of crossed
plants of which measurements are given is 796, and of self-fertilised
809; that is altogether 1,605 plants. Some of the species were
experimented on during several successive generations; and it should be
borne in mind that in such cases the crossed plants in each generation
were crossed with pollen from another crossed plant, and the flowers on
the self-fertilised plants were almost always fertilised with their own
pollen, though sometimes with pollen from other flowers on the same
plant. The crossed plants thus became more or less closely inter-related
in the later generations; and both lots were subjected in each
generation to almost absolutely the same conditions, and to nearly the
same conditions in the successive generations. It would have been a
better plan in some respects if I had always crossed some flowers either
on the self-fertilised or intercrossed plants of each generation with
pollen from a non-related plant, grown under different conditions, as
was done with the plants in Table 7/C; for by this procedure I should
have learnt how much the offspring became deteriorated through continued
self-fertilisation in the successive generations. As the case stands,
the self-fertilised plants of the successive generations in Table 7/A
were put into competition with and compared with intercrossed plants,
which were probably deteriorated in some degree by being more or less
inter-related and grown under similar conditions. Nevertheless, had I
always followed the plan in Table 7/C, I should not have discovered the
important fact that, although a cross between plants which are rather
closely related and which had been subjected to closely similar
conditions, gives during several generations some advantage to the
offspring, yet that after a time they may be intercrossed with no
advantage whatever to the offspring. Nor should I have learnt that the
self-fertilised plants of the later generations might be crossed with
intercrossed plants of the same stock with little or no advantage,
although they profited to an extraordinary degree by a cross with a
fresh stock.
With respect to the greater number of the plants in Table 7/A, nothing
special need here be said; full particulars may be found under the head
of each species by the aid of the Index. The figures in the right-hand
column show the mean height of the self-fertilised plants, that of the
crossed plants with which they competed being represented by 100. No
notice is here taken of the few cases in which crossed and
self-fertilised plants were grown in the open ground, so as not to
compete together. The table includes, as we have seen, plants belonging
to fifty-four species, but as some of these were measured during several
successive generations, there are eighty-three cases in which crossed
and self-fertilised plants were compared. As in each generation the
number of plants which were measured (given in the table) was never very
large and sometimes small, whenever in the right hand column the mean
height of the crossed and self-fertilised plants is the same within five
per cent, their heights may be considered as practically equal. Of such
cases, that is, of self-fertilised plants of which the mean height is
expressed by figures between 95 and 105, there are eighteen, either in
some one or all the generations. There are eight cases in which the
self-fertilised plants exceed the crossed by above five per cent, as
shown by the figures in the right hand column being above 105. Lastly,
there are fifty-seven cases in which the crossed plants exceed the
self-fertilised in a ratio of at least 100 to 95, and generally in a
much higher degree.
If the relative heights of the crossed and self-fertilised plants had
been due to mere chance, there would have been about as many cases of
self-fertilised plants exceeding the crossed in height by above five per
cent as of the crossed thus exceeding the self-fertilised; but we see
that of the latter there are fifty-seven cases, and of the former only
eight cases; so that the cases in which the crossed plants exceed in
height the self-fertilised in the above proportion are more than seven
times as numerous as those in which the self-fertilised exceed the
crossed in the same proportion. For our special purpose of comparing the
powers of growth of crossed and self-fertilised plants, it may be said
that in fifty-seven cases the crossed plants exceeded the
self-fertilised by more than five per cent, and that in twenty-six cases
(18 + 8) they did not thus exceed them. But we shall now show that in
several of these twenty-six cases the crossed plants had a decided
advantage over the self-fertilised in other respects, though not in
height; that in other cases the mean heights are not trustworthy, owing
to too few plants having been measured, or to their having grown
unequally from being unhealthy, or to both causes combined.
Nevertheless, as these cases are opposed to my general conclusion I have
felt bound to give them. Lastly, the cause of the crossed plants having
no advantage over the self-fertilised can be explained in some other
cases. Thus a very small residue is left in which the self-fertilised
plants appear, as far as my experiments serve, to be really equal or
superior to the crossed plants.
We will now consider in some little detail the eighteen cases in which
the self-fertilised plants equalled in average height the crossed plants
within five per cent; and the eight cases in which the self-fertilised
plants exceeded in average height the crossed plants by above five per
cent; making altogether twenty-six cases in which the crossed plants
were not taller than the self-fertilised plants in any marked degree.
[1. Dianthus caryophyllus (third generation).
This plant was experimented on during four generations, in three of
which the crossed plants exceeded in height the self-fertilised
generally by much more than five per cent; and we have seen under Table
7/C that the offspring from the plants of the third self-fertilised
generation crossed by a fresh stock profited in height and fertility to
an extraordinary degree. But in this third generation the crossed plants
of the same stock were in height to the self-fertilised only as 100 to
99, that is, they were practically equal. Nevertheless, when the eight
crossed and eight self-fertilised plants were cut down and weighed, the
former were to the latter in weight as 100 to 49! There can therefore be
not the least doubt that the crossed plants of this species are greatly
superior in vigour and luxuriance to the self-fertilised; and what was
the cause of the self-fertilised plants of the third generation, though
so light and thin, growing up so as almost to equal the crossed in
height, I cannot explain.
2. Lobelia fulgens (first generation).
The crossed plants of this generation were much inferior in height to
the self-fertilised, in the proportion of 100 to 127. Although only two
pairs were measured, which is obviously much too few to be trusted, yet
from other evidence given under the head of this species, it is certain
that the self-fertilised plants were very much more vigorous than the
crossed. As I used pollen of unequal maturity for crossing and
self-fertilising the parent-plants, it is possible that the great
difference in the growth of their offspring may have been due to this
cause. In the next generation this source of error was avoided, and many
more plants were raised, and now the average height of the twenty-three
crossed plants was to that of the twenty-three self-fertilised plants as
100 to 91. We can therefore hardly doubt that a cross is beneficial to
this species.
3. Petunia violacea (third generation).
Eight crossed plants were to eight self-fertilised of the third
generation in average height as 100 to 131; and at an early age the
crossed were inferior even in a still higher degree. But it is a
remarkable fact that in one pot in which plants of both lots grew
extremely crowded, the crossed were thrice as tall as the
self-fertilised. As in the two preceding and two succeeding generations,
as well as with plants raised by a crossed with a fresh stock, the
crossed greatly exceeded the self-fertilised in height, weight, and
fertility (when these two latter points were attended to), the present
case must be looked at as an anomaly not affecting the general rule. The
most probable explanation is that the seeds from which the crossed
plants of the third generation were raised were not well ripened; for I
have observed an analogous case with Iberis. Self-fertilised seedlings
of this latter plant, which were known to have been produced from seeds
not well matured, grew from the first much more quickly than the crossed
plants, which were raised from better matured seeds; so that having thus
once got a great start they were enabled ever afterwards to retain their
advantage. Some of these same seeds of the Iberis were sown on the
opposite sides of pots filled with burnt earth and pure sand, not
containing any organic matter; and now the young crossed seedlings grew
during their short life to double the height of the self-fertilised, in
the same manner as occurred with the above two sets of seedlings of
Petunia which were much crowded and thus exposed to very unfavourable
conditions. We have seen also in the eighth generation of Ipomoea that
the self-fertilised seedlings raised from unhealthy parents grew at
first very much more quickly than the crossed seedlings, so that they
were for a long time much taller, though ultimately beaten by them.
4, 5, 6. Eschscholtzia californica.
Four sets of measurements are given in Table 7/A. In one of these the
crossed plants exceed the self-fertilised in average height, so that
this is not one of the exceptions here to be considered. In two other
cases the crossed equalled the self-fertilised in height within five per
cent; and in the fourth case the self-fertilised exceeded the crossed by
above this limit. We have seen in Table 7/C that the whole advantage of
a cross by a fresh stock is confined to fertility, and so it was with
the intercrossed plants of the same stock compared with the
self-fertilised, for the former were in fertility to the latter as 100
to 89. The intercrossed plants thus have at least one important
advantage over the self-fertilised. Moreover, the flowers on the
parent-plants when fertilised with pollen from another individual of the
same stock yield far more seeds than when self-fertilised; the flowers
in this latter case being often quite sterile. We may therefore conclude
that a cross does some good, though it does not give to the crossed
seedlings increased powers of growth.
7. Viscaria oculata.
The average height of the fifteen intercrossed plants to that of the
fifteen self-fertilised plants was only as 100 to 97; but the former
produced many more capsules than the latter, in the ratio of 100 to 77.
Moreover, the flowers on the parent-plants which were crossed and
self-fertilised, yielded seeds on one occasion in the proportion of 100
to 38, and on a second occasion in the proportion of 100 to 58. So that
there can be no doubt about the beneficial effects of a cross, although
the mean height of the crossed plants was only three per cent above that
of the self-fertilised plants.
8. Specularia speculum.
Only the four tallest of the crossed and the four tallest of the
self-fertilised plants, growing in four pots, were measured; and the
former were to the latter in height as 100 to 98. In all four pots a
crossed plant flowered before any one of the self-fertilised plants, and
this is usually a safe indication of some real superiority in the
crossed plants. The flowers on the parent-plants which were crossed with
pollen from another plant yielded seeds compared with the
self-fertilised flowers in the ratio of 100 to 72. We may therefore draw
the same conclusion as in the last case with respect to a cross being
decidedly beneficial.
9. Borago officinalis.
Only four crossed and four self-fertilised plants were raised and
measured, and the former were to the latter in height as 100 to 102. So
small a number of measurements ought never to be trusted; and in the
present instance the advantage of the self-fertilised over the crossed
plants depended almost entirely on one of the self-fertilised plants
having grown to an unusual height. All four crossed plants flowered
before their self-fertilised opponents. The cross-fertilised flowers on
the parent-plants in comparison with the self-fertilised flowers yielded
seeds in the proportion of 100 to 60. So that here again we may draw the
same conclusion as in the two last cases.
10. Passiflora gracilis.
Only two crossed and two self-fertilised plants were raised; and the
former were to the latter in height as 100 to 104. On the other hand,
fruits from the cross-fertilised flowers on the parent-plants contained
seeds in number, compared with those from the self-fertilised flowers,
in the proportion of 100 to 85.
11. Phaseolus multiflorus.
The five crossed plants were to the five self-fertilised in height as
100 to 96. Although the crossed plants were thus only four per cent
taller than the self-fertilised, they flowered in both pots before them.
It is therefore probable that they had some real advantage over the
self-fertilised plants.
12. Adonis aestivalis.
The four crossed plants were almost exactly equal in height to the four
self-fertilised plants, but as so few plants were measured, and as these
were all "miserably unhealthy," nothing can be inferred with safety with
respect to their relative heights.
13. Bartonia aurea.
The eight crossed plants were to the eight self-fertilised in height as
100 to 107. This number of plants, considering the care with which they
were raised and compared, ought to have given a trustworthy result. But
from some unknown cause they grew very unequally, and they became so
unhealthy that only three of the crossed and three of the
self-fertilised plants set any seeds, and these few in number. Under
these circumstances the mean height of neither lot can be trusted, and
the experiment is valueless. The cross-fertilised flowers on the
parent-plants yielded rather more seeds than the self-fertilised
flowers.
14. Thunbergia alata.
The six crossed plants were to the six self-fertilised in height as 100
to 108. Here the self-fertilised plants seem to have a decided
advantage; but both lots grew unequally, some of the plants in both
being more than twice as tall as others. The parent-plants also were in
an odd semi-sterile condition. Under these circumstances the superiority
of the self-fertilised plants cannot be fully trusted.
15. Nolana prostrata.
The five crossed plants were to the five self-fertilised in height as
100 to 105; so that the latter seem here to have a small but decided
advantage. On the other hand, the flowers on the parent-plants which
were cross-fertilised produced very many more capsules than the
self-fertilised flowers, in the ratio of 100 to 21; and the seeds which
the former contained were heavier than an equal number from the
self-fertilised capsules in the ratio of 100 to 82.
16. Hibiscus africanus.
Only four pairs were raised, and the crossed were to the self-fertilised
in height as 100 to 109. Excepting that too few plants were measured, I
know of nothing else to cause distrust in the result. The
cross-fertilised flowers on the parent-plants were, on the other hand,
rather more productive than the self-fertilised flowers.
17. Apium petroselinum.
A few plants (number not recorded) derived from flowers believed to have
been crossed by insects and a few self-fertilised plants were grown on
the opposite sides of four pots. They attained to a nearly equal height,
the crossed having a very slight advantage.
18. Vandellia nummularifolia.
Twenty crossed plants raised from the seeds of perfect flowers were to
twenty self-fertilised plants, likewise raised from the seeds of perfect
flowers, in height as 100 to 99. The experiment was repeated, with the
sole difference that the plants were allowed to grow more crowded; and
now the twenty-four tallest of the crossed plants were to the
twenty-four tallest self-fertilised plants in height as 100 to 94, and
in weight as 100 to 97. Moreover, a larger number of the crossed than of
the self-fertilised plants grew to a moderate height. The
above-mentioned twenty crossed plants were also grown in competition
with twenty self-fertilised plants raised from the closed or cleistogene
flowers, and their heights were as 100 to 94. Therefore had it not been
for the first trial, in which the crossed plants were to the
self-fertilised in height only as 100 to 99, this species might have
been classed with those in which the crossed plants exceed the
self-fertilised by above five per cent. On the other hand, the crossed
plants in the second trial bore fewer capsules; and these contained
fewer seeds, than did the self-fertilised plants, all the capsules
having been produced by cleistogene flowers. The whole case therefore
must be left doubtful.
19. Pisum sativum (common pea).
Four-plants derived from a cross between individuals of the same variety
were in height to four self-fertilised plants belonging to the same
variety as 100 to 115. Although this cross did no good, we have seen
under Table 7/C that a cross between distinct varieties adds greatly to
the height and vigour of the offspring; and it was there explained that
the fact of a cross between the individuals of the same variety not
being beneficial, is almost certainly due to their having been
self-fertilised for many generations, and in each generation grown under
nearly similar conditions.
20, 21, 22. Canna warscewiczi.
Plants belonging to three generations were observed, and in all of three
the crossed were approximately equal to the self-fertilised; the average
height of the thirty-four crossed plants being to that of the same
number of self-fertilised plants as 100 to 101. Therefore the crossed
plants had no advantage over the self-fertilised; and it is probable
that the same explanation here holds good as in the case of Pisum
sativum; for the flowers of this Canna are perfectly self-fertile, and
were never seen to be visited by insects in the hothouse, so as to be
crossed by them. This plant, moreover, has been cultivated under glass
for several generations in pots, and therefore under nearly uniform
conditions. The capsules produced by the cross-fertilised flowers on the
above thirty-four crossed plants contained more seeds than did the
capsules produced by the self-fertilised flowers on the self-fertilised
plants, in the proportion of 100 to 85; so that in this respect crossing
was beneficial.
23. Primula sinensis.
The offspring of plants, some of which were legitimately and others
illegitimately fertilised with pollen from a distinct plant, were almost
exactly of the same height as the offspring of self-fertilised plants;
but the former with rare exceptions flowered before the latter. I have
shown in my paper on dimorphic plants that this species is commonly
raised in England from self-fertilised seed, and the plants from having
been cultivated in pots have been subjected to nearly uniform
conditions. Moreover, many of them are now varying and changing their
character, so as to become in a greater or less degree equal-styled, and
in consequence highly self-fertile. Therefore I believe that the cause
of the crossed plants not exceeding in height the self-fertilised is the
same as in the two previous cases of Pisum sativum and Canna.
24, 25, 26. Nicotiana tabacum.
Four sets of measurements were made; in one, the self-fertilised plants
greatly exceeded in height the crossed, in two others they were
approximately equal to the crossed, and in the fourth were beaten by
them; but this latter case does not here concern us. The individual
plants differ in constitution, so that the descendants of some profit by
their parents having been intercrossed, whilst others do not. Taking all
three generations together, the twenty-seven crossed plants were in
height to the twenty-seven self-fertilised plants as 100 to 96. This
excess of height in the crossed plants, is so small compared with that
displayed by the offspring from the same mother-plants when crossed by a
slightly different variety, that we may suspect (as explained under
Table 7/C) that most of the individuals belonging to the variety which
served as the mother-plants in my experiments, had acquired a nearly
similar constitution, so as not to profit by being mutually
intercrossed.]
Reviewing these twenty-six cases, in which the crossed plants either do
not exceed the self-fertilised by above five per cent in height, or are
inferior to them, we may conclude that much the greater number of the
cases do not form real exceptions to the rule,--that a cross between two
plants, unless these have been self-fertilised and exposed to nearly the
same conditions for many generations, gives a great advantage of some
kind to the offspring. Of the twenty-six cases, at least two, namely,
those of Adonis and Bartonia, may be wholly excluded, as the trials were
worthless from the extreme unhealthiness of the plants. Inn twelve other
cases (three trials with Eschscholtzia here included) the crossed plants
either were superior in height to the self-fertilised in all the other
generations excepting the one in question, or they showed their
superiority in some different manner, as in weight, fertility, or in
flowering first; or again, the cross-fertilised flowers on the
mother-plant were much more productive of seed than the self-fertilised.
Deducting these fourteen cases, there remain twelve in which the crossed
plants show no well-marked advantage over the self-fertilised. On the
other hand, we have seen that there are fifty-seven cases in which the
crossed plants exceed the self-fertilised in height by at least five per
cent, and generally in a much higher degree. But even in the twelve
cases just referred to, the want of any advantage on the crossed side is
far from certain: with Thunbergia the parent-plants were in an odd
semi-sterile condition, and the offspring grew very unequally; with
Hibiscus and Apium much too few plants were raised for the measurements
to be trusted, and the cross-fertilised flowers of Hibiscus produced
rather more seed than did the self-fertilised; with Vandellia the
crossed plants were a little taller and heavier than the
self-fertilised, but as they were less fertile the case must be left
doubtful. Lastly, with Pisum, Primula, the three generations of Canna,
and the three of Nicotiana (which together complete the twelve cases), a
cross between two plants certainly did no good or very little good to
the offspring; but we have reason to believe that this is the result of
these plants having been self-fertilised and cultivated under nearly
uniform conditions for several generations. The same result followed
with the experimental plants of Ipomoea and Mimulus, and to a certain
extent with some other species, which had been intentionally treated by
me in this manner; yet we know that these species in their normal
condition profit greatly by being intercrossed. There is, therefore, not
a single case in Table 7/A which affords decisive evidence against the
rule that a cross between plants, the progenitors of which have been
subjected to somewhat diversified conditions, is beneficial to the
offspring. This is a surprising conclusion, for from the analogy of
domesticated animals it could not have been anticipated, that the good
effects of crossing or the evil effects of self-fertilisation would have
been perceptible until the plants had been thus treated for several
generations.
The results given in Table 7/A may be looked at under another point of
view. Hitherto each generation has been considered as a separate case,
of which there are eighty-three; and this no doubt is the more correct
method of comparing the crossed and self-fertilised plants.
But in those cases in which plants of the same species were observed
during several generations, a general average of their heights in all
the generations together may be made; and such averages are given in
Table 7/A; for instance, under Ipomoea the general average for the
plants of all ten generations is as 100 for the crossed, to 77 for the
self-fertilised plants. This having been done in each case in which more
than one generation was raised, it is easy to calculate the average of
the average heights of the crossed and self-fertilised plants of all the
species included in Table 7/A. It should however be observed that as
only a few plants of some species, whilst a considerable number of
others, were measured, the value of the mean or average heights of the
several species is very different. Subject to this source of error, it
may be worth while to give the mean of the mean heights of the
fifty-four species in Table 7/A; and the result is, calling the mean of
the mean heights of the crossed plants 100, that of the self-fertilised
plants is 87. But it is a better plan to divide the fifty-four species
into three groups, as was done with the previously given eighty-three
cases. The first group consists of species of which the mean heights of
the self-fertilised plants are within five per cent of 100; so that the
crossed and self-fertilised plants are approximately equal; and of such
species there are twelve about which nothing need be said, the mean of
the mean heights of the self-fertilised being of course very nearly 100,
or exactly 99.58. The second group consists of the species, thirty-seven
in number, of which the mean heights of the crossed plants exceed that
of the self-fertilised plants by more than five per cent; and the mean
of their mean heights is to that of the self-fertilised plants as 100 to
78. The third group consists of the species, only five in number, of
which the mean heights of the self-fertilised plants exceed that of the
crossed by more than five per cent; and here the mean of the mean
heights of the crossed plants is to that of the self-fertilised as 100
to 109. Therefore if we exclude the species which are approximately
equal, there are thirty-seven species in which the mean of the mean
heights of the crossed plants exceeds that of the self-fertilised by
twenty-two per cent; whereas there are only five species in which the
mean of the mean heights of the self-fertilised plants exceeds that of
the crossed, and this only by nine per cent.
The truth of the conclusion--that the good effects of a cross depend on
the plants having been subjected to different conditions or to their
belonging to different varieties, in both of which cases they would
almost certainly differ somewhat in constitution--is supported by a
comparison of the Tables 7/A and 7/C. The latter table gives the results
of crossing plants with a fresh stock or with a distinct variety; and
the superiority of the crossed offspring over the self-fertilised is
here much more general and much more strongly marked than in Table 7/A,
in which plants of the same stock were crossed. We have just seen that
the mean of the mean heights of the crossed plants of the whole
fifty-four species in Table 7/A is to that of the self-fertilised plants
as 100 to 87; whereas the mean of the mean heights of the plants crossed
by a fresh stock is to that of the self-fertilised in Table 7/C as 100
to 74. So that the crossed plants beat the self-fertilised plants by
thirteen per cent in Table 7/A, and by twenty-six per cent, or double as
much, in Table 7/C, which includes the results of the cross by a fresh
stock.
TABLE 7/B.
A few words must be added on the weights of the crossed plants of the
same stock, in comparison with the self-fertilised. Eleven cases are
given in Table 7/B, relating to eight species. The number of plants
which were weighed is shown in the two left columns, and their relative
weights in the right column, that of the crossed plants being taken as
100. A few other cases have already been recorded in Table 7/C in
reference to plants crossed by a fresh stock. I regret that more trials
of this kind were not made, as the evidence of the superiority of the
crossed over the self-fertilised plants is thus shown in a more
conclusive manner than by their relative heights. But this plan was not
thought of until a rather late period, and there were difficulties
either way, as the seeds had to be collected when ripe, by which time
the plants had often begun to wither. In only one out of the eleven
cases in Table 7/B, that of Eschscholtzia, do the self-fertilised plants
exceed the crossed in weight; and we have already seen they are likewise
superior to them in height, though inferior in fertility, the whole
advantage of a cross being here confined to the reproductive system.
With Vandellia the crossed plants were a little heavier, as they were
also a little taller than the self-fertilised; but as a greater number
of more productive capsules were produced by the cleistogene flowers on
the self-fertilised plants than by those on the crossed plants, the case
must be left, as remarked under Table 7/A, altogether doubtful. The
crossed and self-fertilised offspring from a partially self-sterile
plant of Reseda odorata were almost equal in weight, though not in
height. In the remaining eight cases, the crossed plants show a
wonderful superiority over the self-fertilised, being more than double
their weight, except in one case, and here the ratio is as high as 100
to 67. The results thus deduced from the weights of the plants confirm
in a striking manner the former evidence of the beneficial effects of a
cross between two plants of the same stock; and in the few cases in
which plants derived from a cross with a fresh stock were weighed, the
results are similar or even more striking.
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.
GREATER CONSTITUTIONAL VIGOUR OF CROSSED PLANTS.
As in almost all my experiments an equal number of crossed and
self-fertilised seeds, or more commonly seedlings just beginning to
sprout, were planted on the opposite sides of the same pots, they had to
compete with one another; and the greater height, weight, and fertility
of the crossed plants may be attributed to their possessing greater
innate constitutional vigour. Generally the plants of the two lots
whilst very young were of equal height; but afterwards the crossed
gained insensibly on their opponents, and this shows that they possessed
some inherent superiority, though not displayed at a very early period
in life. There were, however, some conspicuous exceptions to the rule of
the two lots being at first equal in height; thus the crossed seedlings
of the broom (Sarothamnus scoparius) when under three inches in height
were more than twice as tall as the self-fertilised plants.
After the crossed or the self-fertilised plants had once grown decidedly
taller than their opponents, a still increasing advantage would tend to
follow from the stronger plants robbing the weaker ones of nourishment
and overshadowing them. This was evidently the case with the crossed
plants of Viola tricolor, which ultimately quite overwhelmed the
self-fertilised. But that the crossed plants have an inherent
superiority, independently of competition, was sometimes well shown when
both lots were planted separately, not far distant from one another, in
good soil in the open ground. This was likewise shown in several cases,
even with plants growing in close competition with one another, by one
of the self-fertilised plants exceeding for a time its crossed opponent,
which had been injured by some accident or was at first sickly, but
being ultimately conquered by it. The plants of the eighth generation of
Ipomoea were raised from small seeds produced by unhealthy parents, and
the self-fertilised plants grew at first very rapidly, so that when the
plants of both lots were about three feet in height, the mean height of
the crossed to that of the self-fertilised was as 100 to 122; when they
were about six feet high the two lots were very nearly equal, but
ultimately when between eight and nine feet in height, the crossed
plants asserted their usually superiority, and were to the
self-fertilised in height as 100 to 85.
The constitutional superiority of the crossed over the self-fertilised
plants was proved in another way in the third generation of Mimulus, by
self-fertilised seeds being sown on one side of a pot, and after a
certain interval of time crossed seeds on the opposite side. The
self-fertilised seedlings thus had (for I ascertained that the seeds
germinated simultaneously) a clear advantage over the crossed in the
start for the race. Nevertheless they were easily beaten (as may be seen
under the head of Mimulus) when the crossed seeds were sown two whole
days after the self-fertilised. But when the interval was four days, the
two lots were nearly equal throughout life. Even in this latter case the
crossed plants still possessed an inherent advantage, for after both
lots had grown to their full height they were cut down, and without
being disturbed were transferred to a larger pot, and when in the
ensuing year they had again grown to their full height they were
measured; and now the tallest crossed plants were to the tallest
self-fertilised plants in height as 100 to 75, and in fertility (i.e.,
by weight of seeds produced by an equal number of capsules from both
lots) as 100 to 34.
My usual method of proceeding, namely, to plant several pairs of crossed
and self-fertilised seeds in an equal state of germination on the
opposite sides of the same pots, so that the plants were subjected to
moderately severe mutual competition, was I think the best that could
have been followed, and was a fair test of what occurs in a state of
nature. For plants sown by nature generally come up crowded, and are
almost always exposed to very severe competition with one another and
with other kinds of plants. This latter consideration led me to make
some trials, chiefly but not exclusively with Ipomoea and Mimulus, by
sowing crossed and self-fertilised seeds on the opposite sides of large
pots in which other plants had long been growing, or in the midst of
other plants out of doors. The seedlings were thus subjected to very
severe competition with plants of other kinds; and in all such cases,
the crossed seedlings exhibited a great superiority in their power of
growth over the self-fertilised.
After the germinating seedlings had been planted in pairs on the
opposite sides of several pots, the remaining seeds, whether or not in a
state of germination, were in most cases sown very thickly on the two
sides of an additional large pot; so that the seedlings came up
extremely crowded, and were subjected to extremely severe competition
and unfavourable conditions. In such cases the crossed plants almost
invariably showed a greater superiority over the self-fertilised, than
did the plants which grew in pairs in the pots.
Sometimes crossed and self-fertilised seeds were sown in separate rows
in the open ground, which was kept clear of weeds; so that the seedlings
were not subjected to any competition with other kinds of plants. Those
however in each row had to struggle with the adjoining ones in the same
row. When fully grown, several of the tallest plants in each row were
selected, measured, and compared. The result was in several cases (but
not so invariably as might have been expected) that the crossed plants
did not exceed in height the self-fertilised in nearly so great a degree
as when grown in pairs in the pots. Thus with the plants of Digitalis,
which competed together in pots, the crossed were to the self-fertilised
in height as 100 to 70; whilst those which were grown separately were
only as 100 to 85. Nearly the same result was observed with Brassica.
With Nicotiana the crossed were to the self-fertilised plants in height,
when grown extremely crowded together in pots, as 100 to 54; when grown
much less crowded in pots as 100 to 66, and when grow in the open
ground, so as to be subjected to but little competition, as 100 to 72.
On the other hand with Zea, there was a greater difference in height
between the crossed and self-fertilised plants growing out of doors,
than between the pairs which grew in pots in the hothouse; but this may
be attributed to the self-fertilised plants being more tender, so that
they suffered more than the crossed, when both lots were exposed to a
cold and wet summer. Lastly, with one out of two series of Reseda
odorata, grown out of doors in rows, as well as with Beta vulgaris, the
crossed plants did not at all exceed the self-fertilised in height, or
exceeded them by a mere trifle.
The innate power of the crossed plants to resist unfavourable conditions
far better than did the self-fertilised plants, was shown on two
occasions in a curious manner, namely, with Iberis and in the third
generation of Petunia, by the great superiority in height of the crossed
over the self-fertilised seedlings, when both sets were grown under
extremely unfavourable conditions; whereas owing to special
circumstances exactly the reverse occurred with the plants raised from
the same seeds and grown in pairs in pots. A nearly analogous case was
observed on two other occasions with plants of the first generation of
Nicotiana.
The crossed plants always withstood the injurious effects of being
suddenly removed into the open air after having been kept in the
greenhouse better than did the self-fertilised. On several occasions
they also resisted much better cold and intemperate weather. This was
manifestly the case with some crossed and self-fertilised plants of
Ipomoea, which were suddenly moved from the hothouse to the coldest part
of a cool greenhouse. The offspring of plants of the eighth
self-fertilised generation of Mimulus crossed by a fresh stock, survived
a frost which killed every single self-fertilised and intercrossed plant
of the same old stock. Nearly the same result followed with some crossed
and self-fertilised plants of Viola tricolor. Even the tips of the
shoots of the crossed plants of Sarothamnus scoparius were not touched
by a very severe winter; whereas all the self-fertilised plants were
killed halfway down to the ground, so that they were not able to flower
during the next summer. Young crossed seedlings of Nicotiana withstood a
cold and wet summer much better than the self-fertilised seedlings. I
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