Darwin and Modern Science [287]
from the same parent, shows no superiority over self- fertilisation.) Darwin, however, considered it possible that this may sometimes be the case. "The sexual elements in the flowers on the same plant can rarely have been differentiated, though this is possible, as flower-buds are in one sense distinct individuals, sometimes varying and differing from one another in structure or constitution." ("Cross and Self fertilisation" (1st edition), page 444.)
As regards the importance of this question from the point of view of the significance of cross-fertilisation in general, it may be noted that later observers have definitely discovered a difference between the results of autogamy and geitonogamy. Gilley and Fruwirth found that in Brassica Napus, the length and weight of the fruits as also the total weight of the seeds in a single fruit were less in the case of autogamy than in geitonogamy. With Sinapis alba a better crop of seeds was obtained after geitonogamy, and in the Sugar Beet the average weight of a fruit in the case of a self-fertilised plant was 0.009 gr., from geitonogamy 0.012 gr., and on cross-fertilisation 0.013 gr.
On the whole, however, the results of geitonogamy show that the favourable effects of cross-fertilisation do not depend simply on the fact that the pollen of one flower is conveyed to the stigma of another. But the plants which are crossed must in some way be different. If plants of Ipomoea purpurea (and Mimulus luteus) which have been self-fertilised for seven generations and grown under the same conditions of cultivation are crossed together, the plants so crossed would not be superior to the self- fertilised; on the other hand crossing with a fresh stock at once proves very advantageous. The favourable effect of crossing is only apparent, therefore, if the parent plants are grown under different conditions or if they belong to different varieties. "It is really wonderful what an effect pollen from a distinct seedling plant, which has been exposed to different conditions of life, has on the offspring in comparison with pollen from the same flower or from a distinct individual, but which has been long subjected to the same conditions. The subject bears on the very principle of life, which seems almost to require changes in the conditions." ("More Letters", Vol. II. page 406.)
The fertility--measured by the number or weight of the seeds produced by an equal number of plants--noticed under different conditions of fertilisation may be quoted in illustration.
On crossing On crossing On self- with a fresh plants of the fertilisation stock same stock Mimuleus luteus (First and ninth generation) 100 4 3
Eschscholzia californica (second generation) 100 45 40
Dianthus caryophyllus (third and fourth generation) 100 45 33
Petunia violacea 100 54 46
Crossing under very similar conditions shows, therefore, that the difference between the sexual cells is smaller and thus the result of crossing is only slightly superior to that given by self-fertilisation. Is, then, the favourable result of crossing with a foreign stock to be attributed to the fact that this belongs to another systematic entity or to the fact that the plants, though belonging to the same entity were exposed to different conditions? This is a point on which further researches must be taken into account, especially since the analysis of the systematic entities has been much more thorough than formerly. (In the case of garden plants, as Darwin to a large extent claimed, it is not easy to say whether two individuals really belong to the same variety, as they are usually of hybrid origin. In some instances (Petunia, Iberis) the fresh stock employed by Darwin possessed flowers differing in colour from those of the plant crossed with it.) We know that most of Linneaus's
As regards the importance of this question from the point of view of the significance of cross-fertilisation in general, it may be noted that later observers have definitely discovered a difference between the results of autogamy and geitonogamy. Gilley and Fruwirth found that in Brassica Napus, the length and weight of the fruits as also the total weight of the seeds in a single fruit were less in the case of autogamy than in geitonogamy. With Sinapis alba a better crop of seeds was obtained after geitonogamy, and in the Sugar Beet the average weight of a fruit in the case of a self-fertilised plant was 0.009 gr., from geitonogamy 0.012 gr., and on cross-fertilisation 0.013 gr.
On the whole, however, the results of geitonogamy show that the favourable effects of cross-fertilisation do not depend simply on the fact that the pollen of one flower is conveyed to the stigma of another. But the plants which are crossed must in some way be different. If plants of Ipomoea purpurea (and Mimulus luteus) which have been self-fertilised for seven generations and grown under the same conditions of cultivation are crossed together, the plants so crossed would not be superior to the self- fertilised; on the other hand crossing with a fresh stock at once proves very advantageous. The favourable effect of crossing is only apparent, therefore, if the parent plants are grown under different conditions or if they belong to different varieties. "It is really wonderful what an effect pollen from a distinct seedling plant, which has been exposed to different conditions of life, has on the offspring in comparison with pollen from the same flower or from a distinct individual, but which has been long subjected to the same conditions. The subject bears on the very principle of life, which seems almost to require changes in the conditions." ("More Letters", Vol. II. page 406.)
The fertility--measured by the number or weight of the seeds produced by an equal number of plants--noticed under different conditions of fertilisation may be quoted in illustration.
On crossing On crossing On self- with a fresh plants of the fertilisation stock same stock Mimuleus luteus (First and ninth generation) 100 4 3
Eschscholzia californica (second generation) 100 45 40
Dianthus caryophyllus (third and fourth generation) 100 45 33
Petunia violacea 100 54 46
Crossing under very similar conditions shows, therefore, that the difference between the sexual cells is smaller and thus the result of crossing is only slightly superior to that given by self-fertilisation. Is, then, the favourable result of crossing with a foreign stock to be attributed to the fact that this belongs to another systematic entity or to the fact that the plants, though belonging to the same entity were exposed to different conditions? This is a point on which further researches must be taken into account, especially since the analysis of the systematic entities has been much more thorough than formerly. (In the case of garden plants, as Darwin to a large extent claimed, it is not easy to say whether two individuals really belong to the same variety, as they are usually of hybrid origin. In some instances (Petunia, Iberis) the fresh stock employed by Darwin possessed flowers differing in colour from those of the plant crossed with it.) We know that most of Linneaus's