Darwin and Modern Science [286]
were then pollinated with their own pollen, others with pollen from another plant of the same species. The seeds were germinated on moist sand; two seedlings of the same age, one from a cross and the other from a self-fertilised flower, were selected and planted on opposite sides of the same pot. They grew therefore under identical external conditions; it was thus possible to compare their peculiarities such as height, weight, fruiting capacity, etc. In other cases the seedlings were placed near to one another in the open and in this way their capacity of resisting unfavourable external conditions was tested. The experiments were in some cases continued to the tenth generation and the flowers were crossed in different ways. We see, therefore, that this book also represents an enormous amount of most careful and patient original work.
The general result obtained is that plants produced as the result of cross- fertilisation are superior, in the majority of cases, to those produced as the result of self-fertilisation, in height, resistance to external injurious influences, and in seed-production.
Ipomoea purpurea may be quoted as an example. If we express the result of cross-fertilisation by 100, we obtain the following numbers for the fertilised plants.
Generation. Height. Number of seeds.
1 100 : 76 100 : 64 2 100 : 79 - 3 100 : 68 100 : 94 4 100 : 86 100 : 94 5 100 : 75 100 : 89 6 100 : 72 - 7 100 : 81 - 8 100 : 85 - 9 100 : 79 100 : 26 (Number of capsules) 10 100 : 54 -
Taking the average, the ratio as regards growth is 100:77. The considerable superiority of the crossed plants is apparent in the first generation and is not increased in the following generations; but there is some fluctuation about the average ratio. The numbers representing the fertility of crossed and self-fertilised plants are more difficult to compare with accuracy; the superiority of the crossed plants is chiefly explained by the fact that they produce a much larger number of capsules, not because there are on the average more seeds in each capsule. The ratio of the capsules was, e.g. in the third generation, 100:38, that of the seeds in the capsules 100:94. It is also especially noteworthy that in the self-fertilised plants the anthers were smaller and contained a smaller amount of pollen, and in the eighth generation the reduced fertility showed itself in a form which is often found in hybrids, that is the first flowers were sterile. (Complete sterility was not found in any of the plants investigated by Darwin. Others appear to be more sensitive; Cluer found Zea Mais "almost sterile" after three generations of self-fertilisation. (Cf. Fruwirth, "Die Zuchtung der Landwirtschaftlichen Kulturpflanzen", Berlin, 1904, II. page 6.)
The superiority of crossed individuals is not exhibited in the same way in all plants. For example in Eschscholzia californica the crossed seedlings do not exceed the self-fertilised in height and vigour, but the crossing considerably increases the plant's capacity for flower-production, and the seedlings from such a mother-plant are more fertile.
The conception implied by the term crossing requires a closer analysis. As in the majority of plants, a large number of flowers are in bloom at the same time on one and the same plant, it follows that insects visiting the flowers often carry pollen from one flower to another of the same stock. Has this method, which is spoken of as Geitonogamy, the same influence as crossing with pollen from another plant? The results of Darwin's experiments with different plants (Ipomoea purpurea, Digitalis purpurea, Mimulus luteus, Pelargonium, Origanum) were not in complete agreement; but on the whole they pointed to the conclusion that Geitonogamy shows no superiority over self-fertilisation (Autogamy). (Similarly crossing in the case of flowers of Pelargonium zonale, which belong to plants raised from cuttings
The general result obtained is that plants produced as the result of cross- fertilisation are superior, in the majority of cases, to those produced as the result of self-fertilisation, in height, resistance to external injurious influences, and in seed-production.
Ipomoea purpurea may be quoted as an example. If we express the result of cross-fertilisation by 100, we obtain the following numbers for the fertilised plants.
Generation. Height. Number of seeds.
1 100 : 76 100 : 64 2 100 : 79 - 3 100 : 68 100 : 94 4 100 : 86 100 : 94 5 100 : 75 100 : 89 6 100 : 72 - 7 100 : 81 - 8 100 : 85 - 9 100 : 79 100 : 26 (Number of capsules) 10 100 : 54 -
Taking the average, the ratio as regards growth is 100:77. The considerable superiority of the crossed plants is apparent in the first generation and is not increased in the following generations; but there is some fluctuation about the average ratio. The numbers representing the fertility of crossed and self-fertilised plants are more difficult to compare with accuracy; the superiority of the crossed plants is chiefly explained by the fact that they produce a much larger number of capsules, not because there are on the average more seeds in each capsule. The ratio of the capsules was, e.g. in the third generation, 100:38, that of the seeds in the capsules 100:94. It is also especially noteworthy that in the self-fertilised plants the anthers were smaller and contained a smaller amount of pollen, and in the eighth generation the reduced fertility showed itself in a form which is often found in hybrids, that is the first flowers were sterile. (Complete sterility was not found in any of the plants investigated by Darwin. Others appear to be more sensitive; Cluer found Zea Mais "almost sterile" after three generations of self-fertilisation. (Cf. Fruwirth, "Die Zuchtung der Landwirtschaftlichen Kulturpflanzen", Berlin, 1904, II. page 6.)
The superiority of crossed individuals is not exhibited in the same way in all plants. For example in Eschscholzia californica the crossed seedlings do not exceed the self-fertilised in height and vigour, but the crossing considerably increases the plant's capacity for flower-production, and the seedlings from such a mother-plant are more fertile.
The conception implied by the term crossing requires a closer analysis. As in the majority of plants, a large number of flowers are in bloom at the same time on one and the same plant, it follows that insects visiting the flowers often carry pollen from one flower to another of the same stock. Has this method, which is spoken of as Geitonogamy, the same influence as crossing with pollen from another plant? The results of Darwin's experiments with different plants (Ipomoea purpurea, Digitalis purpurea, Mimulus luteus, Pelargonium, Origanum) were not in complete agreement; but on the whole they pointed to the conclusion that Geitonogamy shows no superiority over self-fertilisation (Autogamy). (Similarly crossing in the case of flowers of Pelargonium zonale, which belong to plants raised from cuttings