Almost all these plants, as Darwin pointed out, have also chasmogamous flowers which render cross-fertilisation possible. His view that cleistogamous flowers are derived from originally chasmogamous flowers has been confirmed by more recent researches. Conditions of nutrition in the broader sense are the factors which determine whether chasmogamous or cleistogamous flowers are produced, assuming, of course, that the plants in question have the power of developing both forms of flower. The former may fail to appear for some time, but are eventually developed under favourable conditions of nourishment. The belief of many authors that there are plants with only cleistogamous flowers cannot therefore be accepted as authoritative without thorough experimental proof, as we are concerned with extra-european plants for which it is often difficult to provide appropriate conditions in cultivation.
Darwin sees in cleistogamous flowers an adaptation to a good supply of seeds with a small expenditure of material, while chasmogamous flowers of the same species are usually cross-fertilised and "their offspring will thus be invigorated, as we may infer from a wide-spread analogy." ("Forms of Flowers" (1st edition), page 341.) Direct proof in support of this has hitherto been supplied in a few cases only; we shall often find that the example set by Darwin in solving such problems as these by laborious experiment has unfortunately been little imitated.
Another chapter of this book treats of the distribution of the sexes in polygamous, dioecious, and gyno-dioecious plants (the last term, now in common use, we owe to Darwin). It contains a number of important facts and discussions and has inspired the experimental researches of Correns and others.
The most important of Darwin's work on floral biology is, however, that on cross and self-fertilisation, chiefly because it states the results of experimental investigations extending over many years. Only such experiments, as we have pointed out, could determine whether cross-fertilisation is in itself beneficial, and self-fertilisation on the other hand injurious; a conclusion which a merely comparative examination of pollination-mechanisms renders in the highest degree probable. Later floral biologists have unfortunately almost entirely confined themselves to observations on floral mechanisms. But there is little more to be gained by this kind of work than an assumption long ago made by C.K. Sprengel that "very many flowers have the sexes separate and probably at least as many hermaphrodite flowers are dichogamous; it would thus appear that Nature was unwilling that any flower should be fertilised by its own pollen."It was an accidental observation which inspired Darwin's experiments on the effect of cross and self-fertilisation. Plants of Linaria vulgaris were grown in two adjacent beds; in the one were plants produced by cross-fertilisation, that is, from seeds obtained after fertilisation by pollen of another plant of the same species; in the other grew plants produced by self-fertilisation, that is from seed produced as the result of pollination of the same flower. The first were obviously superior to the latter.
Darwin was surprised by this observation, as he had expected a prejudicial influence of self-fertilisation to manifest itself after a series of generations: "I always supposed until lately that no evil effects would be visible until after several generations of self-fertilisation, but now Isee that one generation sometimes suffices and the existence of dimorphic plants and all the wonderful contrivances of orchids are quite intelligible to me." ("More Letters", Vol. II. page 373.)The observations on Linaria and the investigations of the results of legitimate and illegitimate fertilisation in heterostyled plants were apparently the beginning of a long series of experiments. These were concerned with plants of different families and led to results which are of fundamental importance for a true explanation of sexual reproduction.
The experiments were so arranged that plants were shielded from insect-visits by a net. Some flowers 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 -
