412 The Biology of Flowers 



In the book containing the account of heterostyled plants 

 other species are dealt with which, in addition to flowers opening 

 normally (chasmogamous), also possess flowers which remain closed 

 but are capable of producing fruit. These cleistogamous flowers 

 afibrd a striking example of habitual self-pollination, and H. von 

 Mohl drew special attention to them as such shortly after the 

 appearance of Darwin's Orchid book. If it were only a question of 

 producing seed in the simplest way, cleistogamous flowers would be 

 the most conveniently constructed. The corolla and frequently other 

 parts of the flower are reduced ; the development of the seed may, 

 therefore, be accomplished with a smaller expenditure of building 

 material than in chasmogamous flowers ; there is also no loss of 

 pollen, and thus a smaller amount suffices for fertilisation. 



Almost all these plants, as Darwin pointed out, have also chas- 

 mogamous 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 con- 

 cerned 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 chasmo- 

 gamous flowers of the same species are usually cross-fertilised and 

 "their oflspring will thus be invigorated, as we may infer from a 

 wide-spread analogy \" 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 experi- 

 ment has unfortunately been little imitated. 



Another cliapter of this book treats of the distribution of the sexes 

 in polygamous, dioecious, and gyno-dioecious plants (the last term, 

 now in connnon 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 



^ Forms of Flowers (Ist edit.), p. 341. 



