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 
afford 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 offspring 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 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 
1 Forms of Flowers (1st edit.), p. 341. 
