connected with the conditions of the formation of
hybrids, a fact which was formerly overlooked, we can appreciate how
Darwin was able to say in his autobiography: "I do not think anything in
my scientific life has given me so much satisfaction as making out the
meaning of the structure of these plants." ("Life and Letters", Vol. I.
page 91.)

The remarkable conditions represented in plants with three kinds of
flowers, such as Lythrum and Oxalis, agree in essentials with those
in Primula. These cannot be considered in detail here; it need only be
noted that the investigation of these cases was still more laborious.
In order to establish the relative fertility of the different unions in
Lythrum salicaria 223 different fertilisations were made, each flower
being deprived of its male organs and then dusted with the appropriate
pollen.

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 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
proo