f, 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 surprise