unisexual animals. I will also give a full abstract of such observations as have
been published since the appearance of my papers; but only those cases will be
noticed, with respect to which the evidence seems fairly satisfactory. Some
plants have been supposed to be heterostyled merely from their pistils and
stamens varying greatly in length, and I have been myself more than once thus
deceived. With some species the pistil continues growing for a long time, so
that if old and young flowers are compared they might be thought to be
heterostyled. Again, a species tending to become dioecious, with the stamens
reduced in some individuals and with the pistils in others, often presents a
deceptive appearance. Unless it be proved that one form is fully fertile only
when it is fertilised with pollen from another form, we have not complete
evidence that the species is heterostyled. But when the pistils and stamens
differ in length in two or three sets of individuals, and this is accompanied by
a difference in the size of the pollen-grains or in the state of the stigma, we
may infer with much safety that the species is heterostyled. I have, however,
occasionally trusted to a difference between the two forms in the length of the
pistil alone, or in the length of the stigma together with its more or less
papillose condition; and in one instance differences of this kind have been
proved by trials made on the fertility of the two forms, to be sufficient
evidence.

The second sub-group above referred to consists of hermaphrodite plants, which
bear two kinds of flowers--the one perfect and fully expanded--the other minute,
completely closed, with the petals rudimentary, often with some of the anthers
aborted, and the remaining ones together with the stigmas much reduced in size;
yet these flowers are perfectly fertile. They have been called by Dr. Kuhn
cleistogamic, and they will be described in the last chapter of this volume.
(Introduction/3. 'Botanische Zeitung' 1867 page 65. Several plants are known
occasionally to produce flowers destitute of a corolla; but they belong to a
different class of cases from cleistogamic flowers. This deficiency seems to