pollen.--Anemophilous species.--Conversion of anemophilous species into
entomophilous.--Origin of nectar.--Anemophilous plants generally have
their sexes separated.--Conversion of diclinous into hermaphrodite
flowers.--Trees often have their sexes separated.


CHAPTER XI.

THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.

Insects visit the flowers of the same species as long as they
can.--Cause of this habit.--Means by which bees recognise the flowers of
the same species.--Sudden secretion of nectar.--Nectar of certain
flowers unattractive to certain insects.--Industry of bees, and the
number of flowers visited within a short time.--Perforation of the
corolla by bees.--Skill shown in the operation.--Hive-bees profit by the
holes made by humble-bees.--Effects of habit.--The motive for
perforating flowers to save time.--Flowers growing in crowded masses
chiefly perforated.


CHAPTER XII.

GENERAL RESULTS.

Cross-fertilisation proved to be beneficial, and self-fertilisation
injurious.--Allied species differ greatly in the means by which
cross-fertilisation is favoured and self-fertilisation avoided.--The
benefits and evils of the two processes depend on the degree of
differentiation in the sexual elements.--The evil effects not due to the