are small and scale-like. Such a branch-system is
called an inflorescence. The primary function of the flower is to bear
the spores. These, as in Gymnosperms, are of two kinds, microspores
or pollen-grains, borne in the stamens (or microsporophylls) and
megaspores, in which the egg-cell is developed, contained in the
ovule, which is borne enclosed in the carpel (or megasporophyll). The
flower may consist only of spore-bearing leaves, as in willow, where
each flower comprises only a few stamens or two carpels. Usually,
however, other leaves are present which are only indirectly concerned
with the reproductive process, acting as protective organs for the
sporophylls or forming an attractive envelope. These form the perianth
and are in one series, when the flower is termed monochlamydeous, or
in two series (dichlamydeous). In the second case the outer series
(calyx of sepals) is generally green and leaf-like, its function being
to protect the rest of the flower, especially in the bud; while
the inner series (corolla of petals) is generally white or brightly
coloured, and more delicate in structure, its function being to
attract the particular insect or bird by agency of which pollination
is effected. The insect, &c., is attracted by the colour and scent
of the flower, and frequently also by honey which is secreted in some
part of the flower. (For further details on the form and arrangement
of the flower and its parts, see FLOWER.)

_Stamen and pollen._

Each stamen generally bears four pollen-sacs (_microsporangia_)
which are associated to form the anther, and carried up on a stalk
or filament. The development of the microsporangia and the contained
spores (pollen-grains) is closely comparable with that of the
microsporangia in Gymnosperms or heterosporous ferns. The pollen is
set free by the opening (dehiscence) of the anther, generally by means
of longitudinal slits, but sometimes by pores, as in the heath family
(Ericaceae), or by valves, as in the barberry. It is then dropped
or carried by some external agent, wind, water or some member of the
animal kingdom, on to the receptive surface of the carpel of the same
or another flower. The carpel, or aggregate of carpels forming the
pistil or gynaeceum, comprises an ovary containing one or more ovules
and a receptive surface or stigma; the stigma is sometimes carried up
on a style. The mature pollen-grain is, like other spores, a single
cell; except in the case of some