_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 submerged aquatic plants, it has
a double wall, a thin delicate wall of unaltered cellulose, the
endospore or intine, and a tough outer cuticularized exospore or
extine. The exospore often bears spines or warts, or is variously
sculptured, and the character of the markings is often of value
for the distinction of genera or higher groups. Germination of the
microspore begins before it leaves the pollen-sac. In very few cases
has anything representing prothallial development been observed;
generally a small cell (the antheridial or generative cell) is cut
off, leaving a larger tube-cell. When placed on the stigma, under
favourable circumstances, the pollen-grain puts forth a pollen-tube
which grows down the tissue of the style to the ovary, and makes its
way along the placenta, guided by projections or hairs, to the mouth