82 PLANTS AND ANIMALS 
Each stamen consists usually of a slender stalk, the 
filament, bearing an oblong head, the anther, which 
contains four chambers, or pollen sacs, filled with 
pollen grains; these, when mature, escape into the air 
by the rupturing of the walls of the chambers. 
Each carpel contains in its lower part an ovary, 
while its upper part presents to the air a surface 
charged with nutrient substance, the stigma, which is 
often raised on a slender stalk, the style. 
To secure the production of seed, the first neces- 
sary step is pollination, or the transfer of pollen from 
the stamen to the stigma. When this is effected 
—the means will be considered immediately—and a 
pollen grain alights on the surface of the stigma, which 
is usually sticky or hairy to aid its retention there, the 
pollen grain commences growth, and sends out a 
slender tube (the pollen tube), which pursues its way 
through the substance of the stigma, down the style, 
into the ovary, and from its tip a male cell passes 
out and fuses with the ovum. In most flowers the 
pollen tube is not called on to make any great effort 
of growth, the distance between stigma and ovary 
being very small; but occasionally, as in Crocus and 
Lily, this may amount to half a foot. The result of 
this act of fertilization is that the ovum and ovule 
grow, the former forming eventually the embryo, or 
young plant, the latter the seed in which the embryo 
is enclosed. In order that fertile seed may be pro- 
duced it is often necessary, and usually desirable, that 
the pollen which reaches the stigma should not belong 
to the same flower, but to a different flower of the 
same species; cross-pollination being the rule among 
seed plants, self-pollination the exception. To secure 
