FERTILIZATION 
211 
Seeds formed as a result of cross¬ 
pollination produce more vigorous 
plants than those which grow from 
seeds in self-pollinated flowers. 
Self-Pollination. — Some flowers, 
like the cleistogamous flowers of the 
violet, are arranged with a view 
to securing self-pollination. Most 
flowers, on the other hand, have 
adaptations to prevent it. One of 
these is bearing unisexual flowers 
only, pistillate and staminate flowers, 
on different plants as the willow does. 
Another is having pistil and stamens 
mature at different times (dandelion, 
see p. 207); and a third is having stamens and pistils of 
different lengths. When an insect visits such a flower, one 
part of its body is apt to come 
into contact with the stamens 
and another with the pistil. 
For example in the primrose, 
an insect which gets pollen 
from short stamens on its 
body in one flower leaves it 
on the short stigma of an¬ 
other flower. 
192. Fertilization. — The 
union of the pollen nucleus 
with the nucleus of the egg 
cell is called fertilization. 
Without it, the ovule never 
develops into a seed. (Look 
in pea or bean pods for unfertilized ovules; in the seed- 
case of an apple; on an ear of corn.) 
Successful fertilization depends much on thorough pollina- 
Figure 197. — Pollen Tube, En¬ 
larged. 
A, tube ; B, loose cells of style. 
Figure 196. — Pollen 
Grains Sprouting and 
Growing through 
Style. 
