Seeds for the Garden 117 
must be carried by the wind or 
by insects from the flowers with 
only stamens to the flowers with 
only pistils. 
In other garden plants (bean, 
pea, salsify, and tomato are good 
examples) the stamens and pis- 
tils are both present in each 
flower. But even in these the 
wind and the insects very often 
carry pollen from one flower to Fic. 69. qaisapollengrain, and . 
ee 6 and c show pollen tubes which 
the pistils of another. have developed from grains ger- 
The carrying of pollen from meted on suearasne The 
the stamens to the pistils is of the pollen tube is shown near 
called pollination. If the pollen {2e,°"4)0% the longer tubes the 
which reaches a pistil is from ther back in the same tube. 
The grains are here shown 220 
the same flower or another times natural size. 
flower of the same plant, it is 
self-pollination. If it is from a different plant, it is cross- 
pollination. 
Fertilization. After the pollen grains have been 
placed on the stigma, a tiny, thread-like tube sprouts out 
from each pollen grain. These tubes grow downward, 
making their way among the cells of the pistil, until they 
reach the sac-like structures (ovules). The pollen tube 
enters the ovule through a tiny opening that is present 
in its wall and continues growing until it reaches the 
egg. There it bursts open at the tip, and a little cell 
called the sperm cell, which was within the tube, unites 
with the egg cell. This uniting of a sperm and an egg 
cell is called fertilization. 
