s FERTILIZATION 165 
193. Fertilization.— Fertilization in seed-plants means 
the union of a sperm nucleus derived from a generative 
cell of a pollen grain with an 
egg-cell at the apex of the 
embryo sac (Fig. 124). This 
process gives rise to a cell 
ANS 
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islele 
Fic. 128. Germination of Pollen 
Grain of an Angiosperm. (Much 
magnified and somewhat dia- 
grammatic.) 
wa) 
Fic. 122. Pollen Grains producing A, ee kart ae ich 
Tubex, on Stigma of a Lily. siderably advanced. B, tip of pollen 
Maoh ified tube at a much later stage, after the 
(Much magnified.) tube nucleus has disappeared: g, the 
g, pollen grains; ¢, pollen tubes; generative cell beginning to enter the 
p, papille of stigma; ¢c, canal or tube; ¢, the tube nucleus; s, sperm 
passage running toward ovary. cells formed from the generative cell. 
which contains material derived from the pollen and from 
the egg-cell. Ina great many plants the pollen, in order to 
accomplish the most successful fertilization, must come 
from another plant of the same kind, not from the indi- 
vidual which bears the ovules that are being fertilized. 
Pollen tubes begin to form soon after pollen grains lodge 
on the stigma. The time required for the tube to penetrate 
to the ovary varies in different kinds of plants. 
Finally the tube enters the opening at the apex of the 
