214 
FOUNDATIONS OF BOTANY 
226. Fertilization. — By fertilization in seed-plants the 
botanist means the union of a generative cell from a pol- 
ae. 
we! 
L\ 
a 
15) 
Fie. 163. — Pollen Grains producing 
Tubes, on Stigma ofa Lily. (Much 
magnified.) 
g, pollen grains ; t, pollen tubes; p, 
papillz of stigma; c, canal or pas- 
sage running toward ovary. 
Ss 
Fic. 164.— Pollen Grain of Snow- 
flake (Leucoiwm) producing a Pol- 
len Tube with Two Naked Genera- 
tive Cells. 
len grain with that of an egg-cell 
at the apex of the embryo sac 
(Fig. 165). This process gives 
rise to a cell which contains 
material derived from the pollen 
and from the egg-cell. In a 
ereat 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 re- 
quired for the process to begin 
varies in different kinds of 
plants, requiring in many cases 
twenty-four hours or more. The 
length of time needed for the 
pollen tube to make its way 
through the style to the ovary 
depends upon the length of the 
style and other conditions. In 
the crocus, which has a style 
several inches long, the descent 
takes from one to three days. 
Finally the tube penetrates the opening at the apex of 
