POLLINATION AND FERTILIZATION 135 
extremely slender, sticky threads. The mature grain contains 
two nuclei, a yenerative nucleus and a tube nucleus. Contact of 
the pollen grain with the moist stigmatic 
surface brings about osmosis, which results 
in distention of the pollen grain. At some 
point the outer coat becomes ruptured and 
the inner thin coat protrudes through the 
opening and is the beginning of the pollen 
tube. Then a portion of the contents of 
the grain passes out 
into the pollen tube, 
which is developed 
from the thin inner 
coat of the pollen 
grain (fig. 118, -1). 
The pollen tube con- 
tains a tube nucleus 
(¢) near its tip and 
a generative cell (g) 
somewhere farther 
back. At length the gen- 
erative cell divides into 
two male nuclet, these 
develop into male cells 
(fig. 118, B), and the tube 
nucleus sooner or later 
disappears. 
126. Course of the pol- 
len tube. The pollen tube 
readily makes its way 
between the exterior cells 
of the stigma and passes 
onward to the ovary. 
Sometimes it traverses a 
Fie. 118. Germination of the pollen grain 
of a dicotyledon 
A, an early stage in the germination; B, later 
stage, with the tube rather fully developed ; 
g, generative cell; ¢, tube nucleus; s), s., male 
cells formed from the generative cell. It is ap- 
parent that when the growth of the tube is far 
advanced, the tube nucleus (¢) almost disappears. 
Much magnified. After Bonnier and Sablon 
tubular passage and sometimes it penetrates the tissues, aided 
by the corroding action of ferments which it secretes. It is 
