116 



PRACTICAL BOTANY 



Each mature grain contains a generative nucleus and a tube 

 nucleus (# and t, Fig. 107, A). After the pollen grain lodges 

 on the stigma the inner coat of the grain 

 becomes slightly distended by osmosis, pro- 

 duced by contact with the moist stigmatic 

 surface. The distention of the inner coat 

 causes it to protrude through the outer coat 

 and it at length develops into the wall of 

 a pollen tube (Fig. 107). This tube has the 

 nucleus () at its 

 tip and a generative 

 cell (#) somewhere 

 within the tube. 

 Finally the genera- 

 tive cell divides in- 

 to two male nuclei, 

 these develop into 

 male cells (Fig.107, 

 B), and the tube nu- 

 cleus disappears. 



108. Course of the 

 pollen tube: fertilization. 

 The pollen tube makes its 

 way from the stigma to 

 the ovary either through 

 a canal or passage (Fig. 

 108), or by directly trav- 

 ersing the cellular tis- 

 sue of the style, upon 

 which it acts so as to eat 

 its way along by means 

 of ferments (Sect. 36) 

 secreted by the .tube. 

 The tube may require 

 a day or more to reach the ovule. Food materials from the 

 style are dissolved by the enzymes and used in promoting the 



FIG. 107. 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 ; t, tube nucleus ; s 1? s 2 , male 

 cells formed from the generative cell. It is ap- 

 parent that when the growth of the tube is far 

 advanced thetubenucleus (t) almostdisappears. 

 Much magnified. After Bonnier and Sablon 



