POLLINATION AND FERTILIZATION 



135 



extremely slender, sticky threads. The mature^rrajnpnnfging 

 two jiuclei, &_gnerative_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 gram. 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, A). 

 The pollen tube con- 

 tains a tube nucleus 

 (f) near its tip and 

 a generative cell (#) 

 somewhere farther 

 back. At length the gen- 

 erative cell divides into 

 two male nuclei, these 

 develop into male cells 

 (fig. 118, J5), 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 

 tubular passage and sometimes it penetrates the tissues, aided 

 by the corroding action of ferments which it secretes. It is 



FIG. 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 ; t, tube nucleus ; s lt s z , male 

 cells formed from the generative cell. It is ap- 

 parent that when the growth of the tube is far 

 advanced, the tube nucleus (t) almost disappears. 

 Much magnified. After Bonnier and Sablon 



