136 



MORPHOLOGY 



is occupied by a vacuole. The ovule is now ready 

 for fertilization. 



A pollen-grain is a single cell with a cell wall in 

 which may be distinguished a thick cutinized outer 

 layer and a thin inner layer of cellulose. On the 

 outer layer are thin or weak spots for the exit of the 



pollen -tubes. The proto- 

 plasmic contents, formerly 

 called FOViLLA, are un- 

 usually rich in starch 

 or oil, or both, and often 

 contain chloroplasts. Be- 

 fore the pollen-grains are 

 shed the nucleus divides 

 first into two, one of which 

 forms the naked vegeta- 

 tive CELL, and the other 

 again divides into two, 

 which form two naked cells 

 known as generative 

 CELLS. The pollen-grain 

 falling on the stigma and 

 feeding on the sugary juice 

 secreted by it, germinates; 

 in other words, the inner 

 cellulose layer protrudes 

 through the weak spots of 

 the outer cutinized layer, forming what is known as 

 the POLLEN-TUBE. The pollen-tube carries with it 

 the greater portion of the contents of the pollen-grain, 

 including the vegetative and generative cells. The 

 pollen-tube, making its way through the loose tissue 

 of the style, enters into the cavity of the ovary, and, 

 guided by grooves, lines, marks, or hairs within it, 

 reaches the mycropyle of the ovule (fig. 120). The 



Figf. 120. — Longitudinal Section of a 

 Pistil, with a Single Basal and Anatro- 

 pous Ovule, showing the course of the 

 Pollen Tube 



THt micropyle; e, embryo-sac. 



