THE SEED PLANTS 



301 



forces its way through the central softer tissues of the style. It 

 does not make a passageway by forcing the tissue aside, but 

 by means of its own secretions (enzymes) it breaks down these 

 tissues, and they doubtless furnish nourishment to the grow- 

 ing pollen tube (fig. 230). When the tube reaches the cavity 



P 



FIG. 230. Germinating 

 pollen grains 



The pollen grains (g) have been 

 deposited upon the stigma. The 

 roughened surface of the stigma 

 is made by cell extensions, or 

 papillae (p) . Pollen tubes (<) grow 

 from the grains through the tis- 

 sue or along the central canal (c) 

 until they reach the ovule. Only 

 a small part of the stigma and 

 style are shown in this cut 



FIG. 231. Diagram of the ovule of an 



angiospermous plant, showing the parts 



of the ovule 



The outer integument (oi) ; the inner integu- 

 ment (i); the micropyle, which is the opening 

 between the parts of the inner integument ; 

 the pollen tube (pt) , which has grown through 

 the micropyle 



which contains the ovules, it turns across to the micropyle of 

 the ovule. It then grows through the tissue at the tip of the 

 ovule to the end of the embryo sac (fig. 231). In some cases 

 (elm and walnut) the pollen tube grows down to the base of 

 the ovule, then up through it, and finally reaches the egg. 



