326 PRACTICAL BOTANY 



through the central softer tissues of the style. It does not 

 make a passageway by forcing aside the tissue, but by means 

 of its own secretions (enzymes) it breaks down these cells, 

 and they doubtless furnish nourishment to the growing pollen 

 tube (Figs. 107 and 266). The tube usually makes its way 

 down the center of the style, then along the wall of the ovary 

 cavity, and finally turns across to the micropyle of the ovule. 

 It then grows through the tissues to the end of the embryo 

 sac. 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. 



While this long growth of the pollen tube has been taking 

 place, development within it has also occurred. From the single- 

 celled microspore there now have developed three cells, which 

 are contained in the tip of the tube. Two of these are male 

 cells and can function as sperms, though they are non-ciliated. 

 The other is a nutritive cell, which goes forward within the 

 tip of the pollen tube and is an important factor in its growth. 



When the tube reaches the embryo sac it opens, the male 

 cells pass out, and one of them unites with the egg (Fig. 265). 

 In many cases, and perhaps generally, the other male cell 

 passes down and unites with the endosperm nucleus, which 

 was formed by the union of two female gametophyte cells. This 

 results in a cell that is the union of three cells, a phenome- 

 non not known elsewhere in the plant kingdom. The ordinary 

 fertilization of the egg and this added phenomenon of union 

 of a male cell and the endosperm cell is known as double fer- 

 tilization. Its significance is not understood, but it is notably 

 true that angiosperms present the first case in seed plants 

 wherein the female gametophyte has two periods of develop- 

 ment, one before fertilization and one afterward, and it may 

 be that double fertilization is related to this second period of 

 growth of the female gametophyte. During the period while 

 the oospore is growing into the embryo plant, the endosperm 

 cell is growing into the endosperm that is often found in ripe 

 seeds (Figs. 125 and 128). 



