DEVELOPMENT OF PLANTS 393 



mate strength of the solution on the stigma must be determined 

 in order to prepare a solution suitable for their development. 

 The tube cell, stimulated by the secretion of the stigma, ruptures 

 the outer wall of the spore, which is provided with one or more 

 thin places to favor this growth, and protrudes as a delicate tube. 

 This tube, owing to the fact that it is repelled by the oxygen of 

 the air, grows dowJi_intQ_the tissues of the style which are really 

 a continuation of the stigma. These tissues of the style are usu- 

 ally looser and provided with abundant foods which are deposited 

 in the cells just ahead of the elongating tube to nourish and direct 

 it in its growth. In this way the tube is directed down the style 

 to the cavity of the ovary where, owing to the attractive influence 

 of the organic substances in the sporangium, possibly in the syn- 

 ergids, it usually turns out into the cavity of the ovary, enters 

 the micropyle and works its way through the sporangium, and, 

 unlike the Pinales, enters the female gametophyte generally 

 alongside of one of the synergids (Fig. 273). The antheridial cell 

 usually divides, forming directly two motionless male cells during 

 the elongation of the tube, and in other cases these two cells are 

 already formed when the microspores are discharged from their 

 sporangia. In none of these cases is there any indication in the 

 division of the antheridial cell of the formation of a wall cell as in 

 the gymnosperms, so that the male gametophyte of the Spermato- 

 phyta presents a very regular series of reductions from the cycads 

 to the pines and thence to the angiosperms, where it consists of a 

 tubular growth containing three naked cells (Fig. 272, D). This 

 development of the male gametophyte requires from a day to 

 several months and is quite independent, apparently, of the dis- 

 tance that it has to traverse in reaching the female gametophyte. 

 The male cells are often somewhat elongated and even spirally 

 coiled and carried to the end of the tube, as in the Pinales. 



126. Fertilization. The end of the tube finally ruptures, owing 

 to the tension of the fluids that gradually accumulate in it, and 

 the male cells are forcibly expelled into the sac-like cavity 

 of the female gametophyte (Fig. 273). One of the male cells 

 passes over to and fuses with the female gamete, thus forming 

 the gametospore; and the other male cell unites with the polar 

 26 



