320 



SENESCENCE AND REJUVENESCENCE 



In the seed plants the whole gametophyte generation is greatly 

 reduced and represents scarcely more than specialized male and 

 female organs of the plant. In the lower seed plants, the gymno- 

 sperms, a considerable number of nuclear divisions may occur in 

 the development of the gametophyte, and the female gamete is 

 separated from other cells at some stage of this development. 

 In the male gametophyte of the gymnosperms the number of 

 divisions varies, but is always small, and in the course of these 

 divisions the male gamete is separated from non-reproductive 

 cells. 



And finally in the angiosperms, which represent the final stage 

 in reduction of the gametophyte, the development of the male 



gametophyte — the mature poUen 

 grain — from the microspore con- 

 sists, with one exception, of only 

 two nuclear divisions, of which the 

 first separates the primary sper- 

 matogenous cell from the tube 

 nucleus and the second divides 

 the spermatogenous nucleus into 

 two male gametes, so that the 

 male gametophyte contains only 

 three nuclei (Fig. 140). 



The course of development of 

 the female gametophyte, which is 

 the embryo sac within the ovule, 

 is indicated in Fig. 141. The 

 nucleus of the megaspore {A) divides and the two daughter nuclei 

 pass to opposite poles (B); a second division occurs in each (C), 

 and a third follows (D) , so that eight nuclei are present, four at each 

 pole, but without cell boundaries. Two nuclei, one from each group 

 of four, move toward the middle of the embryo sac and fuse to form 

 the primary endosperm nucleus. About the three nuclei at the 

 micropylar end (the upper end in the figures) three naked cell bodies 

 arise, and these three cells are the egg and the two synergids (£). 

 The three nuclei at the opposite pole form the three antipodal cells 

 which are usually ephemeral but may persist. Thus the germ 



Fig. 140. — Pollen grain of Silphium 

 terebinthinaceum, showing rounded 

 vegetative nucleus and the two elon- 

 gated male nuclei. From Merrell, '00. 



