SEXUAL CHARACTERISTICS GIVEN BY HETEROSPORY 353 



homospory they took on certain characteristics of sex. This does 

 not mean that the microspores and megaspores became gametes, 

 for their spore nuclei have never become sexual nuclei in any 

 group of plants. But microspore and megaspore did assume 

 sexual characters to this extent that they always give rise, 

 respectively, to male and female gametophytes. 



Furthermore, the degeneration of the gametophytes steadily 

 reduced the number of the nuclear divisions between the germ- 

 ination of these spores and the formation of the gametes until 

 the gamete nuclei have been brought very close indeed to the 

 spore nuclei. An examination of the figures of the male game- 

 tophytes of Marsilia (Fig. 282, A}, Selaginella (Fig. 290, A), and 

 Isoetes (Fig. 292, A) will show that there can hardly be more 

 than from six to ten nuclear divisions in these types before the 

 sperms are developed. There are even fewer nuclear divisions 

 in some groups of seed plants where the degeneration of the 

 gametophyte is carried much further than in the pteridophytes. 

 Some forms of angiosperms present but a single division of the 

 spore nucleus before the female gamete nuclei are formed, as 

 in the embryo sac of the lily (Sec. 360, note), and there are only 

 two nuclear divisions in the male gametophytes (pollen grain 

 and tube) of the angiosperms. 



This gradual transference of sexual characteristics to portions 

 of the asexual generation, accompanying the reduction of the 

 sexual generation, is one of the most interesting results of the 

 evolution of the sporophyte and degeneration of the gameto- 

 phyte (summarized in Chapter xxix), for it makes clear many 

 puzzling conditions in the seed plants. Thus it shows why the 

 pollen grain (which is a microspore) is really functionally a male 

 reproductive structure and the stamen a male organ ; and why 

 the carpels and pistil are functionally female organs, even though 

 they have had their origin on asexual plants (sporophytes). 



