SEED PLANTS 



163 



of the size of a large plum. The very large macrospore 

 (Fig. 40, B, ma) has a definite thick membrane like that of 

 the ferns, but is retained permanently within the sporan- 

 gium. So far as it is known, the development of the 



F 



ma 





FIG. 40 (CycadacejE). A, a sporophyll of Cyras circinalis, with six 

 ovules (macrosporangia) , ma ; B, longitudinal section of a young ovule 

 of C. revoluta, showing the single large macrospore, ma ; C, a sporophyll 

 from the male cone of (7. revo/uta, showing the lower surface covered 

 with groups or sori of microsporangia, mi; D, a single sorus of live 

 raicrosporangia ; E, a microspore (pollen-spore), showing the rudimen- 

 tary antheridium, an: the larger antlieridial cell later gives rise to 

 two large spermatozoids : F, a young leaf of C. revoluta, showing the 

 fern-like coiling of the divisions; G, a scale from the female cone of 

 Zamia integrifolia, with two ovules, ma; H, section through the ovule 

 at the time of fertilization : pc, the pollen-chamber with three germi- 

 nating pollen-spores ; </, the vegetative tissue of the female gametophyte 

 contained within the macrospore; ar, two archegonia; I, a spermato- 

 zoid of Zamia, showing the numerous cilia, c. (Figs. H, I, after Webber.) 



gametophyte is much like that of Isoetes or Selaginella, 

 but the details are still somewhat imperfectly known. 

 The gametophyte, if fertilization is not effected, may 

 grow out beyond the spore and develop chlorophyll, 



