Ch. VI, 10] MORPHOLOGY OF FLOWERS 



325 





This identification of pollen and 

 ovules with the spores of the lower 

 plants at once throws light on two other 

 features of floral structure. First, the 

 megasporangia and microsporangia of 

 the flowerless plants occur in close asso- 

 ciation with, or upon, certain leaves, 

 somewhat modified accordingly, called 

 sporophylls (Fig. 223) ; and it seems 

 clear that stamens and pistils are the 

 lineal descendants of the sporophylls. 

 As to petals and sepals, it is not yet 

 certain whether they represent ancient 

 sporophylls which have lost their spo- 

 rangia, or green leaves independently 

 specialized, though the latter seems 

 most probable. Second, the pollen 

 grains and embryo sacs (the ancient 

 spores) are not themselves the sex cells, 

 but develop the sperm cells and egg 

 cells through intermediation of some 

 cell divisions which have no apparent 

 meaning under existent conditions (Figs. 

 188, 190, and full account in Part II). 

 Now in the lower plants the spores 

 are not sex cells either, but they pro- 

 duce special and often elaborate struc- 

 tures (including the prothallus stage 



Of the Ferns, the thallus of the Liver- several microspores ; on 



WOrts, and the whole body of the the right megasporangia 



_ .. N l • i ji 11 containing four mega- 



MpSSes), Upon Which the Sex Cells are spores. The sporangia 



developed ; and it is the reduced pro- s * and u p°" sporophylls. 

 thallus, or equivalent, of the lower 

 plants which persists as the seemingly meaningless cell divi- 

 sions within the pollen grain and embryo sac. Thus while 

 ovule and embryo sac, with anther and pollen grain, are parts 



Fiq. 223. — The fruit- 

 ing strobilus of Selagi- 

 nella inoequifolia, a 

 Pteridophyte ; X 12. 



On the left, micro- 



