34 STRUCTURAL BOTANY 



sperms the membrane of the embryo-sac likewise becomes 

 cuticularised. It has also been found that in some plants 

 of this class (Cycadeae) the development of the endosperm 

 is completed while the seeds are lying on the ground, 

 and in a few cases the endosperm has been observed 

 to burst through the embryo-sac and seed-coats and to 

 become green, just like the prothallus of Selaginella. 



A more serious difficulty is that there are four 

 megaspores in Selaginella, and only one embryo- sac in 

 the Fir, though there are Gymnosperms which have more 

 than one, as Gnetum. The most probable explanation 

 is that in most Gymnosperms the embryo-sac represents 

 a mother-cell, which directly becomes a megaspore, 

 without going through the fourfold division which is 

 usual in Cryptogams. 1 



The organ in which the megaspores are produced is a 

 megasporangium ; that in which the embryo-sac develops 

 is the ovule. Both organs arise in the same way from a 

 group of cells near the growing-point. The similarity of 

 their development has already been pointed out (see p. 17). 

 We infer, then, that the megasporangium corresponds to 

 the ovule, or more strictly to the nucellus of the ovule, 

 for the megasporangium has no integument. 



We have found, however, that the megasporangium 

 and microsporangium are just alike in the earlier stages 

 of their growth ; the former, as we have seen, corresponds 

 to the nucellus of an ovule, the latter to a pollen-sac. 

 Hence we must draw the conclusion that a pollen- sac 

 and the nucellus of an ovule are equivalent structures a 



1 In Sequoia, however (the genus to which the famous "Big Trees" of 

 California belong), the embryo-sacs are formed in fours, just like megaspores, 

 though only one comes to maturity. Very likely this also takes place in 

 some of the other Gymnosperms. 



