SPERMATOPHYTA 411 
(Figs. 464, 469). After the embryo reaches a certain size the seed 
is mature. In the mature seed the embryo is surrounded by the 
prothallus, and the prothallus is surrounded by the integument, 
the nucellus having largely disappeared (Figs. 464, 469). The 
prothallus contains a very considerable supply of stored food. 
During germination the embryo lives on this supply of stored 
food until it has developed sufficiently to be independent. 

Frc. 462. Cone of megasporophylls of Zamia floridana 
Left, entire cone; right, cone with some sporophylls removed and 
showing seed. (x 2) 
Alternation of generations. In the cycads there is an alternation 
of generations, just as in ferns and in liverworts. In the cyeads the 
sporophytic generation produces megaspores and microspores. The 
spores give rise to the gametophytes. The female gametophyte is a 
prothallus that is developed within the megasporangium, while the 
male gametophyte develops from a microspore, or pollen grain. 
Just as in the ferns, the number of chromosomes 1s reduced when a 
spore mother cell divides to form spores. Therefore the spores and 
