136 Plant Life and Evolution 



more modern types of seed-plants, but there are 

 still some seventy-five species of these, pretty well 

 distributed over the warmer parts of the earth. Of 

 these, the genus Cycas, represented by the common 

 C. revoluta, the " Sago Palm " of the florist, is espe- 

 cially interesting, as it is a survivor of one of the 

 earliest genera known, and has come down probably 

 from the early Mesozoic with apparently little 

 change. Cycas is in habit very much like a tree- 

 fern. The upright trunk bears at its summit a 

 crown of fern-like leaves, which when young have 

 the leaflets coiled up like those of a young fern- 

 leaf. The fertile leaves, or sporophylls, in Cycas 

 retain the fern-like form (Fig. 15, A), and the 

 enormous ovules, or megasporangia, are borne on 

 the margins of the sporophylls, and later develop 

 into great seeds, which in some species are as big as 

 a hen's egg. 



Although the gametophyte is well advanced in the 

 big seeds, its final development and fertilization 

 take place after the seed has fallen off of the 

 plant. The gametophyte has also been known in 

 some cases, where fertilization was not effected, to 

 continue its growth and develop a green mass of 

 tissue like the gametophyte of the lower ferns, indi- 

 cating that as heterospory developed in the fern an- 

 cestors of the cycads, the reduction of the gameto- 

 phyte was much less than in that of the existing 

 heterosporous ferns. This great development of the 

 gametophyte, together with the presence of motile 



