4IO STRUCTURE AND LIFE HISTORIES 



in frequently not having the embryo develop until after 

 the seed is planted, and in not having the embryo undergo 

 a period of rest between its formation and its final develop- 

 ment into a mature sporophyte ; the absence of an embryo 

 characterizes some of the fossil relatives of Cycas. 



The genus Cycas is the only living genus of plants which 

 produces seeds without developing either a cone or a flower, 

 but produces the seeds on sporophylls borne on the main 

 stem. It is the simplest of all living seed-plants, to this 

 extent, being in this respect as lowly organized as the 

 ferns. 



OUTLINE OF LIFE HISTORY OF A CYCAD 



The Cycad plant 

 (Sporophyte) 



Microsporophylls Megasporophylls 



4" 4- 4- 4- 



Microsporangia Megasporangia (ovules) 



4. < Reduction > ^, 



Microspores Megaspores 



Male gametophyte Female gametophyte 



(Pollen-grain) (Endosperm) 



4. 4. 



Vestigial antheridium Archegonium 

 Generative cell 



4- 



Sperms 



;;on 



I 



Oosperm 



44 



Embryo 



4-4- 



New Cycad plant 

 (Sporophyte) 



