THE GAMETOPHYTES OF THE CYCADS 368 



the micropylar end of the endosperm (Fig. 295, D, a). These are 

 very much reduced in structure, the neck being represented prob- 

 ably by two small cells and the very large eggs lying imbedded 

 in the cells of the endosperm. 



The male gametophyte consists of the protoplasm with 

 several nuclei contained in the pollen grain and tube. Some of 

 the nuclei near the pollen grain end of the tube lie within deli- 

 cate cell walls. One of these cells termed the generative cell 

 develops two sperm mother cells which become organized into 

 two very large motile sperms (Fig. 295, B, C), each with a spiral 

 band or line bearing hundreds of cilia. The two sperms finally 

 begin to move around in the fluid of the pollen tube and are 

 discharged from the end nearest the pollen grain (which now 

 hangs down over the embryo sac) into the fluid within the 

 cavity formed from the disorganized tissue at the tip of the 

 nucellus. The pollen tube in the cycads grows off to one side 

 in the nucellus and seems to be a sort of absorbing organ, so 

 that it does not carry the sperms to the embryo sac as the 

 sperm nuclei are carried in most seed plants. 



The motile sperms are set free in the fluid above the embryo 

 sac, whose female gametophyte at that time bears mature arche- 

 gonia. They have been observed swimming about for many min- 

 utes in sections of the living ovules, and probably have a long 

 motile period in the ovule. One of them is finally able to enter 

 the neck of an archegonium, and fusing with an egg fertilizes it. 



The finding of motile sperms in the cycads and in Ginkgo 1 

 by two Japanese botanists in 1896-1897 proved two of the most 

 interesting botanical discoveries of the past decade. It is very 

 remarkable that the sperm-forming habits of the bryophytes and 

 pteridophytes should have persisted so long after the seed habit 

 became established in a group. The free swimming of these 

 motile sperms is actually a return, such as occurs in the bryo- 

 phytes and pteridophytes, for a short time in the life history of the 

 cycad to the aquatic habits of an algal ancestry of ages ago. 



1 A beautiful Japanese tree, not uncommon under cultivation. 



