GNETALES— GNETUM 



423 



reduction has proceeded until there is only one neck canal cell. 

 There is no neck canal cell in known gymnosperms, and many gym- 

 nosperms have lost the wall between the ventral canal nucleus and 

 that of the egg. In Torreya it is very doubtful whether there is even 

 a mitosis: the central cell becomes the egg. Welwitschia, going a 

 step farther, has lost the mitosis which gives rise to the neck cell and 

 central cell, so that the archegonium initially 

 functions as an egg. In Gnetum there is not 

 even an archegonium initial : a free nucleus, or- 

 ganizing some cytoplasm about itself, functions 

 directly as an egg. 



Without such a series in mind, some have tried 

 to homologize the angiosperm synergids with 

 various lost structures of the archegonium. 

 There is no relation. The synergids are new ad- 

 vanced structures, having no connection with 

 gymnosperm archegonia. 



After fertiHzation the rest of the gametophy te 

 becomes cellular. As yet, there is no evidence 

 that any of the tissue has come from a fusion 

 with a male nucleus, or that a fertilized egg has 

 developed "endosperm" instead of an embryo. 

 It used to be said that the endosperm of gym- 

 nosperms is formed before fertilization, while 

 that of angiosperms was formed after fertiliza- 

 tion. In Gnetum most of this tissue is formed 

 after fertilization, and, in this respect, follows the angiosperm meth 

 od, although no "double fertilization," hke that described by Herz 

 FELD for Ephedra, has been reported. 



Fig. 394. — Gnetum 

 gnemon: development 

 of tissue at the base of 

 the female gameto- 

 phyte; X300. — After 

 Thompson. ''■''5 



FERTILIZATION AND EMBRYOGENY 



The most interesting feature about fertilization in Gnetum is that 

 it occurs while the female gametophyte is still in the free nuclear 

 stage. One or more eggs are organized from the free nuclei, looking 

 like the eggs of angiosperms. The pollen tubes, some of which begin 

 their development in the micropylar tube, at some distance from the 

 nucellus, grow through the nucellus and reach the egg. The two 



