ALTERNATION OF GENERATIONS 443 



The gymnosperms show the end of the series in the reduction of 

 the vegetative part of the male gametophyte. 



The number of sperms is, dominantly, only two. Microcycas and 

 Cupressus are notable exceptions in having a dozen or more sperms. 

 Probably in extinct forms, near their pteridophyte ancestors, sperms 

 were more numerous. 



The female gametophyte is, necessarily, parasitic in all seed 

 plants. In all known gymnosperms its development begins with a 

 period of free nuclear divisions, which is followed by a period of wall 

 formation. This sequence was already established in the heteros- 

 porous pteridophytes. 



Aside from a general reduction in the size of the gametophyte, the 

 main features are the reduction of the archegonium and a delay in 

 wall formation. 



In the most primitive archegonia, like those of Pinus and Ginkgo, 

 there is a neck, consisting of two or more cells, and a ventral canal 

 cell separated from the egg by a definite wall. In most, if not all of 

 the Abietaceae, there is a definite ventral canal cell. In the rest of 

 the Coniferales the wall between the ventral canal nucleus and the 

 egg nucleus is lacking, and is also lacking in the Cycadales. Nothing 

 is known about the ventral canal situation in any extinct gymno- 

 sperm; but we should expect to find a well developed ventral canal 

 cell. 



In Torreya there is not even a ventral canal nucleus, and in Wel- 

 witschia, not even a neck cell, the archegonium initial functioning as 

 an egg. In Gnetum there is not even an archegonium initial. One or 

 more of the free nuclei organize an egg, as in the angiosperms, and 

 fertilization takes place with the upper part of the female gameto- 

 phyte in the free nuclear condition, as in a,ngiosperms. The lower 

 part of this gametophyte, as often in the antipodal region in angio- 

 sperms, is already cellular at the time of fertilization, and, as in angi- 

 osperms, the rest of the gametophyte becomes cellular after fertiliza- 

 tion. 



Consequently, the reduction of the female gametophyte has al- 

 most reached the angiosperm level, the principal difference being 

 that Gnetum has more antipodal cells and more free nuclei. 



Not much is known of the female gametophyte of extinct gymno- 



