THE VASCULAR OUtPTOGAMS 101 



usually divides up so as to form at once a flat plate of 

 tissue ; a distinct apical cell is not always to be found. 

 The development is very variable, but generally the 

 prothallus puts out filamentous branches, and forms a 

 kind of cushion with a midrib in the middle, while the 

 sides remain one cell thick. The male pro thai li often 

 have no definite growing-point. They remain, on the 

 whole, decidedly smaller than the females, and begin to 

 form antheridia very early. Sometimes antheridia are 

 formed at the ends of branches, while in other cases they 

 arise from the thickened cushion. Fig. 45 represents a 

 very large and complicated female prothallus ; the male 

 individuals are much smaller and less branched. The 

 large female prothalli possess a distinct growing-point, 

 which produces a series of lobes on the lower side of 

 the prothallus. Between these lobes the archegouia are 

 placed. 



2. THE SEXUAL ORGANS 



a. The Antheridia 



The antheridium of an Equisetum is a very simple 

 structure (see Fig. 46). It arises from a single cell, 

 which divides into two by a wall parallel to the free 

 external surface. The outer of these two cells simply 

 forms the cover ; the inner, after very numerous divisions, 

 gives rise to all the spermatozoid mother-cells. The 

 cover-cell divides up two or three times, by walls at 

 right angles to the surface, usually forming a triangular 

 cell in the middle of the cover, through which dehiscence 

 takes place. 



In each of the very numerous cells in the interior of 

 the antheridium a single spermatozoid is produced. Its 



