26 



STRUCTURAL BOTANY 



in other cases the lower part of the tissue is added after 

 the spores have fallen on to the ground. 



The tissue which fills the megaspore is called the 

 prothallus. The prothallus gives rise to the archegonia, 

 or female organs. A cell at the apical end of the 

 prothallus grows larger than the rest, and divides into 

 two by a wall parallel to the outer surface. The upper 

 cell divides by two longitudinal walls, crossing each 

 other at right angles into four, and each of these four 

 cells divides by a transverse wall 

 into two. Thus a neck is formed, 

 consisting of eight cells arranged 

 in two tiers (see Fig. 14, n, n). 

 (Of course only four of the cells 

 can be seen in longitudinal section.) 

 In the mean time the lower cell, 

 which has so far remained un- 

 divided, forms an outgrowth which 

 penetrates between the cells of 

 the neck. This outgrowth is cut 

 off as a distinct cell, called the 

 neck canal -cell. Another smaller 

 cell, the ventral canal -cell, is cut 

 off below it ; the remaining lower 

 portion of the original ventral cell is the ovum. We 

 see, then, that the development is just like that in Picea, 

 except that in Selaginella we find two canal- cells instead 

 of one. In Selaginella a real canal is formed, for the 

 neck canal-cell breaks down, and an open passage, con- 

 taining only mucilage, is left between the cells of the 

 neck leading down to the ovum within (see Fig. 14). 



The first archegonium is sometimes formed before 

 the megaspore is shed. After the dehiscence of the 



O 



FIG. 14. Selaginella ; 

 archegonium ready for 

 fertilisation. 0, ovum ; 

 n, cells of neck ; m, 

 mucilage in canal. Mag- 

 nified about 500 dia- 

 meters. ( After Pfeffer.) 



