DIV. I 



PTERIDOPHYTA 



531 



The development of the sexual generation is accomplished in the same way as 

 in Selaginella. The reduced male prothallium (Fig. 500) arises similarly within 



Fio. 4it. Isoetes lacustris. (i nat. size.) 



FIG. 500. A-F, Isoetes setacea (x 640). A, Microspore 

 seen from the side. B-D, Segmentation of the spore ; 

 p, prothallial cell ; w t the four cells of the wall ; 

 spennatogenous cells. E, The four spermatozoid 

 mother cells are surrounded by the disorganised cells 

 of the wall ; surface view. F, The same in side view. 

 G, Isoetes Mcdinverniana, spermatozoid (x 780)i 

 (After BELAJEFF.) 



the spore, by the formation of a small, lenticular, vegetative cell '(p\ and a larger 

 cell, the rudiment of a single antheridium. The larger cell divides further into 



cot 



FIG. 501. Isoetes echinospora. A, Female pro- 

 thallium ; ar, archegonium ; o, egg-cell. B, C, 

 Development of the archegonium from a super- 

 ficial cell ; h, neck-cells ; hk, neck-canal-cell ; 

 b, ventral canal-cell; o, egg -cell, (x 250. 

 After CAMPBELL.) 



FIG. 502. Isoetes echinospom. Embryo before 

 breaking out from the prothallus in longi- 

 tudinal section ; cot, cotyledon ; I, ligule ; v, 

 sheath at the base of the cotyledon in the axil 

 of which the apex of the stem arises ; w, 

 root ; /, foot. ( x 200. After CAMPBELL.)) 



four sterile peripheral cells, which completely enclose two central spermatogenous 

 cells. From each of the latter arise, in turn, two spermatozoid mother cells, four 

 in all, each of which, when liberated by the rupture of the spore wall, gives rise 



