236 



PLANT MORPHOLOGY 



into an outer and inner layer that also separate as a result of a more rapid 

 growth of the outer layer, thus forming a second fluid-filled space (Figs. 

 \9\B and 1925). The female gametophyte begins to develop by free- 

 nuclear division. Its protoplast enlarges until it comes in contact with 

 the inner spore coat, which later comes in contact with the outer coat, 

 thereby obliterating both cavities. The nuclei in the young gametophyte 



Fig. 193. Sections through the male gametophyte of Selaginella kraussiana in different 

 stages of development. A, early stage, consisting of a small prothallial cell and an antherid- 

 ial cell; B, later stage with prothallial cell and antheridium consisting of four primary 

 spermatogenous cells surrounded by eight jacket cells; C, mature gametophyte with nearly 



ripe sperms lying free inside microspore wall. {After Slagg.) 



» 



lie in a peripheral layer of cytoplasm surrounding a large central vacuole 

 (Fig. 194A). 



After the female gametophyte has undergone a series of free-nuclear 

 divisions, wall formation begins at the apical (pointed) end. At this 

 place the spore wall ruptures and the gametophytic tissue protrudes 

 slightly, developing archegonia and in some species rhizoids also, or rhi- 

 zoids and chlorophyll. The main portion of the gametophyte, lying 

 within the megaspore wall, acts as a large food reservoir. In many spe- 

 cies there is a marked differentiation between the deeper nutritive region 

 and the exposed portion, and often the former is not divided into cells 

 (Fig. 194B). The development of the archegonium is similar to that of 

 Lycopodium except that the neck is very short and no basal cell is formed. 

 There is usually only one neck canal cell. 



Although, in most species, the early development of the female game- 

 tophyte occurs while the megaspore is still within the megasporangium, 



