18 



STRUCTURAL BOTANY 



way as in the pollen mother-cells of Dicotyledons (see 

 Part I. Fig. 39, B, p. 110). The four daughter-cells 

 become the microspores. Each group of four is tetra- 

 hedrally arranged. The microspore acquires a cell-wall 



of its own. the outer laver of 



' t/ 



which is thickened and cuti- 

 cularised. 



The microsporangium is now 

 ripe, and the space within the 

 sporangium-wall is filled with 

 thousands of microspores (see 

 Figs. 4 and 10). 



We see that in every respect 

 the microsporangium, through- 

 out its whole development, 

 closely resembles a pollen-sac, 

 while the microspores, in their 

 structure and mode of origin, 

 precisely correspond to the 

 pollen- grains. 



We will consider the further 

 o. 10. Selaginella spinosa; , . . ,, . . 



microsporangium in radial destiny of the microspores later 



section. L, sporophyll ; lig, on an( J W JU now p asg {- t ] 1Q 

 ligule of sporophyll ; mi, 



"microspores still grouped in megasporangium. 

 tetrads inside the sporangium; Up to the time when the 



t, persistent tapetum ; a, 



place of dehiscence. Magni- spore mother - cells become 

 tied about 40 diameters, isolated from one another, the 



(it. o. ) 



development of the mega- 

 sporangium goes on in just the same way ns that of 

 the microsporangium. Now, however, a striking differ- 

 ence manifests itself. In the megasporangium, out of all 

 the numerous mother-cells, only a single one undergoes 

 division ; all the rest remain undivided and are abortive 

 (see Fig. 4). 



