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 layer of 

 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 



FIG. 10. Selaginella spinosa ; , . . . 



microsporangium in radial destiny ot the microspores later 



on, and will now pass to the 

 megasporangium. 



Up to the time when the 

 spore mother - cells become 

 isolated from one another, the 

 development of the mega- 

 sporangium goes on in just the same way as 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. 



section. L, sporophyll ; lig, 

 ligule of sporophyll ; mi, 

 microspores still grouped in 

 tetrads inside the sporangium; 

 t, persistent tapetum ; d, 

 place of dehiscence. Magni- 

 fied about 40 diameters. 

 (R. S.) 



