20 STRUCTUEAL BOTANY 



great dimensions at the expense of the abortive mother- 

 cells. 



We cannot understand the relation of the microspores 

 and megaspores to each other, or to the reproductive 

 cells of Flowering Plants, until we are made acquainted 

 with their further history. We will therefore now go 

 on to describe the changes which take place in the spores, 

 on their germination. 



b. Germination of the Microspores 



The microsporangium, when ripe, opens by transverse 

 dehiscence, the walls splitting along a line parallel to 

 the surface of the adjoining leaf (see Fig. 10). The 

 microspores are thus set free, and if they fall onto damp 

 earth germination takes place. 



The first change that happens is that the spore 

 divides into two cells of very unequal size. A very 

 small cell is cut off on one side of the spore (see Fig. 12, 

 A, p). This little cell takes no further part in the 

 development, and may be called the prothallas-cell. The 

 large cell now divides into two equal parts (see Fig. 

 12, A)', each half undergoes several further divisions. 

 The final result is that we find the spore divided up 

 into about a dozen cells. One of these is the little 

 prothallus - cell first formed, which remains unaltered. 

 Of the remainder, eight form an external layer, within 

 which the central cells are enclosed, their number being 

 either two or four according to the species. In S. Kraus- 

 siana there are four (Fig. 12, B, c). While these divisions 

 are in progress, the spore grows a little, becomes more 

 spherical, and bursts its outer hard membrane, so that 

 its contents are now enclosed only by its inner cellulose 

 wall. The central cells next undergo several successive 



