174 



Practical Plant Biology. 



which become the wall of the capsule. The central globular mass 

 now forms a layer of cells lying against the inside of the wall. 

 This layer is called the tapetum inasmuch as by the products of 

 its deliquescence the spores are nourished. In the microsporangium 

 each of the sporogenous cells becomes rounded and by two suc- 

 cessive divisions gives \rise 'to four, spores. The remains of the 

 mother-cells and of the tapetum are -used upiin the development 



FIG. 47. Selaginella, base of cone, longitudinal section, x 80. a, micro- 

 sporangium; b, megasporangium ; c, sporophyll ; d, axis of cone ; e, 

 ligule ; /, wall of sporangium ; g, microspores ; h, megaspores. (After 

 Chamberlain.) 



of these spores and in the formation of their thick and warty coat. 

 The development of the megasporangia is quite similar to that of 

 the microsporangia except that only one mother-cell divides and 

 gives rise to spores, while the others and the tapetum are absorbed 

 by the growth of these four which attain a much greater size than 

 the microspores ; the microspores being about 0*025 mm. and the 

 megaspores about 0-55 mm. in diameter. 



The germination of the spores may begin before they are shed 



