134 



MORPHOLOGY 



extensive that usually only one mother cell functions, all the others 

 contributing to its nutrition (fig. 302). This results in a relative/ y very 



300 



FIGS. 298-300. Sporangium of Selaginetta: 298, section through tip of strobilus, 

 showing young sporangium (two shaded cells) on stem, and below it a young sporo- 

 phyll; 299, further development of sporangium; the superficial cell of the sporophyll 

 containing a nucleus is to give rise to the ligule ; 300, sporangium in the mother cell 

 stage. 298 and 299, after Miss LYON. 



large mother cell and a tetrad of four very large spores. In some 

 cases, although a tetrad of spores is started, two or three of them may 

 not develop further, resulting in a sporangium containing only one or 



two spores. It is this 

 condition of dissimilar 

 spores that is called 

 heterospory, in con- 

 trast with the condi- 

 tion of similar spores 

 (as in Lycopodium), 

 which is called homo- 

 spory. Selaginella, 

 therefore, is hetero- 

 s porous, while Lycopo- 

 dium is homosporous. 



FIGS. 301, 302. Sporangia of Selaginetta: 301, micro- The terminology 



iporangium, showing all the mother cells forming tetrads; a ppli e( J to the hetero- 



302, megasporangium, showing one functioning mother cell ... 



(shaded), the other mother cells acting as nutritive cells. sporous Condition IS 



Alter Miss LYON. simple, The small 



