THE CLUB-MOSSES 163 



organs moist in the bud. The ligule is con- 

 stantly present on all leaves, whether vegeta- 

 tive or fertile, both in Selaginella and Isoetes, 

 and extends to a large proportion of their Palaeo- 

 zoic allies. It is thus an important character 

 and serves at once to distinguish the Selaginel- 

 lacese and related fossil groups from the homo- 

 sporous Lycopodiaceaa. 



In Selaginella, as in all other Lycopods, there 

 is one sporangium to each fertile leaf; in Selagi- 

 nella, however, the sporangium is axillary, i. e. 

 seated in the angle between the fertile leaf and 

 the stem, instead of on the leaf itself a dis- 

 tinction, however, of comparatively small 

 importance. 



The two kinds of sporangia are commonly 

 formed in the same cone, those producing the 

 small spores in the upper part of the cone and 

 those producing the large spores in the lower 

 part. It appears that the megasporangia are 

 produced on the lower side of the cone, what- 

 ever its position. Up to a certain point the two 

 kinds of sporangia develop alike, forming a large 

 number of spore mother-cells. If the small 

 spores are to be produced, each of the mother- 

 cells divides into four, and a great output of 

 microspores is the result. But in order to form 

 the large spores, only one of the mother-cells 

 divides, so that only four megaspores result; 



