1(52 PLANT STKUCTUKES 



sporangia, which produce spores of but one kind, hence 

 these plants are homosporous ; and as the sporangia origi- 

 nate in eusporangiate fashion, Equisetum has the homospo- 

 rous-eusporangiate combination shown by one of the Fern 

 groups. It is interesting to know, however, that some of 

 the ancient, more highly organized members of this group 

 were heterosporous, and that the present forms have 

 dioecious gametophytes (Fig. 136). 



LYCOPODIALES ( Club-mosses) 



87. General characters. This group is now represented 

 by about five hundred species, most of which belong to 

 the two genera Lycopodium and Selaginella, the latter 

 being much the larger genus. The plants have slender, 

 branching, prostrate, or erect stems completely clothed 

 with small foliage leaves, having a general moss-like 

 appearance (Fig. 137). Often the erect branches are 

 terminated by conspicuous conical or cylindrical strobili, 

 which are the " clubs " that enter into the name " Club- 

 mosses." There is also a certain kind of resemblance 

 to miniature pines, so that the name " Ground-pines " is 

 sometimes used. 



Lycopodiales were once much more abundant than now, 

 and more highly organized, forming a conspicuous part of 

 the forest vegetation of the Coal-measures. 



One of the distinguishing marks of the group is that the 

 sperm does not resemble that of the other Pteridophytes, 

 but is of the Bryophyte type (Fig. 140, F). That is, it 

 consists of a small body with two cilia, instead of a large 

 spirally coiled body with many cilia. Another distinguish- 

 ing character is that there is but a single sporangium pro- 

 duced by each sporophyll (Fig. 137). This is in marked 

 contrast with the Filicales, whose leaves bear very numer- 

 ous sporangia, and with the Equisetales, whose sporophylls 

 bear several sporangia. 



