144 EVOLUTION OF PLANTS 



lacese, or smaller club-mosses, is closely related to the 

 Lycopodiacese, and includes a single genus Selaginella, 

 with several hundred species, mostly tropical (these are 

 common in greenhouses, where they are usually mis- 

 named "Lycopodium "). The third order, Psilotacese, 

 includes two peculiar tropical genera, Psilotum and 

 Tmesipteris, evidently closely related genera, but doubt- 

 fully associated with the other Lycopods, and possibly 

 more nearly allied to certain extinct Pteridophytes. 



The gametophyte is at present known only in Lyco- 

 podium and Selaginella, and until its character in the 

 other genera is known, it will be impossible to assign 

 them their proper place in the system. In Lycopodium 

 the gametophyte varies greatly in different species, in 

 some being a green lobed thallus somewhat like the 

 gametophyte in Equisetum, while in others it is desti- 

 tute of chlorophyll, at least in its older stages, and is 

 apparently truly saprophytic in its habits. The earliest 

 stages of these colorless gametophytes are not known, 

 and it is possible that they may at first possess chloro- 

 phyll. The sexual organs are much like those of Equi- 

 setum and the eusporangiate ferns, but the spermatozoids 

 have only two cilia, as in the Bryophytes. 



The embryo in the club-mosses differs from that of the 

 other Pteridophytes in being derived from one only of 

 the two cells resulting from the first transverse division 

 of the egg-cell. The other cell forms a structure known 

 as the suspensor (Fig. 38, G, MS), which is much like the 

 similar organ found in the embryo of most flowering 

 plants. The embryo in Lycopodium remains for a long 

 time dependent upon the gametophyte, and may develop 

 several leaves before the first root is formed. 



