144 EVOLUTION OF PLANTS 
laces, or smaller club-mosses, is closely related to the 
Lycopodiacez, 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, Psilotacee, 
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. 88, G, sus), 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. 
