THE PTERIDOPHYTES 291 
Other species, however, like H. sylvaticwm (Fig. 211) and 
E. hiemale, have part or all of the stem both green and 
fertile. 
The gametophytes of Hquisetum are produced from the 
germinating spores very much as are those of ferns. The 
prothallia are always of one sex only, and the female ones 
are of a very irregular shape, having many lobes (Fig. 212). 
From the manner in which the spores cling together by 
means of their elaters, male and female prothallia are likely 
to grow side by side and thus insure fertilization. 
While the ferns, horsetatls, and club-mosses are essen- 
tially land plants, they all show their aquatic ancestry (as 
do the bryophytes) by the possession of ciliated sperms. 
That period of their life history which is concerned with 
sexual reproduction is distinctly aquatic. The presence 
of water is absolutely necessary in order that the sperms 
or male gametes may swim to and reach the archegonia 
and thus bring about the fertilization of the egg. 
THE CLUB-MOSSES1 
372. Occurrence. — The best-known and most conspicu- 
ous club-mosses belong to the two genera Lycopodium and 
Selaginella. The former is well represented in the woods 
of temperate climates and is well known throughout much 
of the United States as “Christmas evergreen,” largely 
used in holiday decorations. Selaginella occurs to some 
extent in temperate climates, but most of the five hundred 
species? are tropical. 
1 As the detailed study of the Lycopodinez is mainly interesting on account 
of its bearing on the evolution of plants, no laboratory work is here given. 
2 This number is only approximate. 
