THE PTERIDOPHYTES 291 



Other species, however, like E. sylvatioum (Fig. 211) and 

 IE. hiemale, have part or all of the stem both green and 

 fertile. 



The gametophytes of Equisetum 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, horsetails, and dub-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-MOSSES 1 



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 Lycopodineas is mainly interesting on account 

 of its bearing on the evolution of plants, no laboratory work is here given. 

 - This number is only approximate. 



