126 THE "CLUB-MOSS: 



ANNOTATIONS. 



In general appearance Selaginella is more like higher 

 plants than any Pteridophyte yet studied, and this general 

 resemblance is borne out in detail in most of its struc- 

 tures. The stem is not so complex as in some of the true 

 ferns, but has similar concentric vascular bundles. The 

 horizontal stem bears four rows of leaves, those of the 

 two uppermost rows being small and so arranged as 

 not to shade the larger lower ones. The lower leaves 

 are twisted on their petioles so as to expose their flat 

 surfaces to the light. 



The aerial roots serve to support the stem from which 

 they come, and also to absorb nourishment from the 

 earth. 



In the Pteridophyte groups previously studied the 

 asexual spores are all of one kind. In Pteris it was 

 seen that any spore possessed the power of producing a 

 gametophyte that could form both kinds of sex-organs 

 and gametes. It was observed in that connection that 

 a small or poorly nourished gametophyte produced 

 antheridia and no archegonia. In those ferns that 

 have only one kind of asexual spore (homosporous 

 ferns) the question of nutrition seems to determine 

 whether a gametophyte can produce both sex-organs. 

 In Selaginella, as in all higher plants, there are two 

 kinds of asexual spores, each of which produces a certain 

 kind of gametophyte. The microspore upon germina- 

 tion produces a male gametophyte, which in turn bears 

 the male sex-organ containing male gametes. The 

 megaspore produces the female gametophyte on which 



