PTERIDOPHYTES 



91 



to the working cells. Leaves differ in the arrangement of 

 their veins, but every arrangement means an effective dis- 

 tribution of water to the working cells. There are main 

 veins (often only one) that give rise to smaller ones, and these 

 in turn to still smaller ones, until the system of veins forms 

 a complete network 

 through the green tissue 

 (Fig. 155). The system 

 may be likened to the 

 water-pipe system of a 

 house, with its main 

 pipe, which gives off 

 smaller pipes, and these 

 in turn still smaller 

 ones, until every room 

 in a large house may be 

 supplied with water. 

 / The vein system of 

 / a leaf, in addition to 

 / carrying water, inci- 

 dentally forms a stiff 

 framework with its 

 woody fibers, which 

 helps to support the 

 I delicate green tissue 

 and keep it from col- 

 lapsing. 



It is known to every 

 one that leaves are ex- 

 tremely variable in size 

 and form. For exam- 

 ple, among the Pteridophytes, the first great group of vas- 

 cular plants, they are very small in the Club-mosses (Fig. 

 71), and are often very large among the Ferns (Fig. 72). 

 Also in the Club-mosses the whole leaf is a single small blade 



FIG. 71. Branch of a club-moss (Selaginella,), 

 showing the numerous simple leaves; the 

 leaves at the tips of the branches bear spo- 

 rangia, and therefore are sporophylls, so that 

 each branch-tip in this case is a strobilus. 



