306 ESSENTIALS OP BOTANY 



show the differentiation of the plant body into root,^ stem, 

 and leaf so familiar among seed-plants. In most ferns the 

 aerial leaves are all nearly alike, but in some genera, as 

 Onoclea (Fig. 209), certain leaves of peculiar form are set 

 aside for spore-production. In horsetails and club-mosses 

 the spore-bearing leaves are quite unlike the others, and 

 are often grouped into flower-like clusters (Figs. 211, 213). 



Fig. 220. Coleochxte, a Green Alga. 

 The disk-like thallus is one layer of cells thick. 



The fibro-vasaular bundles of ferns contain bast-fibers, sieve- 

 tubes, thick-walled wood cells (tracheids), and vessels. The 

 leaves are well provided with stomata. 



It is not necessary to go into much detail in regard 

 to the differentiation of parts and division of labor in 

 seed-plants, since these topics have already been treated 

 (Chapters lil-xvi). As every one knows, seed-plants have 



1 Rhizoids, though they perlorm the fuuctious ol roots, have not their 

 structure. 



