806 ESSENTIALS OF BOTANY 
show the differentiation of the plant body into root,! stem, 
and leaf so familiar among seed-plants. In most ferns the 
aérial 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). 
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Fic. 220. Coleochete, a Green Alga. 
The disk-like thallus is one layer of cells thick. 
The fibro-vascular 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 111-xv1). As every one knows, seed-plants have 
1 Rhizoids, though they perform the functions of roots, have not their 
structure. 
