TYPES OF CRYPTOGAMS; PTERIDOPHYTES 247 
FERNS 
317. Structure, Form, and Habits of Ferns. — The struc- 
ture of ferns is much more complex than that of any of 
the groups of cryptogamous plants discussed in the earlier 
portions of the present chapter. They are possessed of 
well-detined fibro-vascular bundles; they form a variety of 
parenchymatous cells; the leaves have a distinct epidermis 
and are provided with stomata. 
Great differences in size, form, and habit of growth are 
found among the various genera of ferns. The tree ferns 
of South America and of many of the islands of the Pacitic 
Ocean sometimes rise to a height of forty feet, while the 
most minute species of temperate and colder climates are 
not as large as the largest mosses. Some species climb 
freely, but most kinds are non-climbing plants of moderate 
size, with well-developed rootstocks, which are often, as 
in the case of the bracken-fern, or brake and in Osmuida, 
very large in proportion to the parts of the plant visible 
above ground. 
318. Economic Value of Ferns. — Ferns of living species 
have little economic value, but are of great interest, even 
to non-botanical people, from the beauty of their folage. 
During that vast portion of early time known to geolo- 
gists as the Carboniferous Age, the earth’s surface in many 
parts must have been clothed with a growth of ferns more 
dense than is now found anywhere. These ferns, with 
other flowerless herbs and tree-like plants, produced the 
vegetable matter out of which all the principal coal beds 
of the earth have been formed. 
1 Pteris aquilina. 
