[Chap. XLIX FERNS, CLUB MOSSES AND EQUISETUMS 659 



reported from the frigid zone; but the number of species there, as well 

 as of individuals, is small. As one goes from cool temperate regions 

 toward the equator the number of species increases rapidly. Ferns attain 

 their greatest size and variety in the moist tropics, where some of the 

 species are trees. In warm temperate regions the largest and most striking 

 ferns occur in swamps and in rich soil along streams. Most species of 

 ferns grow in acid soils, but many of them grow on both acid and alka- 

 line soils. 



Geologically ferns are very old. Fossil remains of ferns are reported 

 from the Devonian rocks, but their greatest abundance occurred during 

 Carboniferous times. Some of these early ferns were trees, and some of 

 them reproduced by seeds (Figs. 356A and 360). 



Aside from their use as decorative plants, the commercial value of 

 ferns is almost negligible. Certain species that grow in masses and in- 

 vade bare soil areas contribute to soil development and the protection 

 of soil from wind and water erosion. The young rhizomes of a few spe- 

 cies are sometimes used as a source of food by animals and man; and 

 the leaves have been used in the making of tea and liquors. The aquatic 

 species are eaten by ducks, some fish, and by other water animals. The 

 greatest common interest in ferns, however, is undoubtedly esthetic. 

 They are cultivated for ornamental purposes, and the supply and sale 

 of cut ferns have become an important industry. 



The fern plant. Ferns are generally recognized by their leaves. Thev 

 are the most conspicuous and characteristic parts of the plant and bear 

 characteristic reproductive structures. They vary in length from an inch, 

 or less, in the smallest species to 100 feet in species of climbing ferns. 

 The spirally twisting petiole ( rachis ) of the climbing fern of the Eastern 

 States seldom exceeds 3 feet in length. Some of the common ferns of 

 temperate zones have a blade a yard or more in length supported by a 

 prominent petiole often mistaken for a stem by the novice. Sessile leaves 

 are rare among ferns. 



The leaves may be simple, but they are more often compound. The 

 leaflets of compound leaves may be further divided and dissected, re- 

 sulting in a very complex frond. ^ The leaf develops from a primordium 

 near the apex of the stem and, owing to slightly more rapid enlargements 

 on the outer side, becomes coiled. As development proceeds, growth on 

 the inner side catches up with the outer, and the apex uncoils ( Fig. 308 ) . 



^ Some writers use an entirely different set of terms to describe the parts of a fern 

 plant: the leaves are called "fronds," the leaflets "pinnae," and the divisions of the 

 leaflets "pinnules." 



