662 TEXTBOOK OF BOTANY 



grow on the surface of the soil or below. The life duration of any part 

 of a horizontal stem is relatively short, for the rhizome continues to 

 elongate at the growing tip and die at the other end. A few of our 

 common ferns, such as the cinnamon fern, have erect stems, sometimes 

 extending a foot or more above the soil in swamps. The upright stems 

 of tree ferns in the tropics may be 20 to 30 feet high. Some ferns are 

 epiphytes (Fig. 309). 



The ferns, in marked contrast to all bryophytes and thallophytes, have 

 highly differentiated conducting systems with both phloem and xylem 

 tissues. Special elongated cells known as tracheids are the fundamental 

 structures of the xylem. Xylem tubes, or vessels, occur rarely. The 

 phloem is composed of sieve tubes and phloem parenchyma. Cambium, 

 though common in the fossil ancestors, is very rare in living ferns, hav- 

 ing been found in only two genera. The "bundles" of xylem and phloem 

 mav be few and scattered, or sometimes numerous and arranged in a 

 cylinder. Pith is absent in stems of some species and conspicuous in that 

 of others. The cortex is usually prominent and may have several outer 

 la vers of thick-walled cells. 



The roots of ferns in comparison to leaf surface are smaller, shorter, 

 and less branched than the roots of seed plants. In the herbaceous ferns 

 they originate adventitiously at the nodes and along the internodes of 

 the rhizome. In the tree ferns the root systems are more complex, but 

 they do not attain the size and spread of the root systems of seed plants. 

 Their internal structure is quite similar to that of the roots of seed plants. 

 Roots may be lacking in some aquatic species, and in the filmy ferns 

 only rhizoids develop. 



Ferns multiply vegetatively by means of their branching rhizomes, 

 from which a succession of new leaves develops each season. New in- 

 dividuals may also develop vegetatively from adventitious buds (bul- 

 bils) on the surface or in the axils of the leaves of certain species. New 

 plants of the walking fern develop from buds formed at the tips of the 

 leaves in contact with the soil (Fig. 310). Some tropical species multiply 

 vegetativelv from the swollen leaf bases. 



The sporophyte. The conspicuous fern plants described abo\'e are all 

 sporophytes. They are much larger and far more complex than the 

 sporophvtes of the mosses and liverworts. Among the bryophytes the 

 conspicuous phase in the life history is the gametophyte; among the 

 ferns it is the sporophyte phase. The sporophyte of ferns has definite 

 leaves, stems, and roots with a well-developed vascular system and other 

 tissues comparable to those of seed plants. 



