498 A TEXTBOOK OF BOTANY [Ch. XII 



and above them, as will presently appear, the gametophyte 

 is dependent upon the sporophyte. In this respect the 

 Ferns are a pivotal group. 



While the life cycle here described is usual in the Ferns, 

 various deviations occur. Thus sometimes a sporophyte 

 buds out from the tissue of the prothallus, fertilization being 

 omitted, a condition called apogamy ; and prothallia some- 

 times bud out from the leaves of the sporophyte, producing 

 apospory. Also, in some kinds little bulblets appear on the 

 fronds, and develop young plants, even (in Asplenium bul- 

 biferum), while still in place. Commonly the sori are 

 spaced apart in regular order on the under sides of the fronds, 

 but in some species they are confined to segments which 

 are wholly devoted to their formation. In others they 

 are confined to fertile fronds, likewise wholly devoted to 

 them, the ordinary fronds being sterile, as in the common 

 Sensitive Fern (Onoclea sensibilis). The sori themselves are 

 very diversely shaped, — round, kidney-shaped, elongated, 

 and even in lines under the overturned edge of the leaf, as 

 in the Maidenhair and the common Brake. The fronds 

 are in some kinds entire, in others once compound, in others 

 several times compounded, often in forms of great grace, 

 as our Maidenhair and Tree Ferns especially illustrate. 

 It is, indeed, in the form of their fronds and the arrangement 

 of their sori that the true Ferns find that play of varia- 

 tion which in some form or other is exhibited by all highly 

 specialized groups of plants. 



While most Ferns are terrestrial some are epiphytic, as 

 Platy cerium (page 186), and Filmy Ferns (Hymenophyl- 

 lacece), remarkable for the delicacy of their very thin 

 leaves. There are climbing (twining) Ferns and a Walking 

 Fern, which advances by rooting at the tip of its elongated 

 leaves, on the principle of the traveling stems already de- 

 scribed (page 188). Most striking of all are the Tree Ferns 

 (Fig. 31), which grow to forty feet in height, with great 

 crowns of frondose foliage, and rough trunks clothed with 



