CH. II] VARIETY OF FORM OF SHOOT 27 



{Polypodiuni) Linnaeana, C.Chr.)(Fig.32). Further, while in Onoclea Strtithio- 

 pteris the basket-form is seen, in O. sensibilis there is an elongated creeping 

 rhizome. In the former species, however, there are runners, which connect 

 its habit of shoot with that of O. sensibilis. These facts illustrate what applies 

 generally for Ferns, viz. that species which appear alike in general habit 

 may not be really akin : and that those which are akin as indicated by other 

 and more important characteristics, may differ in habit. In fact, the general 

 form of the plant is no safe guide to affinity. 



What has thus been noted for the general form of the shoot applies with 

 even greater force to the outline of the leaf For instance, the reniform blade 

 borne on a long petiole is a very marked feature of some Ferns. It is seen 

 in Phyllitis {Scolopendriiini) Delavayi, in AdianUmi 7'enifornie (Fig. 33), in 

 TricJwinanes renifornie (Fig. 74, p. 83), and in Pterozoimmi renifornie (Fig. 34). 

 But all of these Ferns, apparently so similar in their habit and particularly in 



Fig. 32. Rhizome oi Dryopteris Liniiaeana C Chr. { = Polypodiiiin 

 Dryopteris, L. ) dichotomously branched, with alternating leaves. 

 The roots are omitted. (After Velenovsky.) Reduced. 



the outline of their leaves, belong to genera widely distinct in the character of 

 their sori, as well as in their anatomy and other features. Again, the widely 

 expanded lamina characteristic of Christensenia {Kaulfussid) aesculifolia 

 appears to resemble that of Hypoderris Brownii in outline and venation ; 

 but the two are widely different when compared as regards other and more 

 essential characters. These examples are mentioned in order to illustrate 

 how similarity of some external feature, even when it is a marked one, does 

 not in itself indicate affinity. Illustrations might be indefinitely extended, 

 and might be taken not only from leaves of simple form such as those named, 

 but from Ferns with various degrees of that pinnation which is so character- 

 istic of their leaves. These will, however, suffice to show at the opening of 

 this Chapter how limited is the value to be attached to habit in the Classi- 

 fication of Ferns. This fact greatly increases the difficulty of the systematic 

 treatment of this large and varied Class. But by an exact analysis not only 

 of form, but of structure and development, many features are brought to 



