64 ON GENERA AND SPECIES. 
"This is particularly the case in the extensive tribes - 
Polypodiee, Acrostichee, Phegopteridee, and Aspidiew, each | 
of which contains groups of species possessing similar - 
venation, and agreeing so perfectly in habit and mode of 4 
growth, that, in the absence of the fructification, it is often E 
difficult to determine to which genus and tribe they should E 
be referred. For instance, Phegopteris and Lastrea, Poly- 4 
betrya and Polystichum, Goniopteris and Nephrodium, Dryo- — 
menes and Aspidium, respectively agree both in habit and — 
venation, and all, with the exception of Polybotrya, have a 
punctiform sori, and, as already stated, these and kindred — 
genera can only be known from one another by the - 
presence or normal absence of the indusium, or by the 
fertile fronds being contracted, as in Polybotrya. There- 
fore, taking all these points into consideration, it appears 
to me that, in order to arrive at the true affinity of Ferns, 
the vascular structure, in conjunction with mode of growth, 
must be viewed as forming the groundwork of any at- 
tempted natural classification, It must, however, be under- 
stood that differences in venation do not always indicate 
distinct modes of growth or habit ; for example, Phegopteris 
. and Goniopteris, Polybotrya and Saromanes, Pteris and Lito- 
 brochia, Adiantum and Hewardia, Lastrea and Nephrodium, 
Lindswa and Synaphlebium, respectively contain species. 
perfectly analogous in habit and other characters, and only 
generically distinguished by the venation being free, or 
anastomose. Another circumstance to be considered is 
whether free or anastomose venation is always constant in 
the same species. In any uncertainty upon this point it is 
necessary to be first assured that the specimens under 
examination have arrived at sufficient age to have th 
normal structure of the species fully developed, for 
fronds of young plants the venation is often free while 
D 
