64 ON GENERA AND SPECIES. 



This is particularly the case in the extensive tribes 

 Polypodiece, Acrostiahere, Pli&jopteridea;; and Aspidiece, each 

 of which contains groups of species possessing similar 

 venation, and agreeing so perfectly in habit and mode of 

 growth, that, in the absence of the fructification, it is often 

 difficult to determine to which genus and tribe they should 

 be referred. For instance, Phegopleris and Lastrea, Pohj- 

 betrya and Polystichuin, Ooniopleris and Nephrodium, P)rijo- 

 menes and Aspidium, respectively agree both in habit and 

 venation, and all, with the exception of Polybotrya, have 

 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 iudusium, or by the 

 fertile fronds being contracted, as in Polyholrya. 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 bo viewed as forming the groundwork of any at- 

 tempted natural classification. It must, however, be under- 

 stood that difTeroncos in venation do not always indicate 

 distinct modes of growth or habit ; for example, Pliegopteris 

 and Goniopterie, Polijlotrya and Saromanes, Pteris and Lito- 

 hroclua, Adianlum and Hewardia, Laitrea and Nephrodium, 

 Lindscea and Synaplilebium, 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 the 

 normal structure of the species fully developed, for in 

 fronds of young plants the venation is often free while in 



