and this I believe can be done by studying the plants 

 with respect to their essential differences. Let us. there- 

 fore, glance at some of the characters upon which fern 

 species are founded and note their values. 



The least varying characters of ferns — what may be 

 called their essential characters — are found in the vena- 

 tion, the rootstock. the fruit-dots, the indusium and the 

 outline of the frond about in the order named. We may 

 now examine these characters separately. 



Venation. This, the most constant feature of ferns 

 because it concerns the very framework of the species, 

 has been rightly selected by Prof. Underwood as of the 

 first importance in distinguishing ferns. He has applied 

 it only to the separation of genera, but it may well be 

 carried further, since if important in distinguishing 

 genera, it should be still more important in distinguish- 

 ing species. This we find to be true, for by this means 

 alone one can readily distinguish Nephrodium patens 

 from N. molle, or A*. Thelypteris from N. simulate!. 

 While the type of venation in each species is the same, 

 we may expect a few minor variations, even in the same 

 frond, especially in the case of species with anastomosing 

 veins. Two adjoining veins may not always be connected 

 at exactly the same place, nor if free veins are included 

 in the areoeae. are they always of the same number. 



Rootstock. The rootstock is practically unvarying in 

 each species, and the articulation or lack of articulation 

 between stipe and rootstock are also constant. So well 

 known is the constant character of the rootstock that it 

 has even been suggested that our lady-fern be made a 

 variety of the true Afkyrium filLv-famina, because its 

 rootstocks exhibit slight differences. By the rootstock, 

 too. the closely resembling Nephrodium molle and N. 

 patens may be separated. 



Fruit-dots. The position of the fruit-dots upon the 

 veins is one of great importance, and from it safe and 

 reliable characters may be drawn. From this it follows 

 that the position of the fruit-dots upon the frond is also 

 constant. The single feature of the position of the fruit- 



