10 
GENERAL DESCRIPTION. 
indusium. Its presence or absence therefore affords 
yet other means of help toward correcter classification. 
What classification itself is, how far from being 
exact to the wonderfully arranged variety of nature, 
however necessary it is to assist the memory and the 
understanding of the botanist, may be seen from the 
interpenetration and confusion of characteristics even 
in the Table of British Eerns which follows here, and 
may be gathered also from a few remarks by Dr. 
Lindley qualifying an attempt to precisely describe 
only the three great primary orders of Exogens, En- 
dogens, and Acrogens. Having spoken of the prin- 
cipal differences between the three classes as to be 
briefly expressed thus : — 
Exogens, — wood growing concentrically — leaves 
with reticulated veins — flowers with their 'parts ar- 
ranged in fours or fives — embryo , or germ, dicotyledonous 
(or two-leafed ), — 
Endogens, — wood confused — leaves with parallel 
veins — flowers with their parts in threes — germ mono- 
cotyledonous (or one-leaf ed ), — 
Acrogens, — wood sinuous — leaves fork-veined or 
altogether unbranched — no flowers — and no germ, — 
he adds : — 
“ In applying these differences to practice, it is 
necessary to attend to the following rules : — 
“ The classes are not absolutely distinguished from 
