514 
PHYTOGRAPHY. 
BOOK IV. 
CHAPTER II. 
OF DESCRIPTIONS. 
We have seen that the principal characters of a plant can be 
comprehended in the essential and differential characters. 
But, as these contain only such peculiarities as are supposed 
to be most essential, a great number of circumstances are 
omitted from them which, in the view of the botanist drawing 
them up, may appear unessential, but which to another may 
seem of the first importance. On this account, a plant cannot 
be considered completely known until a full description of 
every part shall have been obtained. In this description 
every circumstance connected with the external or internal 
organisation should be included, and a full statement made 
of all the peculiarities of every part, however obscure or diffi- 
cult to observe. It is upon descriptions of this kind that 
systematic botany is based. Essential and differential cha- 
racters are only relative to the degree of knowledge of the 
person who prepares them : a description is independent of 
all relative knowledge; it exhibits a plant as it actually is, 
without reference to its resemblances or differences. The 
former are adapted to the state of knowledge of a particular 
era ; the latter, if complete, to that of all eras. 
Notwithstanding their importance, descriptions of this kind 
are very rare : they occupy too much space in books to be 
inserted conveniently ; they are difficult to draw^ up ; and it 
seldom happens that an observer has the means of describing 
every part of a plant : the root, or the fruit, or the flower, or 
some other part, is probably not to be procured ; and this 
renders a description, even in the best hands, necessarily 
imperfect. 
In drawing up a description, care must be taken that every 
term is used in its strict sense ; that all is perspicuous and free 
from ambiguity ; and that the different parts are described in 
