PRINCIPLES OF CLASSIFICATION 123 
more important it is for classification.” Such a part will be 
less liable to rapid variation and modification. The vegeta- 
tive organs, therefore, are comparatively useless in diagnosing 
the larger groups. There are, however, vegetative characters 
which are not so liable to modification and are therefore 
useful ; such are the alternate or opposite arrangement of 
the leaves, their veining, &c. The embryonic characters of 
the vegetative organs, e.g. the number of cotyledons, are 
very important. Vestigial organs are often of great import- 
ance in classification. The characters of flower and fruit 
are the most important for diagnosis of large groups. It is 
necessary to recognise, however, the kind of floral character 
to use. We have seen (p. 70) the importance of cohesion of 
parts. Once acquired, it is not likely to be lost and may be 
handed down to a large number of descendants ; the amount 
of cohesion, on the other hand, is a very variable character. 
In the same way, the existence or non-existence of adhesion, 
the shape of the receptacle, irregularity, number of members 
in a whorl, introrse or extrorse opening of anthers, &c. are 
all important. The characters of the gynceceum are very 
useful, e.g. the placentation, the position of the raphe of the 
ovule, the anatropy, &c. of the ovules, and so on. The 
number of cotyledons, the shape of the embryo, and the 
presence or absence of endosperm in the seed, are also good 
characters. All these are largely used for the diagnosis of 
the larger groups, as will be seen by a study of the tables 
below and of the characters of the orders in Part II. The 
characters of vegetative organs and the more easily modified 
floral characters are chiefly used in the diagnosis of genera 
and species. 
A very important point to recognise is this, that a given 
character may be good in one group of plants, and in 
another may be nearly useless and exhibit a great deal of 
variation ; this is easily understood on the theory of descent. 
For example, whether a flower is perigynous, epigynous or 
hypogynous is usually an important character, used in 
diagnosing natural orders, but in the genus Saxifraga all 
three conditions may be found. Other illustrations are 
Nigella in Ranunculaceae with its syncarpous gynceceum, 
Vaccinium in Ericaceae, and so one. 
