INTRODUCTION. 31 
which a part is occupied by albumen; whilst in the orders especially 
characterized by it, there is probably not a single genus in which 
it is absent. It must be remembered that albumen exists in all 
seeds at an early period of their formation; and that the subsequent 
difference will depend upon the degree in which it is absorbed by the 
embryo. 
36. The student who has given attention to the preceding statements, 
is not unlikely to feel some perplexity, on account of the constant un- 
certainty which has been stated to attend the value of the several 
characters that have been enumerated. But as he proceeds further, he 
will find that this uncertainty is greater in appearance than in reality ; 
and that it necessarily results from the properties of a Natural group, 
as already described. In dividing the vegetable kingdom in an artificial 
method, it seems very easy to lay down a small number of characters as 
the standard ; and to bring together, or to separate plants, according to 
their conformity or variety in these. But, as has been already shown, 
when we come to apply this plan, numerous difficulties are met with, 
in consequence of the differences which are of constant occurrence, 
among plants belonging to the same genus or even to the same species 
(§ 9;) so that even here the Botanist must be guided by general resem- 
blance. Now, although it is quite true that no single characters, when 
traced throughout the vegetable scale, can be relied on, as indicating the 
natural affinities of plants, yet experienced Botanists have little difficulty 
in defining each order, by a certain combination of characters, which are 
peculiar to it, and not unfrequently, the plants belonging to one order 
may be separated from those of all other groups, by some evident and 
well-marked peculiarity. 
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DE CANDOLLE’S CLASSIFICATION. 
37. On the foregoing principles, the class of Exogens is divided by 
De Candolle in the following manner :— . 
The first group consists of those, of which the flowers possess both 
calyx and corolla, and in which the petals of the latter are distinct, and 
which are therefore Polypetalous. This group is divided into two sub- 
classes, according to the mode of insertion of the stamens. 
Sus-cuass I. Thalamiflore. Polypetalous Exogens, in which the 
stamens arise from the disk,—that is, are hypogynous. Sometimes the 
stamens adhere slightly to the sides of the ovary, but they are never 
epigynous, nor perigynous. (§ 32.) : | 
Sus-ciass Il. Calycifore. Polypetalous Exogens in which the 
stamens arise from the calyx or corolla,—that is, are perigynous. 
In the next sub-class, the flowers still possessing both calyx and 
corolla, have the latter formed of united petals, or are Monopetalous. 
