12 INTRODUCTION. 
different genera, when they agree in what have been termed the essential 
characters. Thus, for instance, the general outline of the leaf has been 
stated to be often subject to great variety, in accordance with the degree 
in which the space between the veins is filled up with fleshy paren- 
chyma; and in most cases, a difference in the outline of the leaves of 
two plants, the distribution of the veins remaining the same, would not 
alone serve to cause two plants exhibiting it to rank even as distinct 
species. But any considerable alteration of the veining would be held 
sufficient for such a separation; though the two plants, if agreeing in 
the structure of their organs of fructification, would still be placed in 
the same genus. On the other hand, a marked and constant difference 
in the organs of fructification would be rightly held sufficient to place the 
two species in different genera, even though the form and veining of 
the leaves might be precisely the same. On‘the relative value of the 
characters furnished by the different organs, more will hereafter be 
stated. 
FORMATION OF ORDERS AND CLASSES. 
6. Even when thus grouped together into genera, however, the num- 
ber of objects, which the Botanist has to study, remains by far too great 
for convenience; and he next forms his genera into orders, and combines 
these orders into classes, according to their respective correspondence 
and difference in certain characters of a still more general nature. Now 
in this process he may follow two very different plans; and upon these are 
founded the two systems of classification which are nowinvogue. One of 
these is termed the Linnean system, after its founder; or the Artificial 
system, from its character; the other is termed the Natural system. Inthe 
Linnean system, a small number of characters—chiefly the number of 
stamens and pistils—is taken as the standard ; and the whole vegetable 
kingdom is distributed under classes and orders, according to the corres- 
pondences and differences among the several genera in these respects,— 
no regard whatever being had to any other characters. In the Natural 
system, ai/ the characters of the genera are studied; and those are united 
into orders, which present the greatest correspondence in the characters 
that are regarded as of the most importance: on the same ptinciple, the , 
orders are united into classes. If the former plan be followed, genera 
most widely differing in their structure and physiological characters are 
often brought together, and others which are nearly allied are frequently 
separated to a great extent, so that in fact, it js very common to find, 
that nothing can be stated as true of all the plants included in a Linnean 
order, except that they have a similar number of stamens and pistils, 
On the other hand, in the Natural system, the number of characters in 
which there is a general agreement among all the plants of a particular 
