INTRODUCTION. 27 
happens that it is of much greater value in some orders than it is in others. 
Thus in Lamiacee (Dead-nettles) they are uniformly opposite: so that 
no plant can belong to the order, in which they are alternate or verti- 
cillate. In Urticacee (the Nettle tribe,) on the other hand, they are 
constantly alternate ; so that no opposite Jeaved plant can belong to the 
order. In this manner the common Dead-Nettles and Stinging-Nettles 
may -be at once known from each other. But in many others, one 
arrangement is prevalent, and yet the other sometimes occurs. The 
degree of division of the leaves, again is subject to considerable uncer- 
tainty in many orders, from causes already mentioned; yet, in others 
notwithstanding a constant form is maintained; thus, leaves with 
teeth or jagged edges are never found in the pa Cinchonacee. (from 
which the Bovgvian bark is supplied) and they are very rare in Endogens. 
The particular characters afforded by the veining of leaves are much 
more constant, than those derived from their form; and it is probable 
that, as they have only been recently attended to, much assistance will 
be obtained in classification from an increased knowledge of them. 
A character which would not at first sight appear of much importance, 
is afforded by the presence or absence of those little dots in the leaves, 
which are reservoirs of oily secretions; yet these being connected as it 
would seem with some important differences in the general economy, 
are extremely characteristic of certain Natural orders, such as Myrtacee 
(the Myrtle tribe,) and Awrantiacea (the Orange tribe,) serving to dis- 
tinguish all their members from those of other orders nearly allied to 
them. In other orders, however, there are some genera with, and 
others without these pellucid dots. The cleat or milky character of 
the juices of the leaves and. stalks, indicating as it does, the absence or 
presence of certain secretions which are characteristic of particular 
orders, will often prove of much use in distinguishing their members. 
At the base of the leaf-stalks are often found little leafy appendages 
(which are in fact leaves, in an imperfect state of development) termed 
stipules; the presence or absence of these frequently enables the 
Botanist to distinguish the plants of two allied orders, of which one pos- 
sesses them, whilst the other does not, and certain peculiarities in them, 
are occasionally very characteristic of particular groups. 
31. Passing on to the flowers, we first have to notice the characters 
afforded by the bracts; these are seldom of any use in distinguishing 
‘orders, on account of their constant variation within the limits of each; 
but they are often valuable in separating genera and species. The 
calyx is used in a variety of ways to distinguish orders, but the charac- _ 
ters it affords are far from being of equal or uniform importance 
throughout. The number of sepals is sometimes a very useful and — 
constant mark of a particular order; thus, in Crucifera, the Cabbage 
