76 The Natural System of Botany. 



no plant can belong to either of these orders, which has alter- 

 nate or verticillate leaves. In Urticacese, the Nettle tribe, in 

 Umbelliferae, to which belong such plants as the Parsnip and 

 Carrot, and in some others, they are as uniformly alternate. 

 In many other orders, however, both these arrangements may 

 be found ; as in Composita?, which includes the Aster and the 

 Daisy, and which generally has opposite leaves, are some 

 plants whose leaves are alternate ; and in Leguminosae, the Pea ' 

 tribe, which commonly has alternate leaves, they are sometimes 

 opposite. Some orders have only compound leaves, some 

 only simple leaves, and some both simple and compound. 

 The degree in which the leaf is divided, is not a certain 

 character, since in many orders the leaves are both divided 

 and entire ; but in others, this is a sure mark of distinction. 

 Leaves with a toothed or serrated margin are never found in 

 Cinchonaceoe, the Coffee tribe, and are very rare among 

 endogenous plants. The characters afforded by the veining 

 of leaves are of great value, and will probably be of more 

 assistance in determining orders, when more knowledge of 

 them is obtained. All plants of the order Melastomaceoe, have 

 about four strong ribs extending from the base to the apex of 

 the leaf, and connected by transverse bars, very like some 

 Endogenoe. Myrtaceae, the Myrtle tribe, all have a delicate 

 vein running all around the leaf, just inside the margin ; and in 

 Cupuliferae, the Oak tribe, veins extend directly outwards from 

 the midrib to the margin. Another curious character is afforded 

 by the presence or absence of those little dots, which are so 

 apparent in the leaves of some species of Mint, and which 

 secrete a peculiar oil. All Aurantiaceae, the Orange tribe, 

 possess these, and are thereby distinguished from allied orders. 

 The nature of the juice of the leaves or stem is of great use in 

 distinguishing particular orders. In all the Asclepias tribe 

 the juice is white, thick, and milky ; and in all the Ranuncu- 

 lus tribe it is thin, clear, and acrid. The little imperfect 

 leaves, called stipules, often found at the base of the true 

 leaves, sometimes are a remarkable indication of affinity, and 

 their presence or absence is frequently sufficient to distinguish 

 the order to which a plant belongs. They are always present 

 in Salicineae, the Willow tribe, and always absent in GutU- 



