THE LEAVES OF VASCULAR PLANTS. 263 



culations, so that each of these portions can be detached, 

 at least when mature. The name of Simple is given to 

 all those the parts of which are continuous, whether they 

 are entire, or divided in the highest degree ; and in con- 

 tradistinction, all those are called Compound, which 

 present joints which can be separated at some period of 

 their existence. 



The distinction between compound leaves and those 

 with separate segments, is often difficult in practice, 

 especially when they are young. The names of simple 

 and compound, although convenient in practice, are not 

 perhaps correct in reality ; for one might say, with as 

 much reason, that the leaves called simple are formed 

 by the constant uniting together of leaflets into a single 

 limb. The example of the leaves of Gleditsia, and 

 others, which often have the leaflets united, might 

 authorize this manner of considering it. 



In leaves said to be compound, the general petiole 

 wliich bears all the joints receives the name of the Com- 

 mon Petiole, and each joint that of Leaflet, when it 

 has the nature of a foliaceous limb. When the leaflets 

 are themselves provided with a proper petiole, this 

 receives the name of Petiolule ; and if upon the com- 

 mon petiole others are found which are furnished with 

 an articulation at the base, and bear leaflets, the name 

 of Partial Petioles is given. 



Let us observe, at the commencement, that compound 

 leaves are only found in the angulinerved class, or, what 

 is nearly the same thing, among Dicotyledons. All the 

 leaves of the other classes are simple, even when they 

 resemble in their divisions compound ones, — as, for 

 example, in the Ferns. 



In order to give a just idea of Compound Leaves, we 

 must return to what has been said above of the distri- 

 bution of the nerves, and the formation of the lobes. 



