THE LEAVES OF PLANTS. 



99 



of our ordinary trees the transverse development commences on one side whilst it is 

 absent on the other. Such is shown in the elm leaf, Fig. 153. 



Compound Leaves have already been defined to consist of several pieces connected 

 together at one extremity by the petiole, the whole of which taken together constitutes 

 the leaf. There is also another explanation of the terra, to which we shall refer pre- 

 sently. Compound leaves, then, are lobed or pinnatifid leaves, with the divisions 

 carried down to the midrib or petiole. In their first development they appear as sim- 

 ple leaves only; and in their subsequent progress may still be regarded as simple 

 leaves with extreme subdivision. This may be at once appreciated if Fig. 164 be con- 

 trasted with Fig. 174 ; or Fig. 170 with 17-5 ; or Fig. 168 with the Strawberry 



Fig. 174. Fig. 175. 



Fig. 174. A leaf of the GLUDITSIA (one of the Acacias), showing pinnate leaves becoming bipinnate, 



and clearly exhibiting the mode in which many leaves are formed from one simple leaf. 

 Fig. 175. A pedate compound leaf of the HORSB CHESTNUT (Fagiis castanea). 



leaf, Fig. 176 ; in all of which the reader cannot fail to observe that this mode of 

 division of leaves into simple and compound is purely artificial. The divisions of 



Fig. 176. Fig. 177; 



Fig. 176. A STRAWBTSRRY leaf, divided into three leaflets. 

 Fig. 177. Opposite pinnate leaves, with terminal leaflet. 

 Fig. 178. An interruptedly pinnate leaf. 



Fig. 178. 



a compound leaf are termed leaflets ; and, for the most part, each leaflet is of smaller 



