VII.] LEAVES. 75 



more seasons ; in some Pines, indeed, they persist for 

 several years. 



6. In the fully developed leaf we have already distin- 

 guished petiole and blade. The mode in which the blade 

 is folded while enclosed in the bud is spoken of as the 

 vernation of the leaf. 



The blade is divided into symmetrical halves by a 7fiidrib, 

 which, continuous with the petiole, runs from the base of 

 the blade to its apex. In the Begonias, several of which 

 are wild, and others cultivated in India for the sake of their 

 beautiful, variegated leaves, the sides of the leaf are more or 

 less oblique or unequal. 



The arrangement of the veins in the blade is made a 

 special study by botanists who concern themselves with 

 fossil plants, for the venation of leaves is almost the sole 

 character left them of importance in fossil impressions, upon 

 which to base comparisons with species still living. 



7. In describing plants, the form or outline of the leaf 

 must be noted, and an appropriate adjective term selected 

 to express it. As the forms assumed by leaves are infinitely 

 varied, it necessarily follows that numerous terms must be 

 used to denote them. The same terms apply, however, to 

 any organs with plane surfaces, whether foliage or flower- 

 leaves. The more important only we can note here. 



In the first place, compare the leaf of a Mango or Banyan 

 with one from the Rose, Litchi, or Sterculia foetida. You 

 observe that in the two former the leaf is in one piece ; in the 

 three latter the petiole bears several distinct pieces. These 

 distinct pieces are called leaflets^ and leaves which are thus 

 divided into distinct leaflets are termed compound. Leaves, 

 on the other hand, which are not divided into separate 

 leaflets are termed simple. Simple leaves are frequently 

 deeply divided, but the divisions do not extend to the base 



