THE LEAVES. 



[CHAP. 



halves by a midrib, which, continuous with the petiole, 

 runs from the base of the blade to its apex. In the 

 Begonias, some of which are grown in plant-houses for 

 the sake of their variegated leaves, and, less obviously, 

 in the Common Lime or Linden-tree and Elm, 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, besides their form., 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 infi- 

 nitely varied, it necessarily follows that numerous terms 



FIG. 40. Simple leaf of Apple. FIG. 41. Compound (pinnate) 



leaf of Rose. 



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. 



