528 CONSERVATIVE ORGANS. [LECT. X. 



those which have alienated leaves, as exempli- 

 fied in Mimosa verticillata (see 102, p. 527 ; a. the 

 forn^of the earlier leaves, b. those which succeed 

 them) ; for, in these, the leaves, which are alien- 

 ated or different from those which first appeared 

 on the plant, and from those peculiar to the 

 genus, are ultimately the only leaves of the plant. 



Besides those characteristics of leaves which 

 have been described, there are others common both 

 to simple and to compound leaves, whatever may 

 be their form and structure. These refer to the 

 situation of the leaves ; to their disposition on the 

 stem and branches ; to the mode of their attach- 

 ment or insertion ; and to the direction of their 

 surfaces with respect to the stem and the branches 

 as well as to the plane of the horizon. 



In point of SITUATION the majority of leaves 

 may be regarded as aerial; being suspended on 

 the stem or the branches, in such a manner that 

 the air is applied to both surfaces of every leaf. 

 The position of such leaves in relation to the hori- 

 zon, is determinate, or always the same in the 

 same description of plants, but varies in different 

 kinds of plants ; and this constitutes, in Botanical 

 language, their direction: in ascertaining which, 

 the plant must be in a healthy state, for an un- 

 healthy state of the plant and the natural decay 

 of the leaf in autumn, alter the ordinary direction 

 of this organ. The following are the determinate 



