THE LEAVES OF VASCULAR PLANTS. 291 



Section X. 

 Of the Irregularity of foliaceous Organs. 



The ordinary state of leaves is symmetrical ; that is to 

 say, that, in general, the two sides of the leaf separated 

 by the middle nerve have a tendency to be equal and 

 alike. This tendency to symmetry is remarked both in 

 simple and compound leaves, and it is generally observed 

 likewise in each lobe or leaflet considered separately ; 

 but to form an exact idea of this symmetry, we must not 

 seek for a too geometrical regularity, which never exists 

 in organized beings. The two sides of leaves are con- 

 sidered as symmetrical or regular when they differ but 

 little in the same dimensions, or when they do so in 

 a variable and accidental manner. But there is a certain 

 number of plants in which the two sides of the leaves, or 

 leaflets, are constantly unequal, and they are designated 

 by the names of Inequilateral or Oblique. Thus, 

 in leaves, the two sides of those of Begonia are very 

 remarkable for their inequality; several species of 

 Greivia present analogous examples ; it is also visible 

 in Pterospermum semisagittatum, &e. This inequality 

 generally exists only in alternate leaves, and I cannot 

 find in my memoranda any example of an inequilateral 

 opposite leaf. This fact tends to prove that this ine- 

 quality ought to be referred to the position of the leaf 

 upon the plant favouring the development of one of its 

 sides more than the other ; and in this case it is always 

 the lower wdiich is developed most. 



This law is still more evident in the leaflets of pin- 

 nate leaves : when they are imequal, which frequently 

 is the case, and are met with indifferently in alternate or 



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