390 



RELATION TO ENVIRONMENT. 



evident that compound leaves are only extreme forms of lobed 

 or cut leaves and that the form of all bears a definite relation 

 to the primary venation. There has been a reduction of meso- 

 phyll and of the area of smaller venation. 



759. These forms of leaves probably have some definite sig- 

 nificance. It is not quite clear why they should have developed as 



they have; though it is 

 possible to explain several 

 important relations of these 

 forms to their environ- 

 ment, (i) The reduction 

 of the surface of the leaf, 

 with the retention of the 

 firmer portions, allows 

 freer movement of the air 

 and affords the leaf greater 

 protection from injury dur- 

 ing violent winds, just as 

 the finely dissected leaves 



tion of air through the' large 'leaf . of Some Water - plants 



are less liable to injury from movement of the more dense 

 medium in which they live. It is possible that here we may 

 have an explanation of one of the factors involved in this 

 reduction of leaf surface. (2) In trees with compound leaves, 

 like the hickory, walnut, locust, ailanthus, etc., the midvein, 

 and in the case of the Kentucky coffee-tree (Gymnocladus) the 

 primary lateral veins also, serve in place of terminal branches 

 of the stem. By the increase in the outline of the leaf and 

 the reduction of its surface between the larger veins, the tree 

 has attained the same leaf development that it would were the 



So leaves are palmately bifoliate, etc., pinnately bifoliate, etc. Decom- 

 pound leaves are those where they are more than twice compound, as 

 ternately decompound in the common meadow rue (Thalictrum). 



Perfoliate leaves are seen in the bellwort (Uvularia), connate perfoliate, 

 as in some of the honeysuckles where the bases of opposite leaves are joined 

 together around the stem. Equitant leaves are found in the iris, where the 

 leaves fit over one another at the base like a saddle. 



Fig. 434- 



Twice compound leaf. Leaflets arranged in 

 one plane, but open spaces 



;rmit free circula- 



