148 



ORGANOGRAPHY. 



veins are connected, as in those of the last variety, by unbranched 

 veinlets. The Banana, the Plantain, and allied plants, furnish 

 us with examples of such leaves. 



Besides the above varieties of reticulated and parallel venation 

 as found in Dicotyledonous and Monocotyledonoiis plants, the 

 leaves of Ferns, and those of other Acotyledonous Plants which 

 possess veins, present us with a third ; thus, in these the pri- 

 mary venation may be feather-veined or radiated-veined, but 

 the whole of their principal veins either divide afterwards in a 

 forked manner {fig. 293), or their terminal ramifications are 

 thus divided. Such a variety of venation may be therefore 

 called Furcate ov forked. 



Fig. 

 292. 



Fig. 29S. 



Fig. 292. Curve-veined variety of 

 parallel venation, as seen in the 

 Banana. F'uj. 293. Forked ve- 

 nation of a Pern leaf. The margin 

 is crenated. 



The leaves of the three great classes of plants present uf, 

 therefore, with three different modes of venation : thus, those of 

 Dicotyledons are reticulated ; those of Monocotyledons, para/ZtZ ; 

 and those of Acotyledons, forked. 



Dr. McCosh has endeavoured to show that there is a general 

 correspondence between the ramification of the tree and the 

 venation of the leaf, in the fact that the angle formed by the 

 branch with the stem, is the same as the angle of venation in 

 the leaf. He states, however, that while it is comparatively easy 

 to determine the angle of venation of the leaf, it is very difficult 

 to ascertain the normal ramification of the tree, for the angle at 

 which the branch is given off is frequently modified by a great 

 number of circumstances both natural and artificial. His re- 

 ?<earches have been almost confined to reticulated leaves, and he 



