1 54 ORGAXOGRAPIIT 



Fig. 296.' 



Fig. 297. 



Fig.l^t. Leaf showin? the varietj' of parallel venation called straight- 

 veined Fig. 29'. Straight- veined variety of parallel venation, as 



seen in the leaf of the Fan Paim {Chamvbrops). 



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

 whole of their principal veins either divide in a forked manner 

 {fig. 299), or their terminal ramifications are thus divided. 

 Such a vai'iety of venation may be therefore called Furcate or 

 forked. 



The leaves of the three ^rreat classes of plants present us, 

 therefore, with three different modes of venation ; 

 Fig 298. thus, those of Dicotyledons are reticulated; those 

 of Monocotyledons, /?ara//e/; and those of Acotyle- 

 dons,/*?/^. 



p- 299 Dr. M'Cosh has 



^ * endeavoured to 



show that there is 

 a general corre- 

 spondence between 

 tlie ramification of 

 the tree and tlie 

 venation of the leaf, 

 in the fact that the 

 angle formed by the 

 l)ranch with the 

 stem, is the same 

 as the angle of ve- 

 nation in the leaf, 

 lie states however 

 that while it is 

 comimrntively easy 

 to determine the 

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



Fig. 20S. Curve-veined variety of pa- 

 riillel venation, aa seen in the 

 IJanona Fig- 2:)9. Forked ve- 

 nation of a Fern leaf. The margin 

 18 crenated. 



