54 TYPES OF VENATION [CH. 



They may be opposite or alternate, or both mixed ; 

 many or few ; distant or crowded, &c. all characters of 

 possible value in diagnosis. 



The finer venation, as already said, I do not propose 

 to distinguish in detail ; but it is worth noting that the 

 finer meshes of the reticulation, bounded by the ter- 

 tiaries, &c., and in which the terminals end, differ 

 sufficiently in form and size to be of some diagnostic 

 value. They may be rounded-polygonal e.g. Beech and 

 Oak or oval, linear, angular e.g. Salix cinerea, &c. ; the 

 longer axis being oblique, horizontal or longitudinal in 

 various cases. The visible meshes also vary much in size, 

 though we need not refer to other than large or small ; 

 and the same refers to their distinctness the network 

 being conspicuous in Salix Caprea and Herberts, for in- 

 stance, and inconspicuous in Robiwia, &c. 



From the application of such particulars as those 

 treated above, it is found that various types and sub- 

 types of simple, pinnate and palmate venations are to be 

 observed in the leaves of our trees and shrubs. The 

 principal of these may be summarised as follows. The 

 student should note that all the broader leaves here 

 concerned have reticulated venation, but great differences 

 are observable according to the relative prominence of 

 the primaries and secondaries, &c., and the relatively 

 sudden or gradual breaking up into the network. Most 

 of the following types depend on this arid on the direction 

 of the principal ribs. 



In the simplest case a single primary rib only is visible, 

 the midrib, and secondaries are either absent or are so 

 small and buried in the tissues as to be invisible. It 

 sometimes happens that a rudimentary network really 

 exists, but so immersed in the leathery or succulent tis- 

 sues that no signs are visible externally. 



