CHAPTER V. 



CHARACTERS OF THE VENATION IN DETAIL. 



Ribs and veins Primary, secondary and tertiary ribs, &c. 

 Terminals Cross-ties and outer branches Angles of di- 

 vergence Midrib or primary Secondaries Looping Infra- 

 marginal vein Leaf-areas Tertiaries and veins of higher 

 order Reticulation Examples of simple venation Pinnate 

 and strict-pinnate Pinnate-looped Pinnate-arcuate Pinnate- 

 reticulate Palmate venation and its varieties. 



THE vascular bundles extending through the leaf- tissues 

 are, as already stated, termed ribs and veins, and con- 

 stitute the so-called venation. The principal ribs and 

 veins usually project on the lower surface of the leaf, 

 and are therefore easily visible there, but the finer veins 

 are often invisible or inconspicuous, except in thin leaves 

 and by transmitted light, because they are buried in the 

 soft tissues. In leathery and fleshy leaves all but the 

 strongest ribs are usually so buried and obscured. 



The distribution and arrangement of the venation is 

 by no means devoid of order, and is often very regular. 



Bearing in mind that the ribs and veins are merely 

 the upper ends of the vascular bundles of the stem, coming 

 up the petiole and thence branching in the lamina, we 

 distinguish these branches as of different orders, according 

 to their thickness and relations one to another. 



