CH. III] FUNCTIONS OF VENATION 33 



veneration attached to long usage of the terminology 

 has to be respected, and botanists all over the world term 

 the network referred to, the venation of the leaf. 



As a rule we can distinguish in the larger leaves of 

 trees and shrubs a midrib, giving off secondary ribs, which 

 break up into smaller and smaller tributaries or subordi- 

 nate branches termed veins ; these latter usually breaking 

 up into still smaller ramifications arranged in a more or 

 less definite and complex network, the ultimate ends of 

 which lose themselves as blind ends in the soft tissues, of 

 the leaf. Hence any watery liquids brought up the 

 petiole and midribs from the stem can be distributed to 

 every part of the leaf by smaller and smaller veins, just 

 as the water system of a town distributes the liquid 

 through more and more numerous and narrower pipes, 

 from the great supply pipes to the houses; or, in the 

 reverse direction, the smaller veins gather up liquids from 

 the leaf-tissues, and discharge them through the ribs to 

 the other parts of the plant, much as the small drains of 

 the houses collect liquids and discharge them through 

 larger and larger drains into the main sewers. Only, it 

 must be noticed, first, that the liquids thus collected in 

 the leaf are not mere refuse, but are nutritive in character; 

 and, secondly, that the supporting fibres in the leaf ac- 

 company the conducting vessels, an arrangement not 

 common in engineering, unless we compare it with what 

 occurs in viaducts and similar structures, where the girders, 

 &c., would represent the fibres. 



The venation in the vast majority of leaves conforms 

 to two principal types. It is either parallel, where all the 

 principal veins run for relatively long distances in straight 

 or nearly straight and parallel lines, as in Grasses, Lilies, 

 Orchids and Monocotyledons generally; or reticulated, 

 where they exhibit an evident network made up of 



w. ii. 3 



