50 FINER VENATION [CH. 



connections which serve as the boundaries of segmental 

 areas which are the first larger meshes of the reticulations 

 (Figs. 14 and 15c); these meshes are then divided into 

 smaller and smaller areas by further cross-connections in 

 all directions of the branches of the tertiaries, and so on, 

 the terminals standing out as blind ends in the ultimate 

 meshes. 



When, as often happens, the connecting tertiaries run 

 across the area bounded by nearly parallel secondaries, 

 and meet so as to form nearly straight ties, more or less 

 at right angles to the secondaries, we may speak of them 

 as cross- ties (Fig. 14 c). 



Tertiaries from secondaries, or secondaries from pri- 

 maries, are often stronger on the outer side of their parent 

 strand than on the inner, and are worth distinguishing as 

 outer branches, secondaries or tertiaries as the case may 

 be (Figs. 14 and 15 e). 



The relative thicknesses, lengths, distances apart, 

 angles of divergence, course and direction of the second- 

 aries and tertiaries, as well as the shapes of the areas or 

 meshes enclosed by them, often afford useful characters in 

 distinguishing leaves, but it is in very rare cases only 

 that the characters of ribs or veins of higher order can 

 be utilised. 



Confining our attention solely to such trees and shrubs 

 as are concerned in this book, the following details of 

 venation will be found instructive and useful for our 

 purposes. 



The primary rib may be strong, as is usually the case 

 with the midrib of ordinary leaves, or weak, and fre- 

 quently tapers from a strong basal to a capillary apical 

 portion. It is usually straight in course, but not always, 

 and may be undulating. In by far the majority of cases 

 it runs right up to the tip, and may be prolonged into an 



