394 



Angles the secondary or lateral veins make with the midrib. 



This subject has been already referred to at p. 378. In their " Research, &c," 

 Messrs. Baker and Smith have unduly pressed the correlation between the angles the 

 lateral or secondary veins make with the midrib, and the constituents of the oils 

 distilled from each species. 



I have already pointed out that it is impossible to say what is the angle 

 pertaining to a particular tree (much less to a particular species), mainly because of 

 the shape of the leaf (usually tapering towards apex and base), and because one of 

 the functions of veins is the mechanical support of the leaf. 



The nearest approach to uniformity is secured by the selection of that portion 

 of the leaf where the sides are most parallel, say, the middle third. The upper parts 

 of the leaf taper too much, while, in some forms, the secondary veins cluster towards 

 the petiole, and do not approach uniformity until at some considerable distance from 

 the petiole. 



Another point is that when, in the same species, we have considerable difference 

 in the width of a leaf, the angle the secondary vein makes with the midrib changes 

 by reason of this narrowness. In other words, the secondary veins, being more 

 crowded laterally, exhibit a change of angle. This, therefore, is a secondary cause of 

 variation. 



We may know more about relationships by laboriously measuring the angles 

 of every species, taking every precaution to take representative leaves for each species. 

 I have made a beginning, only a beginning, of this work. (See also Part LVI, p. 294). 



It would be desirable in descriptions to state the approximate angle the 

 secondary veins make with the midrib. Messrs. Deane and Maiden introduced the 

 practice, and I have continued it to some extent, but the practice ought always to be 

 followed. 



As already explained, the accepted terms for types of venation are— 



1. Longitudinal (call them Longitudinales). 



2. Oblique (call them Obliquae). 



3. Transverse (call them Transversa}). 



1. LONGITUDINALES. 



(Angles 0-25 degrees.) 



It is impossible to say, in the case of almost every species, that the angle the 

 lateral veins make with the middle third of the midrib is a definite number of degrees, 

 consequently in some of them I have given two inclusive figures, which may or may 

 not be sufficiently inclusive. Doubtless two figures should be given in all cases. 



