V] TERTIARIES 53 



when necessary in terms of fractions of the length of the 

 midrib. For instance, they are distant in Quercus Robur 

 and close in Oleander. The average distance apart, in 

 fractions of the length of the midrib, is about ^ in Birch, 

 y 1 ^ in Hornbeam, and ^ in Oleander. 



It is sometimes useful to characterise the shape of 

 the leaf-area cut out by two successive secondaries, and 

 bounded by them above and below, and by the midrib 

 and margin (or loop, &c.) respectively to right and left : 

 such an area is of course the first or principal mesh of the 

 reticulation. It is approximately rectangular and straight 

 in the Beech, Alder, Chestnut, &c. ; curved in Salix Caprea, 

 and so forth. 



The tertiaries are the veins of distinctly finer calibre 

 given off from both sides of the secondaries, and sometimes 

 from primaries also, and which at once divide up the larger 

 meshes into smaller reticulations : they are to be dis- 

 tinguished from the stronger branches of the secondaries 

 already referred to, though it is not here worth while to 

 attempt to distinguish them from the finer capillaries to 

 which they give rise, and which end in the terminals. 



The rule is that the tertiaries leave the secondaries at 

 more acute angles on the outer than on the inner sides, 

 but there are exceptions, especially in exotic plants. The 

 tertiaries may be strong or weak in all degrees; long 

 e.g. Alder. Birch, or short e.g. Elm, but they are almost 

 always much shorter than the secondaries e.g. Horn- 

 beam. Though sometimes nearly equal e.g. Elm, Vibur- 

 num Lantana they usually vary much in length (Fig. 14). 



Their course is usually curved e.g. Elm, Birch, Beech, 

 &c. but not necessarily so, and in many leaves they run 

 nearly straight and parallel across from one secondary to 

 another, forming cross-ties e.g. Viburnum Lantana a 

 very characteristic form (Fig. 14). 



