52 LOOPING, ETC. [OH. 



They may run as simple strands to the margin, e.g. 

 Beech, or soon branch at the ends and rapidly break up 

 into a network of fine capillaries, e.g. Pear. In many cases 

 the secondaries, or some of them, give rise on their outer 

 sides to branches but little weaker than themselves, and 

 these outer veins are very characteristic e.g. Alder, Elm, 

 Hazel, Lime : they sometimes give a forked appearance to 

 the secondaries, e.g. Wayfaring Tree (Fig. 14). 



Even more characteristic is the common case where 

 each secondary curves forwards and inwards at its upper 

 end, and joins on to the next anterior secondary, as by a 

 loop, more or less strongly convex to the margin, e.g. 

 Prunus spinosa, Honeysuckle, and since this looping of 

 the secondaries is often a conspicuous feature, and absent 

 from or obsolete in leaves with straight secondaries running 

 direct to the margin, or breaking up there into reticula- 

 tions, it serves as a character of some value. 



The loops may be strong or weak, many or few, close 

 beneath the margin or some distance in, and so on. Some- 

 times the tertiaries and veins of higher orders form series 

 of superposed loops between the loops of the secondaries 

 and the margin : in other cases the loops run so close to 

 the margin, and are so slightly convex to it, that an infra- 

 marginal vein running nearly parallel to the latter is 

 formed by their union e.g. Walnut (Fig. 17), and there 

 is a tendency to something of the same kind in Clematis, 

 Holly and Fig. 



In the vast majority of cases the secondary ribs come 

 off alternately in pinnate venation, e.g. Hornbeam, Quercus 

 Cerris, Beech, Chestnut, though here and there a few may 

 be opposite : it is rare to find them all opposite, though 

 less uncommonly the basal pairs are so. 



Characters of some value can be obtained from the 

 average distances separating pairs of secondaries, expressed 



