Trees; a handbook of forest-botany for the woodlands and the laboratory . Fig. 17. Leaflet of Walnut, Juglans regia, showing pinnate-loopedvenation. The secondary ribs b leave the midrib a in pinnate order,but curve forwards (near d and e) and loop with the next in front. Inthis case the looping is so near the margin that an infra-marginal veinmay result, c tertiaries, forming cross-ties and breaking up into thenetwork (Ett). 58 PINNATE-ARCUATE VENATION [CH. aries, which do not end in the margin, but take a sinuousor curved, or more rarely straight, course, and then bendforwards at their tips
Trees; a handbook of forest-botany for the woodlands and the laboratory . Fig. 17. Leaflet of Walnut, Juglans regia, showing pinnate-loopedvenation. The secondary ribs b leave the midrib a in pinnate order,but curve forwards (near d and e) and loop with the next in front. Inthis case the looping is so near the margin that an infra-marginal veinmay result, c tertiaries, forming cross-ties and breaking up into thenetwork (Ett). 58 PINNATE-ARCUATE VENATION [CH. aries, which do not end in the margin, but take a sinuousor curved, or more rarely straight, course, and then bendforwards at their tips and join in a loop, convex to themargin, with the next upward secondary (Fig. 17). Wemay term this pinnate-looped venation. The following afford examples of pinnate-looped vena-tion :— WalnutRobiniaSpindle TreeAppleHoneysuckle. Travellers Joy Ailanthus Rose Sweet-briar Lilac Holly Blackthorn Alder Buckthorn. Fig. 18. Leaf of Buckthorn, Rhamnus Catharticus, showing piunate-arcuate venation. The few pinnate secondary ribs curve forward andlose themselves in the reticulum near the apex (D). V] PINNATE-RETICULATE VENATION 59 In a fourth case, appropriately termed pinnate-arcuatevenation, the single midrib gives off secondaries, whichstart in pinnate order, but curve forwards towards theapex of the leaf and there suddenly disappear in thenetwork, without forming distinct loops or reaching themargins. The secondaries are typically much strongerthan the tertiaries, and are few and distant. The following are examples of pinnate-arcuate vena-tion :— Buckthorn Dogwood. In the following cases, which we may term pinnate-reticulate venation, the single midrib gives off pinnatesecondaries at various angles and distances, but thesesoon break up into the general network, long beforereaching the margin, and without forming distinct loops— Crack Willow (Fig. 19), and the following :— Elder Ash Tr
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