Les arbres ; un manuel de botanique pour la forêt-bois et le laboratoire . te-pennées-boucle-pennées arcuate-pennées-réticulé-nervation palmée et ses variétés. Les faisceaux vasculaires s'étend dans la feuille-tissuesare, comme déjà indiqué, appelé côtes et les veines, et con-constituer la soi-disant venation. Le principal projet en général andveins nervures sur la face inférieure de la feuille,et sont donc facilement visibles, mais il y a plus veinsare souvent invisibles ou peu visibles, sauf en leavesand par lumière transmise, parce qu'ils sont enterrés dans les tissus thesoft. Dans l'aspect du cuir et feuilles charnues

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Trees; a handbook of forest-botany for the woodlands and the laboratory . te—Pinnate-looped—Pinnate-arcuate—Pinnate-reticulate—Palmate venation and its varieties. The vascular bundles extending through the leaf-tissuesare, as already stated, termed ribs and veins, and con-stitute the so-called venation. The principal ribs andveins usually project on the lower surface of the leaf, and are therefore easily visible there, but the finer veinsare often invisible or inconspicuous, except in thin leavesand by transmitted light, because they are buried in thesoft tissues. In leathery and fleshy leaves all but thestrongest ribs are usually so buried and obscured. The distribution and arrangement of the venation isby no means devoid of order, and is often very regular. Bearing in mind that the ribs and veins are merelythe upper ends of the vascular bundles of the stem, comingup the petiole and thence branching in the lamina, wedistinguish these branches as of different orders, accordingto their thickness and relations one to another. CH. V] PINNATE VENATION 47. Fig. 14. Leaf of Wayfaring Tree, Viburnum Lantana, showingtypical pinnate venation, a midrib; b secondary ; c tertiaries, here form-ing cross-ties; d terminals; e outer branches of the secondaries (Ett). 48 PRIMARY RIBS [CH. The vascular bundles which come off as main strandsfrom the petiole are termed primary, and these bearsecondary strands, which in their turn branch into ter-tiary, &c, and so on, until the bundles of* higher orders—generally the fourth or fifth—are too fine to follow. As arule we do not enumerate any of higher order than thetertiaries, except that the blind ends of the last ramifica-tions are called terminals. In by far the majority of plants such as we have toconsider, there is but one primary rib, the midrib, runningvertically and continuously from base to apex of thelamina, and becoming thinner and thinner as it passes tothe tip of the leaf; as it runs up it gives off secondaryribs fr

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