PHANEROCAMIA : FOLIAR ORGANS. 277 



nerve traversing the middle line of the leaf, and principal lateral nerves 

 passing off from this. According as the latter make an acute angle, or 

 are convex, toward the central nerve in their departure from it, De 

 Candolle* distinguished folia angulinervia and curvinervia ; the latter 

 he claimed for the Monocotyledons, but they also occur frequently enough 

 in Dicotyledons. When, on the other hand, the leaf is traversed by 

 several equally strong nerves starting from its base, De Candolle called 

 it folium rectinervium. These principal divisions were then further 

 subdivided. Others, for instance Link and Lindley, have other divisions, 

 because they make the principal distinctions depend on other forms. 

 These various, equally valid, opinions, show that there can be no law 

 here. These conditions are also quite inapplicable to the characterisa- 

 tion of plants and vegetable groups, excepting in isolated cases, where 

 certain conditions are constant within the limits of certain groups, e. g. 

 in Melastomacece, Scitaminece, &c., which, on the whole, are very rare. 



2. The vascular bundles of the leaves are progressive bundles, 

 and they are so formed that (regarding the leaf as passing off hori- 

 zontally from the axis) the oldest parts lie above, the youngest 

 below. In the lower part also a cambium layer exists in the 

 Dicotyledons ; in the lower part liber-bundles accompany the vas- 

 cular bundles, and in the under part the vascular bundles, in rela- 

 tively thin and flat leaves, project above the surface (probably in 

 consequence of gradual development), while the upper part of the 

 leaf appears level. 



"We are at present wholly destitute of investigations into the develop- 

 ment of the vascular bundles in the leaf, and need more minute obser- 

 vation of the condition of the unlimited bundles of Dicotyledons, and 

 their condition in lengthened duration of the leaf. In Pinus and Abies 

 I believe that I have been able to distinguish, in leaves two years old, 

 two layers of the vascular bundle (similar to the annual rings). 



3. The parenchyma of the leaf is developed in the most varied 

 manner ; in general, in thick, solid leaves, it is composed externally 

 of small crowded cells containing more chlorophyll, internally, of 

 larger and looser cells filled with aqueous juices. Very often the 

 outer layer passes into a tissue, the cells of which are elongated in 

 a direction vertical to the surface of the leaf, are applied closely, 

 almost without trace of intercellular passages, and thus are pretty 

 sharply distinguished from the rest of the parenchyma, and occur 

 in the whole of the periphery of the leaf, not only in round and 

 triangular leaves, but also in flat ones, as in many New Holland 

 Myrtacea. In flat leaves, especially of Dicotyledons, there is very 

 often a separation into two layers, the upper of which has the cells 

 elongated perpendicularly to the surface, as just mentioned, filled with 

 much chlorophyll, while the lower is composed of looser, globular, or, 

 still more frequently, spongiform parenchyma containing little chlo- 

 rophyll. In thick coriaceous or fleshy leaves, for instance, in species 

 of Ficus and Peperomia, one or more layers of cells containing little 



* Loc. cit. 

 T 3 



