VARIETIES OF VENATION. 5 1 



a merely superficial or formal one, but completely describes the fact itself, 

 since this mechanical arrangement of the venation has a similar mechanical 

 significance to that of the arch-piers of a bridge. We find this double or triple 

 arch system perfectly evident in the leaves of Tobacco, Catalpa, and the 

 Tulip-tree; less evident in those of the Rhubarb, and in Asarum (Fig. 43), 

 and Salix grandifolia (Fig. 46). This arrangement is also well seen in the 

 individual leaflets of the Walnut-tree, as well as in the large leaves of Nymphaa 

 alba, floating on the surface of water : and it is expressly to be remarked here, 

 that leaves floating on or in water must be insured against being torn by the 

 shock of its waves, in the same manner as aerial leaves are protected against 

 the wind. 



In the large thin leaves of the Dicotyledons we find, however, besides the 

 arrangement named of a single, double, or triple arch system, quite another form of 

 distribution of the ribs, in that the side ribs spreading from the primary rib run 

 out straight to the margin, and terminate there. Out of these spring secondary and 

 tertiary lateral ribs, which behave in exactly the same way; so that the leaf 

 margin is met by a very large number of ribs coming from the interior of the 

 leaf and terminating in it, which are supported beneath the margin by feeble cross- 

 connections. It is obvious that this arrangement off'ers less guarantee for the 

 solidity of the leaf margin than that described above. Nevertheless, it is often 

 enough very distinctly developed, e.g. in the foliage leaves of the common Bottle 

 Gourd, in those of Corylus colurna and maxima, and in the floating aquatic leaves 

 of Euryale ferox. This form of venation asserts itself much more distinctly in many 

 Ferns with dichotomously (forked) branched ribs {Scolopendrium, Aspidium spinu- 

 losum, Osmunda regalis, &c.). Mixed systems, according to both the principles 

 named, are also frequently to be observed, 



A much rarer form of distribution of the ribs for protection against incision 

 is found in the magnificent leaves of Cyanophyllum. formosum, and also in a 

 monocotyledonous plant — Smilax sarsaparilla. In the lanceolate lamina, close 

 to the right and left margin, and proceeding from the base of the mid-rib, a 

 thin rib runs on each side, which is again united with the mid-rib at the apex of 

 the leaf. Further removed from the right and left leaf margin, a strong rib 

 then rises on each side, also springing from the base to the apex of the mid-rib. 

 Starting from the mid-rib, numerous ribs pass across right and left through the leaf- 

 substance to the two last named, and in like manner from these over to the marginal 

 ribs; and the fields of the leaf-surface so formed are reticulately penetrated by 

 numberless water-veins. This venation, also occasionally occurring in Dicotyledons, 

 leads us to the majority of the broad monocotyledonous leaves, e.g. of Potamogeton 

 naians, Alismaplaniago, Majanihemum bifolium, Convallaria latifolia, and others, where, 

 proceeding from the base of the mid-rib, two, three, four, or more lateral ribs arise, 

 which run in an arch-like manner to the apex of the leaf, approximately parallel to 

 the leaf margin. Feebler cross veins then ofl:en divide the longitudinal bands of the 

 lamina thus arising into smaller fields. In the commonest form of monocoty- 

 ledonous leaves, however, where the lamina forms a long narrow band, as in 

 the Grasses, Lilies, and Dracanas, a number of weak ribs run parallel with the 

 leaf margin and the mid-rib to the apex of the leaf— an arrangement which 



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