52 



LECTURE IV. 



thoroughly satisfies the requirements in. the face of the clanger of transverse and 

 longitudinal tearing. 



If we now return to the broad dicotyledonous leaves, we find, first, those the 

 margin of which is coarsely toothed, or variously incised apd lobed. In such 

 cases as the leaf of the Vine, Petasites albus, and many others, the course of the 

 ribs resembles the second case described above, e. g. that of Cucurbita. The lateral 

 ribs spreading from the mid-rib, as well as their secondary ramifications, run 

 direct to the leaf margin, and generally end in a prominent tooth of the same : 

 this occurs also in small toothed leaves, e. g. of Viburnum latitana. Inside the 

 depression between the teeth, in this case, a smaller vein generally ends ; or two 



small lateral veins meet here at an obtuse angle. 

 With this arrangement, also, the arch-like mar- 

 ginal connection of the lateral veins may again 

 be mixed. 



If we now pass to the divided and com- 

 pound leaves, the important question is whether 

 the diff'erent parts of the lamina have each a con- 

 siderable extent of surface, as in the Walnut, or 

 whether they are small and narrow. In the 

 first case, the venation has the same mechanical 

 function as in large, entire leaves ; in the second 

 case, on the other hand, as in the finely divided, 

 doubly and triply pinnatisected leaves of many 

 Umbelliferae {Coniuvi, Anthriscus\ in the Milfoil 

 and others, a connected leaf-surface, and a 

 tearing of it from the edge inwards, are 

 hardly to be spoken of, and the corresponding 

 arrangements of the venation dt) not exist. The 

 same is the case also in the great majority of the 

 finely divided leaves of Ferns ; where, moreover, 

 the leaf tissue already possesses considerable 

 soHdity on its own account. 



If the mechanical principles of leaf-venation 

 here made evident are kept in view, it is always 

 easy to understand the distribution of the coarser 

 leaf-ribs, even in cases not here considered. It is further obvious that in 

 very small leaves, as those of the Lycopodiaceae, most Conifers, and many 

 Dicotyledons, the mechanical arrangements described would be quite superfluous ; 

 and consequently they are not present. In the same way it is clear that in very 

 stiff leathery leaves, as those of the Oleander, the mechanical aspect of the venation 

 becomes wholly insignificant in contrast to its importance in connexion with the 

 carrying to and fro of nutritive substances ; and that finally, in the so-called suc- 

 culent plants, the foliage leaves of which are very thick, and sufficiently solid besides, 

 nothing at all is to be noticed of the mechanical venation. These considerations, 

 however, teach us at the same time how fruitful is every conception of organic 

 forms based on the principle of causality, as contrasted with the merely formal com- 



FlG. 47.— Venation of the leaf of C 

 latifolia (after Ettingshausen). 



