5° 



LECTURE IV. 



to protect the leaf from rupture. This problem is not satisfactorily solved m all 

 leaves: the large, sometimes gigantic leaves of the Banana (Musa), Strelitzia, 

 Ravenala, &c., are provided with so inadequate a venation, that not only in our 

 inclement climate, but even in their milder native home, they become torn by 

 the currents of air; so that the broad leaf surface (3-6 m. long and 0-5 to 

 I m. wide) is regularly slit into a number of lobes and shreds, still held together 

 only by the powerful mid-rib. From this, innumerable lateral veins proceed, 

 close to one another, and parallel, and directed perpendicularly towards the 

 margin of the large leaf, to end there without adequate connection. When 

 the wind whips the large leaf surface, it tears from the margin inwards, like 

 an unhemmed flag, and the slits go parallel to the lateral veins up to the mid- 

 rib. The large majority of leaves, on the other hand, and especially very thin 



ones, are protected by the mode of distribu- 

 tiort of their veins, in a remarkably effi- 

 cient manner, against the so-called shearing 

 action of the wind, i.e. against the tearing 

 from the margin inwards. First, it is to 

 be remarked that the epidermis at the leaf 

 margins generally undergoes considerable 

 strengthening ; and that the cuticle especially;^ 

 and perhaps elastic fibres as well, usually form 

 a thick, and often very solid border at the 

 margin of the leaf. The resistance to in- 

 cision is again strengthened by the course of 

 the leaf-ribs in the neighbourhood of the mar- 

 gin of the leaf. Without entering into too much 

 detail, or into an exhaustive survey of all the 

 cases occurring, the subject is still sufficiently 

 interesting to bring forward some at least of 

 the most important examples. In this I com- 

 mence with the most complete arrangements 

 known to me, such as we find in the large thin, 

 and entire leaves of dicotyledonous plants. 

 The commonest case appears to be this. Starting from the strong mid-rib 

 of the leaf, each of the primary lateral ribs, which alternate right and left, runs 

 forwards and outwards at an acute angle towards the apex of the leaf, to become 

 joined, finally, in a curve convex towards the leaf margin, on to the rib next in 

 front. There thus arises a series of arches, which run immediately within the 

 leaf margin [Ftcus acuminata, religiosd). If the leaf margin is compared say with 

 a bridge, the lateral ribs, with their arch-like marginal coimections, represent' 

 the piers with their arches. In very large and delicate leaves, this arrangement 

 is strengthened by the formation of a second system of smaller arches, convex 

 outwards, the piers of which rest on the primary lateral ribs ; or there is 

 even a third system of still smaller pier-arches, with their convexities lying 

 towards the margin. The leaf margin is then comparable to a railway viaduclt 

 constructed of two or three storeys of arches ; a comparison which is by no means 



Fig. 46,— Venation of the leaf of Salix grandi/olia 

 (after Ettingshausen). 



