THE VENATION OF LEAVES. 



49 



If the leaves are small, and already rendered sufiSciently stiff by means of their 

 epidermis, the vascular bundles lose themselves within the mesophyll to fulfil the 

 functions named, and little or nothing of the venation is to be remarked externally. 

 The venation is the more prominent, however, the larger the leaf surface and the 

 thinner the leaf tissue. For in this case it is no longer merely a matter of the con- 

 duction of water and the carrying away of the products of assimilation, as in 

 small, thick and stiff leaves ; it is now, rather, an important function of the 

 venation to keep the thin lamina of the leaf expanded flat, much as the ribs 

 of an umbrella extend its thin covering. This purely mechanical oflBce falls 

 ■essentially to the mid-rib and to its stronger ramifications in the leaf: the vas- 

 cular bundles of these are surrounded by more or less thick layers of succulent 

 and tightly stretched parenchyma, and their epidermis is further supported by 

 elastic tissue. These so-called primary veins of the leaves project on the under- 

 side as strong ribs ; their rigidity (upon which 

 almost everjfthing depends), is due to strong 

 tensions between the succulent parenchyma 

 and the epidermis. By their growth in 

 length, the veins strive to obtain greater linear 

 . dimensions than the thin lamina of mesophyll 

 stretched between them; hence they hold 

 the latter tightly extended, exactly as the 

 ribs of an umbrella hold the silk. From these 

 mechanically effective primary veins which 

 project on the underside of the foliage leaf, 

 arise further, as lateral branchings, thinner 

 vascular bundles, by means of which the spaces 

 between the primary veins become so con- 

 nected, that the lamina of the leaf is divided 

 up into a large number of small and still 

 smaller fields or areolae. Out of these anas- 

 tomoses, finally, in the highly organized thin 

 leaves of the Dicotyledons, ramifications of 



vascular bundles arise which become branched within the smallest areolae of the 

 venation, and at length terminate blindly (Fig. 44). 



The venation of thin and large foliage leaves has thus two chief functions. 

 First, a mechanical one, with the object of keeping the thin lamina stiff and 

 extended flat ; and secondly, it has the object of promoting the processes of 

 nutrition in the leaf, since the finer ramifications of the venation permeate the 

 leaf much in the same way as the smaller furrows of a well-designed irrigation 

 system run through a meadow in all directions, in order to bring and carry off 

 water. Even in large foliage leaves, the space is so closely permeated by the ends 

 of veins, that every square millimeter of mesophyll possesses its canals for bringing 

 and carrying off: these all run into the primary veins of the leaf, and thence 

 into the petiole, or into the shoot-axis. 



In ordinary very thin leaves, the venation has, however, yet a second purely 

 mechanical end to fulfil, in addition to the stretching of the green lamina, namely, 



[3] 



Fig, 45. — ^Venation of the leaf-like shoot of Suscus 

 kyposlossmn, a Monocotyledon (after Ettingshausen) , 



