38 



PART I. MORPHOLOGY. 



12 



The Venation of the Lamina. The mid-rib and other ribs of 

 the lamina indicate the course of the larger vascular bundles ; 

 and from these numerous branches are given off which permeate 

 the tissue of the lamina, constituting its Venation. When the 

 leaf decays, the ribs and the vascular tissue persist longer than 

 than soft parts as a skeleton which retains the general form of the 

 lamina. In Onvirandra fenestralis most of the softer tissue 

 decays whilst the leaf is still living, so that the lamina consists of 

 little more than the vascular skeleton. 



The main features of the venation are determined by the type of 

 development of the lamina. In a pinnate lamina, the venation is 

 pinnate ; in a palmate lamina, palmate ; in a pedate leaf, pedate ; 

 in a dichotomously branching lamina, the venation is also dicho- 

 tomous, or as it is specially termed, 

 furcate. But there is considerable 

 variety in the distribution of the 

 smaller vascular bundle ; thus the 

 venation of the individual segments 

 of a palmate or a pedate leaf is fre- 

 quently pinnate. 



According to the distribution of the 

 veins and their branches, the following 

 varieties of venation may be distin- 

 guished ; they are, however, connected by 

 intermediate forms. 



a. The venation is said to be free when 

 the veins end free, without forming ana- 

 stomoses, at the margin of the leaf ; this 

 is the case in the leaves of many Ferns 

 (Fig. 24) ; of Ginkgo (Salisburia), Arau- 

 caria imbricata and others, among Coniferse ; of most Cycads ; of Water- 

 Crowfoots, etc. 



b. The venation is said to be parallel, when numerous adjacent veins run 

 parallel to each other towards either the apex (Fig. 25) or the margin of 

 the blade, and then unite by curving inwards (Fig. 25 a). They are con- 

 nected in their course by short veinlets (Fig. 25 ) which run usually at 

 right angles to them. This form of venation is to be found in the leaves 

 of most Monocotyledons, such as Grasses, Lilies, and Palms, with various 

 modifications. For example in some cases (e.g. Orchis Mario) many veins 

 enter the blade, but they branch scarcely at all ; in other cases lateral 

 veins spring at an acute angle from a midrib which is prominent at the 

 base at least, and then run towards the apex (e.g. Maize and other Grasses, 

 Dracaenas, etc.) ; in others, again, the lateral veins spring almost perpen- 

 dicularly from the well-developed mid-rib, and run out to the margin 



Fio. 21. Leaf of a Young Fern, 

 with free pinnate venation : m the 

 midrib; the large lower lateral 

 veins; n the weak upper lateral 

 veins, (x 3.) 



