48 



LECTURE IF. 



the most varied kind— prickles, stinging hairs, woolly hairs, and glandular hairs- 

 protect the leaves against too intense sun-light, against too strong cooling, and 

 against numerous attacks of insects, &c., according to the biological relations of 

 the plants concerned. Of great importance to the function of the foliage 

 leaves, and consequently to the existence of the whole plant, is the so-called 

 ve7iation. The essential parts of this are the vascular bundles, which, bending out from 



Fig. 44. — Several meshes from the leaf of A}ithyllts vitlneraria. 7H midrib ; l\ b secondary cross-veins 

 radiating from it; a, a a closed mesh; r, c endings of the finest veins. The figure shows only the spiral cells 

 of the veins ; but the phloem of the vascular bundles runs with these, and the meshes are filled with the parenchy- 

 matous niesophjll. (Strongly magnified.) 



the shoot-axis into the base of the leaf, and running through the petiole when it is 

 present, become branched in the leaf-blade or lamina. It depends solely and 

 simply upon the nature of the leaf what the form and importance of its venation 

 shall be. In the first place, the vascular bundles of the leaf venation have the duty 

 of conveying the w^ater, laden with nutritive matters, to the assimilating mesophyll, 

 and of conducting back a p.irt of the products of assimilation into the shoot-a.xis. 



