124 PLANT LIFE 



thickness and resilience of the epidermis, but 

 much more effectively by the strengthening 

 of the abundant " veins " or vascular bundles 

 which run through it. These form, especially 

 on the underside, a connected system of 

 projecting and supporting strands. 



In the elongated strap-shaped leaves of 

 grasses, irises, palms, and suchlike plants, in 

 which the principal veins pursue a longitudinal 

 course in the leaf, we encounter the most 

 beautiful examples of precise mechanical 

 construction by which the proper form and 

 position of the leaf is maintained, and is again 

 recovered after any displacement that may 

 have occurred. Bands of sclerenchyma run 

 down the leaf, just below the upper and lower 

 epidermis, and they are often placed, girder- 

 wise, opposite one another, with the vein or 

 vascular bundle running down between them. 

 The latter thus occupies the position of the 

 webbing of a girder. Although there is a 

 good deal of difference in the details of differ- 

 ent plants, the general application of sound 

 mechanical principles of construction and 

 arrangement, as well as the presence of suit- 

 able strengthening tissues, is patent to any 

 observer who cares to examine the leaves. 



The netted veined leaves of ordinary 

 dicotyledons are exposed to considerable 

 risks of damage by tearing the margins. 

 The forms of many leaves se^em at first sight 

 almost to invite the risk of tearing, but any 

 one who tries will soon convince himself 



