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 seem at first sight 
almost to invite the risk of tearing, but any 
one who tries will soon convince himself 
