120 PLANT STRUCTURES 
ling factor. The veins, or vascular bundles, act effi- 
ciently as strengtheners of the thin surface; to pre- 
vent tearing at the leaf-edges the veins are often 
looped along the margin; while in indented leaves the 
extremities of the indentations are strengthened with 
special tissue. When one surface of the leaf faces 
the sky, as in most cases it does, this surface is 
strengthened against the weather, and the stomata are 
arranged mostly on the lower surface. Where occa- 
sionally the leaves hang normally in a vertical posi- 
tion, as do the mature leaves of the Gum Trees 
(Eucalyptus), both sides are protected, and the 
stomata are borne on the two faces equally. In the 
Water Lily, again, whose leaves float, the upper face, 
which alone is exposed to the air, bears the stomata, 
which are present in unusual numbers—nearly 300,000 
to the square inch; the leaf surface is toughened to 
resist rain and wind, and waxy to prevent water from 
lying on it and so interfering with transpiration. The 
presence or absence of a leaf-stalk, again, is often 
clearly related to the light question. In the Water 
Lilies the continued lengthening of the elongated 
petiole causes the older leaves to float clear outside of 
the younger ones. In many biennial herbs, where 
food is stored up during the first season in. prepara- 
tion for the flowering effort in the second, a similar 
arrangement prevails—note the leaf-rosettes displayed 
by Spear Thistle (Carduus lanceolatus) and Herb 
Robert (Geranium Robertianum), as also especially in 
winter by perennials like the Dandelion (Taraxacum 
officinale) and Ribwort (Plantago lanceolata). Where 
stems spread horizontally, as the lower branches of 
trees, the leaves are arranged more or less in one 
