FOLIAGE AND SCALE LEAVES. 67 
chyma and by collenchyma. The larger vascular bundles keep 
the lamina stretched and extended, acting like the ribs of an 
umbrella. They and the rest of the stereome (p. 41) are also 
arranged so as to prevent the delicate mesophyll from being 
crushed between the two layers of epidermis. ‘Tearing is largely 
prevented by thickening of the epidermic walls, as especially 
in leathery leaves. This takes place to a greater extent at the 
edge, which is, of course, the place most likely to tear, than 
elsewhere. Additional firmness is given where a wavy course is 
taken by the lateral cell-walls of the epidermis, as an increase in 
amount of supporting substance is thereby gained. The epider- 
mis of some plants (grasses, &c.) is strengthened by silica. The 
remarks made on p. 41 about protection of stems by thorns, &c., 
and hair structures apply to the leaf also. The presence of 
distasteful substances also prevents animals from eating many 
leaves. Protection from the weather is effected as in stems 
(p. 41), with the exception that cork is never present. The 
glossy leaves of evergreens are peculiarly adapted for preventing 
the accumulation of snow upon them. Protection from the sun 
has been mentioned already (p. 54). Buds are often protected 
by means of scale-leaves, and when these secrete blastocolla the 
protection is still more complete. It also often happens that 
young foliage leaves in the bud have a warm covering of woolly 
hairs, which afterwards fall off. 
The main function of foliage leaves is that of nutrition. By 
means of their chlorophyll they are able (cf. p. 10) to build up 
organic compounds from the carbon dioxide of the surrounding 
medium, and the crude sap brought to them by the wood of the 
vascular bundles. This building up or assimilation is effected in 
the chlorophyll granules. ‘The first easily recognizable product 
is starch, and this can readily be detected in leaves which have 
been in strong sunlight for some time. ‘They are first bleached 
with chloral hydrate, and then soaked in a solution of iodine, 
when they turn a bluish-black. ‘This is a well-known colour- 
test for the substance in question. 
The starch and other organic substances formed in the leaf are 
largely converted into a soluble form (generally sugar in the case 
of starch), and travel osmotically in the parenchyma all over 
the plant, compensating waste and rendering growth possible. 
Non-diffusible proteid matters can travel by means of the sieve 
tubes. It may also happen that the various substances com- 
posing the elaborated sap are reconverted into the solid form 
within thickened roots, stems, leaves, or, as we shall see farther 
on, seeds. They then become reserve materials. 
Chlorophyll is absolutely dependent on light for the performance 
