FOLIAGE AND SCALE LEAVES. 6j 



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 bleached with 

 spirit, then made transparent with chloral hydrate, and, lastly, 

 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 



