USES OF LEAVES 51 



passages with openings (stoniata) to the outside, usually on the 

 lower surface. These passages and openings facilitate the trans- 

 piration of water (Page H-i), and the absorption of gases. 

 (Page 115.) The texture of leaves varies largely with the en- 

 vironment in which they live. The surfaces of leaves are also 

 frequently covered with spines, hairs (trichomes), gland hairs, 

 wax, etc., which serve to protect the plant, prevent excessive 

 transpiration and otherwise facilitate its work. 



Uses of Leaves. — Leaves are so abundant, and come with 

 such great regularity that we are likely to fail to appreciate their 

 importance in the life of the plant. But these very common- 

 place facts should convince us that they are invaluable to the 

 plant, that they are something more than elements of beauty 

 in the individual plant or in the landscape. We have already 

 learned that the primary function of leaves is to serve as 

 foliage through which the living, growing plant receives gases 

 from the air and energy from the sun and in which the raw 

 food substances are transformed into true foods (photosyn- 

 thesis, Page 115). But the leaves serve many other useful pur- 

 poses which are important in the life history of the plant: (a) 

 they are the organs of transpiration (Page ll-l), by which are 

 given off quantities of water previously absorbed through the 

 roots; (h) they are organs of respiration, although this func- 

 tion is not confined to the leaves; (c) they may serve as bud 

 scales for the protection of the more typical foliage leaves and 

 flowers (Page 40) ; (d) they may develop a bitter gnim and 

 thus serve as a protection against birds and other small animals 

 which feed on the new growths in the early spring; (e) or they 

 may develop as briars or thorns which, no doubt, serve to some 

 extent as a protection against larger animals. Thistles and other 

 plants which are armed with these protective structures will 

 stand unmolested in the stock pasture, even though the food 



