158 THE LEAVES. 



renchyma begins to be exhausted, the guardian cells, at least 

 equally affected by the dryness, quickly collapse, and by closing 

 these thousands of apertures check the drain the moment it be- 

 comes injurious to the plant. 



269. As a general rule, the stomata wholly or principally belong 

 to the epidermis of the lower surface of the leaf: the mechanism 

 is too delicate to work well in direct sunshine. The position of 

 the stomata, and the loose texture of the lower parenchyma, re- 

 quire that this surface should be shielded from the sun's too direct 

 and intense action ; and show why leaves soon perish when artifi- 

 cially reversed, and prevented from resuming (as otherwise they 

 spontaneously will) their natural position. This general arrange- 

 ment is variously modified, however, under peculiar circumstances. 

 The stomata are equally distributed on the two sides of those 

 leaves, of whatever sort, which grow in an erect position, or pre- 

 sent their edges, instead of their surfaces, to the earth and sky 

 (294), and have the parenchyma of both sides similarly constituted, 

 sustaining consequently the same relations to light. In the Water- 

 Lilies (Nymphsea, Nuphar), and other leaves which float upon the 

 water, the stomates all belong to the upper surface ; and all leaves 

 growing under water, where there can be no evaporation, are des- 

 titute, not only of stomates, but usually of a distinct epidermis also. 



270. The number of the stomata varies in different leaves from 

 800 to about 170,000 on the square inch of surface. In the Apple, 

 there are said to be about 24,000 to the square inch (which is un- 

 der the average number, as given in a table of 36 species by Lind- 

 ley) ; so that each leaf of that tree would present about 100,000 of 

 these orifices. From their great numbers, they are doubtless fully 

 adequate to the office that is attributed to them, notwithstanding 

 their minute size. Their size varies so greatly in different plants, 

 that no safe inference can be drawn of the comparative amount of 

 exhalation in different leaves from the mere number of their sto- 

 mata. When the stomata are not all restricted to the lower sur- 

 face, still the greater portion usually occupy this position. Thus, 

 the leaf of Arum Dracontium is said to have 8,000 stomata to a 

 square inch of the upper surface, and twice that number in the 

 same space of the lower. The leaf of the Coltsfoot has 12,000 

 stomata to a square inch of the lower epidermis, and only 1,200 in 

 the upper. That of the White Lily 60,000 to the square inch on 

 the lower surface, and perhaps 3,000 on the upper. 



