THEIK STOMA.TA OB BKEATHING-PORES. 



151 



is in a moist atmosphere, and is freely supplied with sap, the sto- 

 mates remain open, and allow the free escape of moisture by evap- 

 oration. But when the supply fails, and the parenchyma begins to 

 be exhausted, the guardian-cells, at least equally affected by the dry- 

 ness, promptly collapse, and by closing these thousands of apertures 

 check the drain the moment it becomes 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 mechan- 

 ism is too delicate to work well 

 in direct sunshine. The posi- 

 tion of the stomata, and the 

 loose texture of the lower pa- 

 renchyma, require that this sur- 

 face should be shielded from the sun's too direct and intense action ; 

 and show why leaves soon perish when artificially reversed, and pre- 

 vented from resuming (as otherwise they spontaneously will) their 

 natural position. This general arrangement 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 present 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 (Nymphisea, Nuphar), and 

 other leaves which float upon the water, the stomata all belong to 

 the upper surface. All leaves which live under water, where there i 

 can be no evaporation, are destitute, not only of stomata, 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 under 

 the average number, as given in a table of 36 species by Lindley) ; 

 so that each leaf of that tree would present about 100,000 of these 

 orifices. 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 



FIG. 226. Magnified yie^ of the 10,000tli part of a square inch of the epidermis of the 

 lower surface of the leaf of the White Lily, with its stomates. 227. A single stomate, more 

 magnified. 228. Another stomate, widely open. 



