156 ANATOMY OF THE LEAVES. [leSSON 25. 



pacted, so as to expose as little surface as possible to the direct action 

 of the hot sun. On the other hand, the leaves of marsh plants, and 

 Jf others not intended to survive a drought, have their cells more 

 loosely arranged throughout. In such leaves the epidermis, or skin, 

 is made of only one layer of cells ; while in the Oleander, and the 

 like, it consists of three or four layers of hard and thick-walled cells. 

 In all this, therefore, we plainly see an arrangement for tempering 

 the action of direct sunshine, and for resti-aining a too copious evap- 

 oration, which would dry up and destroy the tender cells, at least 

 when moisture is not abundantly supplied through the roots. 



441. That the upper side of the leaf alone is so constructed as to 

 bear the sunshine, is shown by what happens when their position is 

 reversed : then the leaf soon twists on its stalk, so as to turn again 

 its under surface away from the light ; and when prevented from 

 doing so, it perishes. 



442. A large part of the moisture which the roots of a growing 

 plant are constantly absorbing, after being carried up through the 

 stem, is evaporated from the leaves. A Sunflower-plant, a little 

 over three feet high, and with between five and six thousand square 

 inches of surface in foliage, &c., has been found to exhale twenty or 

 thirty ounces (between one and two pints) of water in a day. Some 

 part of this, no doubt, flies off through the walls of the epidermis or 

 skin, at least in sunshine and dry weather ; but no considerable por- 

 tion of it. The very object of this skin is to restrain evaporation. 

 The greater part of the moisture exhaled escapes from the leaf 

 through the 



443. StomatCS or Brcalhing-porcs. These are small openings through 

 the epidermis into the air-chambers, establishing a direct commu- 

 nication between the whole interior of the leaf and the external air. 

 Through these the vapor of water and air can freely escape, or 

 enter, as the case may be. The aperture is guarded by a pair of 

 thin-walled cells, — resembling those of the green pulp within, — 

 which open when moist so as to allow exhalation to go on, but 

 promptly close Avhen dry, so as to arrest it before the interior of the 

 leaf is injured by the dryness. 



444. Like the air-chambers, the br(>athing-pores belong mainly to 

 the under side of the leaf. In the White Lily, — where they are 

 unusually large, and easily seen by a simple microscope of mod- 

 erate power, — there are about GO.OOO to the square inch on the 

 epidermis of the lower surface of the leaf, and only about 3,000 in 



