62 PLANT PHYSIOLOGY AND ECOLOGY 



the upper and outer, being more or less thickened. Since the guard 

 cell is active, it has a relatively high osmotic pressure, and draw 



water readily from the adjoir 

 ing epidermal cells. The resu' 

 is to cause the inner walls of eac, 

 guard-cell to become more an( 

 more convex. Since the guard 

 cells are firmly joined to eacl 

 other at the ends, the increasec 

 Fig. 16. Diagram of the stoma of turgidity forces them apart ii 



//epoms The shape of the guard- the center, as though each wer. 

 cells when the stoma is open is shown ' 



by the heavy lines. The thick outer pulled bv a string attached to th( 

 and inner walls are shown at a and j^^j ' ^ ^he inner wall. A; 

 cd. (After bchwendener.) 



long as the plant remains fulh 



turgid, the stomata stay open, except of course for the regula 

 closing at night. When the water loss tends to become excessive 

 or the water supply deficient, the osmotic pressure of the epiderma 

 cells exceeds that of the guard-cells. Water is withdrawn fron 

 the latter, the inner wall becomes less convex, and releases the 

 strain upon the two guard-cells, which close in consequence. 

 Closure in its turn is maintained until the usual turgidity is re- 

 stored. In many cases it is probable also that the epidermal cell; 

 adjoining the stomata aid in this process by their shape and move 

 ment. 



Experiment 17. Movement of guard-cells. Strip a small piece 

 epidermis from the leaf of a turgid and of a wilted sunflower. Immersi 

 the strips for a few minutes in a killing solution containing osmic aci( 

 (preferably Flemming's solution), wash and examine the stomata unde 

 the high power. Remove fresh strips from the same sunflowers. Plac( 

 the turgid epidermis in a 5% solution of common salt and the flaccic 

 one in distilled water. Examine under the high power and explain the 

 results. Make a schematic drawing of a stoma when open and when 

 closed. 



Experiment 18. Position of stomata and water loss. Select four 

 similar leaves of sunflower or, better, of a plant which has stomata only 

 on the lower surface. Cover the upper side of one leaf with wax melting 

 at a low temperature. Wax the lower surface of the second and both 

 surfaces of the third. The fourth leaf is not coated, in order to serve as 

 a check. Fix each leaf in a small vial filled with a known amount of 

 water, in such fashion that the water lost is supplied through the petiole. 

 Determine the loss from each leaf by weighing or measuring the water 



