210 



EXPERIMENTS WITH PLANTS 



u 



122. Artificial 

 stoma and 

 guard-cells. 



therefore, they curve away from each 

 other, leaving the stoma open; when they 

 collapse again, it closes. Various modifica- 

 tions of this mode of action occur. The 

 general relations of the guard -cells and 

 surrounding cells may be seen in Fig. 121. 



If we tear off a strip of epidermis from 

 a leaf of a Lily or Amaryllis and mount it 

 on a slide in water, we shall (if the plant 

 is well supplied with water and exposed to 

 sunlight) probably find the stomata open : 

 if now, we lift the cover-glass and intro- 

 duce a drop or two of sugar or salt solu- 

 tion, the water will be withdrawn from the 

 guard- cells and they will at once collapse 

 and close the stomata. 



The principle by which the stomata 

 open and close may be illustrated by means 

 of the model shown in Fig. 122. A piece 

 of quarter -inch, thin -walled, black rubber 

 tubing, which is not .so old as to have lost 

 its elasticity, is reinforced throughout its 

 entire length, with the exception of half an 

 inch in the middle, by a strip of elastic 

 band (about one-eighth inch wide) firmly 

 attached with rubber cement. It is then 

 bent in the middle, as shown in the figure, 

 so as to bring the reinforced walls facing 



