THE STOMATA 



127 



guard cells contain chloroplasts (see Chapter IX), which manu- 

 facture sugar, etc., when the sun is shining. These products 

 become dissolved in the cell sap and increase its osmotic pres- 

 sure. Water is then drawn osmotically into the guard cells 

 from the surrounding epidermal cells until the former become 

 turgid and swollen, and in this condition the 

 guard cells draw apart and leave a passage 

 way between them into the interior of the 

 leaf. This behavior is apparently due to the 

 fact that the wall of the guard cell farthest 

 from the opening is more extensible than the 

 wall bordering the opening and bulges out 

 farther when the turgidity of the cell increases, 

 and drags the wall bordering the opening 

 along with it (Fig. 63, A). This action is 

 made clear by a simple experiment with 

 rubber tubing. Two short pieces of tubing 

 are connected by a Y-tube with a water 

 faucet. The lower ends of the pieces of 

 tubing have been plugged and fastened 

 together, and the exterior side of each piece 

 has been pared thin with a sharp knife or 

 scissors. When the water pressure is turned 

 on the thin outer sides bulge and the pieces 

 of tubing draw apart as shown by the dotted 

 lines in Fig. 65. 



Under exposure to light there may be other 

 influences on the action of the guard cells 

 than the increase in them of osmotic substances; but experi- 

 ments have shown that the stomata do not, as a rule, open in 

 an atmosphere devoid of carbon dioxide, and this points to 

 photosynthesis (see page 14c) in the guard cells, yielding sub- 

 stances that increase the osmotic pressure, as the main factor in 

 stomatal action. It must be stated, however, that experiments 

 of this sort have given apparently contradictory results, for in 

 some experiments with well-nourished plants in an atmosphere 



J I 



Fig. 65. — Diagram 

 of apparatus showing 

 how the guard cells 

 draw apart; j, j, posi- 

 tion of the rubber 

 tubing when the water 

 pressure is turned on. 



