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TEXTBOOK OF PLANT PHYSIOLOGY 



With high water content of the foliage they are usually open; with 

 lack of water, closed. 



The mechanism which controls stomatal movements is a very 

 peculiar one and not yet fully understood in all its details. It is 

 subject to the transformation of starch into sugar, which takes 

 place in the guard cells and is regulated by enzymatic processes. 

 When starch is transformed into sugar, the sap concentration, and 

 consequently the suction tension of the cell, increases. When 

 guard cells imbibe water, their volume increases and their curva- 

 ture changes, resulting in their separation and a widening of the 

 opening between them. But when sugar changes into starch, 



the reverse processes take place 

 and the stomata close (Fig. 

 71.) This connection between 

 the movement of stomata and 

 the transformations of starch 

 has been clearly established by 

 Lloyd and Iljin. It is of 

 interest to note that the influ- 

 ence of external factors on the 

 transformation of starch in the 

 guard cells and in other cells of 

 the leaf is very different. In the mesophyll cells, starch is formed 

 in light and dissolved in darkness. In guard cells, on the con- 

 trary, starch disappears in light and is again accumulated with its 

 absence. Lack of water leads to the transformation of sugar into 

 starch and the stomata close. With a sufficient amount of water 

 in the cell sap, sugar accumulates and the stomata are opened. 



Since stomatal movements are controlled by a rather complex 

 mechanism, their response to changes in the environment is also 

 rather complex. Under extreme influences, as very severe wilting, 

 or a rise of temperature above 40°, as well as the action of Na and 

 K cations, the mechanism undergoes striking disturbances. The 

 hydrolysis of starch then rises rapidly, while the converse process, 

 the transformation of the surplus of sugar into starch, is entirely 

 suppressed. This causes a great increase in osmotic pressure of 

 the guard cells. They become saturated with water, open abnor- 

 mally wide, and often lose the capacity to close again. Under 

 extreme dryness of the air this may lead to a complete desiccation 

 of the leaf. According to the investigations of Zalensky, such 



2 3 



Fig. 71. — Changes in the starch content 

 of guard cells during their movements 



(after lljin). 



