306 PLANT PHYSIOLOGY 



peculiar and not yet fully understood in all their details. Trans- 

 formation of starch into sugar occurs in the guard cells, and this 

 is regulated by enzymatic processes. When starch is trans- 

 formed into sugar, the sap concentration, and consequently the 

 suction tension of the cell, increases. When guard cells imbibe 

 water, their volume and their curvature increase, 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. 97). This connection between 

 the movements of stomata and the transformations of starch 

 has been established by Iljin. It is of interest to note that the 

 influence 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 transformed 

 to sugars in darkness. In guard cells, on the contrary, starch 

 disappears in light and is again accumulated in darkness. 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 complex 

 mechanism, their response to changes in the environment is also 

 quite complex. Under extreme conditions, such as very severe 

 wilting or a rise of temperature above 40°C., as well as under the 

 action of sodium and potassium, the mechanism undergoes 

 striking disturbances. The hydrolysis of starch then rises 

 rapidly, while the converse process, the transformation 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 abnormally wide, and often lose the capacity 

 to close again. Under unusual dryness of the air, this may lead 

 to a complete desiccation of the leaf. According to the investi- 

 gations of Zalensky, such abnormal opening of the stomata is 

 observed during hot desiccating winds, which explains their 

 harmful effect. 



The mechanism of stomata may be studied in various ways. 

 The simplest method would be the direct observation under the 

 microscope of stomatal movements in an uninjured leaf. But on 

 account of their small size and the lack of transparency of the 

 leaves, this method can be used only with a few plants. More- 

 over, it is inconvenient to work with a microscope in a field or 



