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A TEXTBOOK OF THEORETICAL BOTANY 



ends are thin walled. It is the expansion and contraction of these thin walled 

 portions which varies the distance between the cells and hence the width of 

 the stomatal openings. 



The mechanism of stomatal movement is not yet fully understood. The 

 turgor pressure in the guard cells varies and with increasing turgor they 

 become more curved and the opening between them widens, while the reverse 

 occurs with decreasing turgor (Fig. 1004). These changes are associated with 

 corresponding changes in their osmotic potential. 



The guard cells are unique among the superficial cells in containing green 

 plastids. They are present even in plants grown in the dark, and are also 

 present on the colourless portions of variegated leaves. Starch accumulates 

 in these plastids, even in plants which do not form it in the mesophyll cells, 



Fig. 1004. — Vitna rosea. Diagram of a section through a stoma. 

 Thick Hnes show the position of the guard cells when the 

 stoma is open. Thin lines indicate the changes in the 

 position of the cell walls which bring about the closing of • 

 the stomatal pore. {After Kohl.) 



and this starch is subject to reversible hydrolysis into sugars, presumably by 

 enzyme action, without, however, any change in the total carbohydrate 

 content of the cells. The carbohvdrate is apparently not manufactured by 

 the guard cells, but seems to be obtained from the mesophyll and it is even 

 doubtful whether their plastids are photosynthetically functional. The starch 

 content is greatest in the early morning hours and decreases in daylight, 

 with a concomitant rise in the osmotic potential of the cell sap and increased 

 opening of the stomata. The reverse changes occur during the evening and 

 no further change appears to occur during the night. It is supposed that 

 light affects the balance by causing absorption of carbon dioxide by the 

 plastids thus increasing the alkalinity of the cell sap, and thereby activating 

 the amylase enzyme to hydrolyse the starch. With decreasing light, the 

 carbon dioxide produced by respiration begins to accumulate, the cell sap 

 becomes more acid and the resynthesis of starch is favoured, resulting in a 

 drop of osmotic potential and hence of turgor. 



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