igo THE STOMATAL MECHANISM 



it should be noted that in some species such as maize it has never been pos- 

 sible to demonstrate night opening of the stomates. 



Discussion Questions 



1. Why is there usually little or no correlation between number of stomates 



per unit area of a leaf and its rate of transpiration? 



2. Would you expect the amount of diffusion through a multiperforate septum 



to more nearly approach that through an open surface of the same area, if 

 the gradient of the diffusing gas is steep or if it is gradual? 



3. Draw curves that might reasonably be expected to represent the daily varia- 



tion in the "diffusive capacity" of the stomates (a) on a "standard day," 

 (b) under otherwise "standard day" conditions but with the soil water con- 

 tent approaching the wilting percentage, (c) on a cloudy summer's day with 

 the soil water content at approximately the field capacity. 



4. Explain why benzene will penetrate rapidly into the intercellular spaces 



through open stomates when a leaf is brought in contact with it but water 

 will not similarly penetrate unless applied with considerable force. 



5. Suggest some possible explanations of the mechanism of night opening of 



stomates. 



6. Stomatal closure sometimes occurs at times when no appreciable decrease 



takes place in the osmotic pressure of the guard cells. Explain. 



7. Some authorities have considered that imbibitional swelling of colloidal ma- 



terials in the guard cells may be an important factor in causing stomatal 

 opening. What objections can be offered to this concept? 



Suggested for Collateral Reading 



Burgerstein, A. Die Transpiration der Pftanzen. I, H, and HI. Gustav 



Fischer. Jena. 1904, 1920, 1925. 

 Loftfield, J. V. G. The behavior of stomata. Carnegie Inst. Wash. Publ. 



No. 314. Washington. 1921. 

 Maximov, N. A. The plant in relation to water. Edited by R. H. Yapp. 



George Allen and Unwin. London. 1929. 

 Miller, E. C. Plant physiology. 2nd Ed. McGraw-Hill Book Co. New 



York. 1938. 



Selected Bibliography 



Beck, W. A. Variations in the Og of plant tissues. Plant Phj^siol. 6: 315- 



323. 1931- 

 Brown, H. T., and F. Escombe. Static diffusion of gases and liquids in rela- 



tioji to the assiniilatioji of carbon and translocation in plants. Phil. Trans. 



Roy. Soc. (London) B. 193: 223-291. 1900. 

 Copeland, E. B. The rnechanisni of the stomata. Ann. Bot. 16: 327-364. 



1902. 

 Deneke, H. ijber den Einfluss beivegter Luft auf die Kohlensdureassimila- 



tio7i. Jahrb. Wiss. Bot. 74:1-32. 1931. 



