FACTORS CONTROLLING VARIATIONS IN THE 

 RATE OF TRANSPIRATION. 



J. D. Sayre. 



While investigating the relation of hairy leaf coverings to 

 the resistance of leaves to water loss,* a number of experiments 

 were performed which show the relative effects of several of the 

 principal factors controlling transpiration and the rhythm of 

 the transpiration curve in darkness in mullein and tobacco. 

 Because these experiments were not directly related to the 

 subject of hairy leaf coverings and transpiration they are 

 presented separately in this paper. 



Historical. 



Brown and Escombe^ in their researches on static diffusion 

 of gases showed that the stomatal openings of a leaf act the 

 same as a multiperforate diaphragm in allowing water vapor 

 to pass out from the intercellular spaces of the leaf. They 

 showed that, in view of the number and sizes of the stomata, 

 only about one-sixth of the possible diffusion of water vapor 

 from the leaf of a sunflower {Helianthiis sp.) is ever attained 

 under ordinary conditions. Lloyd^ came to the same conclusion 

 in his experiments with ocotillo {Fouquieria splendens) and 

 showed that when the stomata are almost closed their diffusion 

 capacity is much greater than is needed even to allow a max- 

 imum transpiration. He concludes that the movements of 

 the guard cells tending to open and close are not sufficient 

 to account for the variations of water loss, under those 

 conditions. He says, however, that complete closure (if it 

 ever does occur) would function in controlling transpiration 

 by eliminating the evaporation from the internal surface of the 

 leaf and reducing the water loss to that from the cuticular 

 surface. Livingston and Estabrook^ found in a number of 

 plants {Fiinkia, Isatis, Allium Eidiomia, and Oenothera) that 

 the stomatal movements were considerably more pronounced 

 than would be expected from Lloyd's measurements. Other 

 authors have found in their experiments on transpiration that 



*Thesis presented for Degree of Master of Arts at the Ohio State University, 

 Columbus, Ohio. 



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