TRANSPIEATION AND THE ASCENT OF SAP.^ 



By Henry H. Dixon, Sc. D., F. R. S., 

 Professor of Botany in the University of Dublin. 



The water of the transpiration stream enters at the roots, passes 

 np the stem, and is given otT from the leaves. It will be convenient 

 to discuss the processes taking place in each of these organs sepa- 

 rately, so far as they affect the stream, and then consider how thase 

 processes are correlated. 



In regard to the exhalation of water vapor from the leaves, early 

 experimenters have shown that cnticular transpiration is usually 

 insignificant compared with diastomatic diffusion. The efficiency of 

 diastomatic transpiration was first clearly explained by Brown and 

 Escombe.'' 



The minute cross sections of the openings of stomata and the com- 

 paratively large area occupied by the practically impermeable cuticle 

 made it difficult to understand how the observed quantities of water 

 escape from the leaf. These authors showed, however, that an unex- 

 pected law governs the diffusion of water vapor through a number of 

 minute perforations in an impermeable membrane. According to 

 this law it follows that the amount which diffuses through the perfo- 

 rations is not only, as one would on first thoughts expect, proportional 

 to the sum of their areas, but may vastly exceed this proportionality; 

 and consequently the diffusion through a number of minute pores, like 

 the stomata, will be much greater than through one large aperture 

 having a cross section equal to the sum of the areas of the stomata. 



In order to obtain a clearer idea of this remarkable result, we will 

 consider in a general way the state of affairs around one stoma, so 

 far as water vapor is concerned. At the level of the stoma the water 

 vapor has a certain density — i. e., the water molecules are more or less 

 crowded, depending on the state of saturation of the external space 



1 Reprinted, by permission, in abridged form, from Progressus Rei BotanicfB. Heraus- 

 gegeben von der Association Internationale des Botanistes, redigiert von Dr. J. P. Lotsy. 

 Dritter P>and. Verlag von Gustav Fischer in Jena 1900. 



" Brown and Escombe, Static Diffusion of Gases and Liquids in Relation to the Assimi- 

 lation of Carbon and Translocation in Plants. Phil. Trans. Roy. Soc. Lond. B., vol. 193, 

 pp. 223-292, 1900, of which an abstract appeared in the Proc. Roy. Soc, vol. 66, and in 

 Ann. of Bot., vol. 14, Sept., 1900, pp. 537-542. 



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