Transpiration and the Ascent of Sap. 9 



0001 mm thick. The osmotic pressure may rise to 30 atm. or about 

 300 grs per sq. mm, 



^, 300X0-0087 ,__ 



Stress = — o' V n-0 01^ ^^ ^^'^ P^^ ^^" ^™' 



The breaking stress of cellulose is over 50,000 grs per sq. mm. 

 Consequently an osmotic pressure of 30 atm. even if entirely borne 

 by the cell wall will not tax the wall to near its limit. 



Probably the greatest value for the diameter of any of the 

 cylindrical cells of the leaves is under 005 mm, but even in cells 

 having this diameter and having a thickness of wall equal to 0001 mm 

 only, it would require a pressure of 100 atm. to tax the cell wall to 

 its breaking point. Consequently we may take it that the osmotic 

 pressures of the leaves never seriously tax the tensile strength of 

 the cell walls. 



With regard to the energy available in the leaf for doing work 

 in the vaporization of water we are again indebted to the beautiful 

 researches of Brown and E s c o m b e ^) for figures. 



By use of a Callendar radiometer it was found that the maximum 

 amount of energy incident on leaves of plants in Kew in full sunshine 

 amounted to 1"0— 05 calories per sq. cm per min. This is a low 

 figure owing to the low position of Kew and the absorptive action of 

 the attenuated veil of smoke lying over it. The coefficient of ab- 

 sorption of the leaves experimented with averaged about 0'7, con- 

 sequently each sq. cm might absorb 0'5 X 0*7 cal. per min. of radiant 

 energy. Of this it was found about 0'25 cal. might be required for 

 the vaporization of the water given olf (allowing 42 X 10~^ grs to 

 be transpired per sq. cm per min.), while less than 1 % or 0*0035 cal. 

 was used in photosynthesis, leaving over roughly 0"0965 cal. which 

 go to raise the temperature of the leaf above its surroundings. 

 (Allowing for the 'emissivity' of the leaf in a breeze of about 

 25*7 kil. per hour, this temperature rise will be about 0*8 " C.) 



When the temperature is high and the sunshine is veiled the 

 vaporization taking place lowers the temperature so that a relatively 

 large amount of energy must enter the leaf from its surroundings. 

 In an experiment on a leaf of Helianthus annuus under such conditions 

 Brown and Escombe found: — 



Total radiation falling on leaf per sq. cm per min. 0*2746 cal. 



„ „ absorbed by leaf „ „ „ „ „ 0'1884 „ 



Energy expended in Photosynthesis 00033 „ 



„ „ „ Vaporization 0'3668 „ 



1) Brown and Escombe, On the Physiological Processes of Green Leaves. 

 Proc. Hoy. Soc. Lond. B., Vol. 76. 1905. 



