198 TRANSPIRATION AND ASCENT OF SAP ch. 



Inasmuch, as the conditions favouring the formation 

 and storage of soluble carbohydrates are generally best 

 realised in the higher levels of trees, it is not surprising 

 to find that the greater pressures are often developed 

 there, while smaller pressures usually occur in leaves, etc., 

 near the ground level. But as in every case the pressure 

 observed is much greater than the force required to over- 

 come the resistance of the water-tracts ; it is evidently 

 in no way defined by that resistance. 



Relation of osmotic pressure to the breaking 

 stress of the cell-walls. The presence of these high 

 pressures, amounting to about 30 atm. in the mesophyll, 

 naturally raises the question as to how far the tensile 

 strength of the cell- walls is taxed by their distending action. 



When these cells are inadequately supplied with water 

 it is evident that their walls may be barely distended 

 and there will be no tension in them. On other occasions 

 when the supply is sufficient their walls may be stretched 

 by nearly the whole osmotic pressure of their vacuoles. 



The actual tenacity of cell-walls has been determined 

 by various investigators. The following determinations 

 are quoted from PfefTer. Schwendener found that 

 sclerenchymatous fibres could support from 15 to 25 kilo, 

 per sq. mm. before breaking, Weinzierl estimated the 

 tenacity of the fibrous cells in the leaves of Phormium 

 tenax at 20'33 kilo, per sq. mm., and those of Allium porro 

 at 14*71 kilo, per sq. mm. Ambronn gives the breaking 

 strain for the walls of collenchymatous cells at 8 to 12 

 kilo, per sq. mm. 



More recently the author estimated the tenacity of 



leaves on the last growth was 8 - 88 atm. (A = 0'738) ; of the penultimate 

 three leaves on the last growth was 926 atm. (A = - 770 ) ; of the leaves on 

 the penultimate growth, 10 - 61 atm. (A = 0'882~), and of the leaves on the 

 antepenultimate growth, 11 "76 atm. (A = - 978'). If these observations had 

 been made on sap extracted by the liquid-air method considerably higher 

 pressures would have been obtained. But observations made by this latter 

 method fully confirm the rise in pressure with age. 



