8 Henry H. Dixon. 



turgor the forces drawing water into the cell must be at least as 

 great as those tending in the opposite direction towards the tracheae, 

 and in this way the osmotic pressure of the cells gives a major limit 

 to the stress in the w^ater in the leaf-tracheae. 



It will appear later that the osmotic pressures of the evaporating 

 cells are sometimes very large being in fact as much as 30 atm. 

 Hence it becomes of interest to ascertain how far the strength of 

 the cell-walls is adapted to bear the possible stresses arising from 

 these pressures. 



The actual tenacity of cell-walls has been determined by various 

 investigators. Schwendener^) found that sclerenchymatous fibres 

 could support from 15 — 25 kilo per sq. mm before breaking, Wein- 

 zierP) 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—12 kilo per sq. mm. 



More recently the author^) estimated the tenacity of cellulose 

 by loading very gradually a single fibre, detached from the seed of 

 Gossypium, until it broke. By observing the breaking weight and 

 the area of the cross-section where the break occurred the tenacity 

 was obtained. 



Diff'erent fibres gave tenacities of 37 kilo to 60 kilo per sq. mm. 

 The cell- walls of these fibres is of pure cellulose, and having been taken 

 from fresh seeds and soaked in w^ater, were in the imbibed condition, 

 and consequently resembled in their properties the imbibed cellulose 

 walls of the mesophyll cells. As in no case can the fibre support a 

 stress greater than its tenacity w^e must regard the lower results as 

 due to flaws in the fibres and the highest figures as giving the 

 actual tenacity of cellulose. 



The cells of the leaf approximate in form more or less to 



cylinders; they are seldom, if ever, spheres. Therefore the stress in 



the cellulose wall will never exceed the internal pressure P acting 



over an area Ttr^ divided by the sectional area of the cell wall 



2îTrt, where t is the thickness of the wall. 



P?rr^ 

 Stress per sq. mm of cellulose = -^ -. 



In Cytisus Laburnum for, example, the palisade cells are approxi- 

 mately 006 mm long, 0-0175 mm in diameter, and their cell walls are 



•) S. Sch wendener, Das mechanische Prinzip im Bau der Monocotylen. 1874. 



-) Weinzier 1, Sitzungsber. d. Wien. Akad. 1877, Bd. 76 Abt. I p. 411. 



^) Ambronn, Sitzungsber. d. Bot. Vereins f. Brandenburg- 1880, Bd. 22 p. 48. 



*) H. H. Dixon, The Tensile Strength of Cell-Walls. Ann. of Bot, Vol. XI, 

 1897 and Idem, Physics of the Transpiration Current. Notes from Botanical School, 

 Trinity College, Dublin, No. 2, 1897, p. 30. 



