x OSMOTIC PRESSURE IN PLANTS 199 



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. 



Different fibres gave tenacities of 37 kilo, to 

 60 kilo, per sq. mm. The cell-walls of these fibres are 

 of pure cellulose, and having been taken from fresh seeds 

 and soaked in water, 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 we 

 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. There- 

 fore the stress in the cellulose wall will never exceed the 

 internal pressure P acting over an area -n-r 2 divided by 

 the sectional area of the cell-wall 2 -n-rt, where t is the 

 thickness of the wall, and r the radius of the cylindrical 

 or spherical cell. 



Pnr* 



Stress per sq. mm. of cellulose = 



2nrt 



In Cytisus laburnum, for example, the palisade cells 

 are approximately 0'06 mm. long, 0-0175 mm. in diameter, 

 and their cell-walls are 0001 mm. thick. The osmotic 

 pressure may rise to 30 atm., or about 300 gr. per sq. mm. 



300 x 0-0087 ,, ft . 

 s = ' 2 x 0001 = gr " per q ' 



The observations quoted above show that the breaking 

 stress of cellulose is more than 50,000 gr. per sq. mm. 

 Consequently an osmotic pressure of 30 atm., even if 

 entirely borne by the cell-wall, will not tax it to near 

 its limit. 



Probably the greatest value for the diameter of any of 



