vii OSMOTIC PRESSURES OF LEAF-CELLS 141 



up to the upper end, where there is a small bend made to 

 receive it. The J -tube is now set in a vertical position, 

 and its short limb is connected with an air-pump. By 

 the action of the pump the atmospheric pressure is removed 

 from the lower end of the column of water in the tube, 

 and the weight of the lower parts of this column, hanging 

 from the upper parts, puts them in tension. As the piece 

 of tissue occupies the top of the tube, the water in it and 

 around it is in a tensile state. It will be noticed that, 

 although exposed to this tension for a considerable time, 

 the tissue will retain its curvature, indicating, as we have 

 seen, an osmotic pressure in its cells. I have exposed a 

 piece of the peduncle of Doronicum austriacum to a tension 

 of 50 cm. of water for two hours, without being able to 

 detect any diminution of curvature. 



In order to expose the water surrounding the piece of 

 tissue to a greater tension, the lower part of the water 

 column may be replaced by mercury. Working in this 

 way I have submitted the osmotic cells of the peduncle 

 of Doronicum to a tension of 75 cm. of mercury for one 

 hour. During this time the turgor of the cells remained 

 unaltered. 



These experiments show the possibility of realising 

 experimentally the conditions we have assumed of pressure 

 and tension in the transpiring cells of the leaves. 



Osmotic pressure in leaf-cells a gauge of 

 tension in tracheae. From the foregoing considera- 

 tions it is evident that so long as the force applied to the 

 upper ends of the sap in the tracheae of the leaves is less 

 than the osmotic pressure of the vacuoles of the leaf- 

 cells, these cells will remain distended and the leaf 

 will appear fresh and stiff ; whilst if the force drawing 

 off water from the cells is greater than that which they 

 can exert on the water in the tracheae, they will collapse 

 and the leaf will fade. Under normal conditions of trans- 

 piration this collapse does not take place. Hence the 



