v.i OSMOTIC PRESSURES OF LEAF-CELLS 143 



above the osmotic pressure of the cells of the leaf, 

 water will be forced from these cells back into the conduits 

 of the branch and into the vessel beneath. This will 

 become apparent in two ways : first, by the flagging 

 of the leaf, inasmuch as the rigidity of the leaf is due to 

 the internal pressure of these cells, so that when this 

 pressure is overcome by the external gas-pressure the leaf 

 will flag ; secondly, by the increase of weight in the vessel 

 beneath containing the 



the 



water into which 

 branch dips. For every 

 branch, then, we may ex- 

 pect to find a pressure 

 above which water will be 

 forced back from the leaves 

 into the stem by reason of 

 the squeezing out of the 

 osmotic cells, and below 

 which water will rise 

 through the conduits to 

 the leaves, on account of 

 the osmotic attraction of 

 the cell-sap and evapo- 

 ration from the outside of 

 the cells. 



To carry out these observations, the form of apparatus 

 I used consisted of a strong glass cylinder of specially 

 well-annealed glass, 50 cm. long, 10 cm. in diameter, and 

 with walls 1 cm. thick. Such a glass cylinder should, 

 according to calculation, be capable of resisting an internal 

 pressure of at least 100 atmospheres. The ends of this 

 glass cylinder were closed by means of two heavy gun- 

 metal castings, which projected over the side of the 

 cylinder so as to take three long bolts with nuts, which 

 drew the castings together on the cylinder. Leather 

 washers, soaked in bees' wax and turpentine, were inserted 



Fk;. 24. 



