Dixon — On the Osmotic Pressure in the Cells of Leaves. 63 



It is evident that -when the gas- pressure in the glass vessel sur- 

 rounding the branch is raised and maintained above the osmotic pres- 

 sure of the cells of the leaf, that 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 : firstly, 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 water into which the 

 branch dips. For every branch, then, we may expect to find a pres- 

 sure 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 of the osmotic cells. When 

 this critical pressure itself is maintained around the branch, water will 

 remain stationary in the plant. In carrying out these observations, 

 the form of apparatus I have used consists of a strong glass cylinder 

 of specially well-annealed glass, 50 cms. long, 10 cms. 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 are closed by means of two 

 heavy gun -metal castings, which project over the side of the cylinder 

 so as to take three long bolts with nuts, which draw the castings 

 together on the cylinder. Leather-washers, soaked in bees' wax and 

 turpentine, are inserted between the ends which are ground flat and the 

 cylinder to make the joints air-tight. The lower end is perforated 

 centrally, and in the perforation is sealed hermetically a narrow brass 

 tube, about "5 cm. in diameter, projecting into the cylinder. This tube 

 includes the stem of the plant to be experimented with, the lower end of 

 which projects out of the cylinder while its leaves are enclosed. To 

 make an air-tight connection between the tube and the stem, a stout 

 rubber tube is first bound on to the upper end of the brass tube. The 

 branch is then inserted into the rubber tube, and, before it has been 

 pushed completely down, a portion of it just above the rubber is coated 

 witli thick glue, so that when it is shoved down into its final position 

 with reference to the tube, it carries this glue down into the rubber tube. 

 When it is in position, a copper wire is bound tightly round the rubber, 

 and draws it into close contact with the glue. To complete the joint, 

 a little glue is smeared over it. This form of joint is simple and highly 

 satisfactory. The upper end of the cylinder is also perforated centrally 

 to admit the gas coming from the pump or bottle. This is a simple 

 screw-joint, made tight by a leather- washer. To the upper end, and on 



