52 Henry H. Dixon. 



molecules, to set up a stress in the water which may be transmitted 

 to the lower menisci. 



If we suppose, in order to imitate the conditions in the plant 

 more closely, the outer surfaces of the two pots in the model to be 

 covered with osmotic cells, we can readily see that the conditions 

 are not essentially altered. The osmotic pressure in these cells need 

 not chang-e the gradients of pressure in the water. The osmotic 

 pressure is the pressure which the dissolved substance exerts ag'ainst 

 the membranes of the cells, while the tension is in the solvent and 

 is transmitted unaltered across the space in which the pressure of the 

 solutes is also exerted. 



In this respect the osmotic pressure acts just in the same way 

 as a number of internal supports keeping the cell turgid and preventing- 

 it from collapsing under the tension of the solvent which drags the 

 water across the cell. 



The gradient of pressure which causes the passage of water 

 from the soil into the root may be referable, as we have seen, to the 

 concave menisci formed in the cell walls of the root surface due to 

 the tension in the sap. In times of root pressure the gradient of 

 pressures is probably maintained by the osmotic properties of the 

 solutions in the root. High osmotic pressures in the cells of the root 

 will not lead to the entry of water unless the cells are at the moment 

 in a depleted condition so far as water is concerned. In that case 

 water will continue to stream into the cells until a state of turgor 

 is restored and the cell wall becomes tense, in a manner strictly 

 comparable to the restoration of turgor in a leaf cell, already con- 

 sidered. But supposing- the absorbing cells to be turgid, as they 

 usually are, then the entry of water and its passage across these 

 cells may be determined by the presence of an osmotic solution in 

 the root tracheae. As long as the solution in these tubes is so con- 

 centrated as to produce a lower vapour pressure within them than 

 exists in the surrounding soil, a flow across the absorbing cells will 

 evidently be determined and water will accumulate in the tracheae. 

 The water will continue to rise upwards until the dilution is such 

 that the dissolved substances can only generate a force equal to the 

 back pressure. Whether the phenomenon of bleeding from intact leaves 

 can be explained in the same way is open to question, but the 

 presence of large amounts of dissolved substances in the sap exuded 

 by-root pressure seems to render the above the most rational explanation. 

 However produced the occurrence of root-pressure is of great importance 

 in the tensile hypothesis, as it periodically flushes the conducting 

 tracts with water under pressure and so must bring into solution 

 the bubbles which have arisen in time of high tension. This action 



