62 Henry H. Dixon. 



Tilia first at a pressure of 26 — 30 atmospheres. In many cases after 

 the collapse of the leaf has been brought about by raising the pressure 

 in the cylinder surrounding the branch, the turgor of the cells may 

 be restored by reducing the pressure once more. 



It has been stated that this method of obtaining an estimate 

 of the osmotic pressures in the cells is not above criticism ^) ; but no 

 definite objections have been formulated against it. A detailed dis- 

 cussion of the method would have been more helpful. It is difficult 

 to see what is the cause of the collapse of the leaf if it is not the 

 loss of turgescence of the cells. A cell lying next a trachea is 

 exposed to the high pressure on all sides except on that forming the 

 common wall with the trachea. The water in the trachea is continuous 

 with that in the vessel below and is at atmospheric pressure. At first 

 the osmotic pressure of the cell under consideration is able to resist 

 the high air pressure in the cylinder and prevents water being squeezed 

 back into the trachea. When the osmotic pressure is overcome, water 

 is crushed out of the cell into the trachea, and the cell loses turgidity. 

 This leads to a concentration of the solution in the cell, and it 

 becomes able to withstand a higher pressure and consequently it 

 ceases to collapse. Then cells having a lower osmotic pressure lose 

 water and the same sequence of events is repeated. When a number 

 of the mesophyll cells are thus robbed of water and lose turgor the 

 leaf assumes a dull appearance. In this experiment the osmotic forces 

 of the cells are balanced against air pressure, just as, in the ordinary 

 determinations of osmotic pressure, the osmotic pressure is balanced 

 against the head of a liquid column. 



The method is certainly preferable to the plasmolytic treatment 

 of fragments cut from leaves. In the preparation of these pieces such 

 violent stimuli have been inflicted on the leaves that their osmotic 

 properties may have been entirely altered, and the subsequent deter- 

 mination of osmotic pressure probably gives quite erroneous estimates 

 of the osmotic pressures in the transpiring leaf. 



The osmotic pressures determined by Sutherst for the leaves of 

 some herbaceous plants ranging, between 9 and 15 atm. and by the 

 author for some trees ranging from 15 to 30 atm., are quite in harmony 

 with the tension theory. These pressures are ample to keep the cells 

 distended against the tension developed in the solvent and so maintain 

 the turgidity of the leaf during transpiration. 



To take for example a tree 100 m high: The tension developed 

 in its upper tracheae, to support the weight .of the water-column and 

 to overcome the resistance to the flow, would not be more than 



^) L. Jost, Lectures in Plant Physiology. Eng. Trans, by R. J. Harvey 

 Gibson. Oxford 1907. 



