230 SECTIONAL ADDRESSES 



now be obtained which were impossible to acquire twenty or even ten 

 years ago. The katharometer, spectrograph and polarograph are three 

 physical instruments which in particular will prove of the greatest aid to 

 such work. 



One significant fact does, at least, emerge from the information so far 

 acquired, namely, the absorption of dissolved substances by plant cells 

 is as much a vital process as the respiratory function and, like it, depends 

 on the presence of the living substance. On what does this dependence 

 consist .'' On the presence of a protoplasmic membrane, which is broken 

 down when the protoplasm changes in the direction of loss of vitality } 

 On the state of aggregation of the particles in the colloidal complex which 

 constitutes the system, and which certainly changes as the cell becomes 

 moribund .'' On the respiratory process itself ? On the presence of 

 certain enzymes or other substances which are contained in the proto- 

 plasm ? I have indicated how certain suggestions have been made in 

 regard to these various possibilities, but only further research will provide 

 the answer. 



It is a remarkable fact that with the continued application of the prin- 

 ciples of physical chemistry to the investigation of vital plant activities, 

 it has gradually become more and more evident that simple explanations 

 of these activities in terms of physical chemistry are not forthcoming. 

 Even the usually accepted simple explanation of the water relations of the 

 plant cell is now suspect. Ever since the classical investigations of De 

 Vries and Pfeffer it has been supposed that these relations, at any rate for 

 vacuolated cells, were explained with complete satisfaction on what I 

 have called the ' simple osmotic view,' the assumption being made that 

 the protoplast, or the limiting layers of it, functioned as a semi-permeable 

 membrane permeable to water but impermeable to many solutes. Now 

 Bennet-Clark, Greenwood and Barker have found that this explanation 

 is not always valid. They have measured the osmotic pressure of the 

 cell sap of a number of plant cells by the plasmolytic method, and also 

 cryoscopically after extraction of the sap from the tissues. In some cases 

 (petioles of Caladium and Rheum) the values obtained by the two methods 

 are the same, and hence in these cases the simple osmotic view affords a 

 satisfactory explanation of the observed facts, but in other cases (petioles 

 of Begonia and roots of beet and swede) the osmotic value of the sap 

 determined plasmolytically was found to be markedly greater than the 

 value obtained for the expressed sap by cryoscopic determination. This 

 means that in the latter cases the pressure sending water into the vacuole 

 is greater than can be accounted for by the actual osmotic pressure of the 

 sap as determined physico-chemically, and hence such cells possess a 

 power of active secretion of water analogous to the capacity for accumulat- 

 ing salts which I have already discussed. That this is so is confirmed by 

 the fact that cells of such tissues are not plasmolysed by their own sap, 

 whereas in the case of those tissues which do not exhibit this phenomenon 

 approximately half the cells of the tissue are plasmolysed by sap ex- 

 pressed from the tissue. So here also the vital activity of the protoplasm 

 is operative, and it may be presumed that the energy required for this 

 active secretion of water from the external medium is ultimately provided 



