EXPERIMENT STATION BULLETINS. .W? 



1. INTRODUCTION. 



The most importaiit problem in plant physiology is doubtless the 

 general one of nutrition. Under this heading there are numerous sub- 

 sidiary problems which have partially or entirely evaded explanation. 

 Until recent years physiologists have had to depend upon the known facts 

 of plrysics and chemistry, but now the closely allied subject of physical 

 chemistry, which has been making rapid strides in the past ten or 

 fifteen years, reveals many principles which offer invaluable aid. It is 

 true that many subjects in this comparatively new science do not touch 

 directly the activities of plants, but there are others which concern the 

 student of plant physiology very closely. The newer knowledge of 

 solutions is of fundamental importance. In no condition outside and 

 inside the plant does matter play a more important physiological role than 

 in a state of solution in water. Some soil solutions are dilute in respect 

 to the minerals present in them and no doubt the solutes undergo a more 

 or less complete dissociation, the former being true at infinite dilution. 

 These solutes no longer exist as definite cliemical compounds but are split 

 up into part-molecules or ions each of which carries a cliarge of electricity 

 TiUd thereafter whatever properties the solutions may possess are due to 

 the properties of these ions and the undissociated molecules when pres- 

 ent. 



The plasma membrane surrounding the living portion of each cell pre- 

 vents the entrance of any substance not in solution. Thus the knowledge 

 of the behavior of solutions is of greatest value for a proper interpreta- 

 tion of the absorption of electrolytes from the soil. Recent data obtained 

 from conductivity measurements lend support to the theory that the 

 solutes in the soil solution enter the plant in ionic form. The general 

 problem of permeability has received added impetus through the applica- 

 tion of certain principles of physical chemistry. To the plasma membrane 

 has been assigned tlie property of selective permeability and it is here 

 also probable that we are concerned with ion permeability. Perhaps the 

 most accurate and at any rate the simplest method for determining the 

 changes in permeability, since doubt has been cast on the plasmolytic 

 method, is the one of electrical conductivity. In the Imnds of Prof. W. 

 J. V. Osterhout (10-31) the method has brought out many interesting 

 facts. Permeability is altered by a variety of substances and conditions, 

 and the plasma membrane is proteid rather than lipoidal in nature. The 

 cell must be considered as a dynamic entity passing through various 

 transformations and existing in a medium that is continually changing 

 in a chemical and in a physical way. These complex conditions must give 

 rise to new relations between the membrane, the various parts of the 



