112 Journal of Agricultural Research voi. xviii. No. a 



freezing point depressions. It is realized, however, that such expressions 

 are matters of convenience and not necessarily significant. The osmotic 

 pressure of a solution is defined by Findlay {lo) as the "equivalent of the 

 hydrostatic pressure produced when the solution and solvent are separated 

 by a perfectly semi-permeable membrane." The absorbing membranes 

 of the roots are obviously not of this sort. They are permeable to all 

 the ions present in varying degrees and, moreover, as shown by Oster- 

 hout (x, p. 96-147), the permeability is subject to change as a result of 

 antagonistic relations among ions. It is also to be noted that the osmotic 

 pressure and electrolyte concentration in the expressed plant sap are far 

 greater than those of any noninjurious nutrient solution. The effects of 

 solutions are not due to the theoretical osmotic pressures of which the 

 solutions are capable, but rather to alterations in permeability due to 

 specific ions or ionic relations, or to internal derangement of the metabolic 

 processes as a result of a too great absorption of one or all ions. 



SUMMARY 



(i) Sand- and solution-culture experiments were carried out under 

 conditions permitting definite control of the total concentration, com- 

 position, and reaction of the nutrient solutions. Numerous absorption 

 studies were made throughout the growth cycle of the barley plant. 

 Plants were obtained which were fairly comparable in size and develop- 

 ment with those produced by a fertile soil. 



(2) Marked absorption of all the nutrient elements took place at all 

 periods up to the final stage of growth when suitable concentrations of 

 the various ions were continuously maintained. This intense absorption 

 during the later stages led to no important increase in yield of crop, 

 which seemed to be conditioned in large measure on a favorable supply 

 and concentration of nutrients during the first 8 or 10 weeks of the 

 growth cycle. 



(3) With increasing concentrations of the nutrient solution it was 

 found in these experiments that the composition, expressed in percent- 

 ages, and total quantity of N and K per plant were decidedly increased 

 in the tops. This was also true for the roots, but in addition these 

 showed a marked increase in the percentage and total quantity per 

 plant of Ca and PO4. In the tops most of the Ca, Mg, PO4, and K was 

 present in a water-soluble form. In the roots grown in the solutions of 

 the higher concentrations large percentages of insoluble Ca and PO4 were 

 found. 



(4) When plants of uniform development were transferred to nutrient 

 solutions of different concentrations, a greater transpiration took place 

 from solutions of low concentration. Absorption and transpiration took 

 place independently, so that the solution under some circumstances 

 might become either more or less concentrated, depending upon the 

 original concentration of the solution. 



