Oct. IS, 1919 Relation of Nutrient Medium to Plant Absorption 103 



on the sap expressed from the tops and roots of the plants grown in 

 water cultures, series 4. Measurements were made of hydrogen-ion concen- 

 tration, depression of the freezing point, and specific conductivity. It 

 was shown that the total osmotic pressure and conductivity increased 

 with increasing concentration of the nutrient solution. Analyses of the 

 sap of barley plants gave evidence of a very high concentration of ions, 

 many times greater than that found in the soil or nutrient solution. The 

 following figures give an idea of the magnitudes concerned: 



The hydrogen-ion concentration of the sap pressed from the tops of bar- 

 ley plants displayed a great constancy. The Ph value of 6. i was practi- 

 cally the same in the sap of plants from sand, water, and soil cultures, 

 even when the osmotic pressures and conductivities differed greatly. 



GENERAL DISCUSSION 



Having now presented the data obtained in the course of these inves- 

 tigations, it becomes necessary to survey the experiments as a whole, 

 with the intention of considering certain principles of plant nutrition. 

 It will also be desirable to correlate the present results with those of 

 other investigators. No attempt will be made, however, to give any 

 complete bibliography of the general subject. This has already been 

 done by Tottingham (45), Shive {39), and Pantanelli {32), among others. 

 Only such citations will be made as bear directly upon the questions 

 considered in this article. 



Before proceeding further in this discussion, we wish to call attention 

 to one very serious deficiency in the fundamental data necessary to 

 any extension of plant nutrition studies. There appear to be no ade- 

 quate and systematic experiments capable of showing the variability 

 of plants under favorable and unfavorable conditions, yet such a basis 

 of calculation is indispensable to any proper interpretation of yields, 

 when small variations of nutrient solutions are concerned. This factor 

 has been called to the attention of the author by the work of Waynick 

 {53), who has shown how very erroneous may be the conclusions derived 

 »from soil studies when the statistical method of interpretation is neg- 

 lected. These criticisms apply also, at least in large measure, to many 

 investigations in plant nutrition. In the interpretation of the data 

 obtained in our investigations there has been no intention of assigning 

 relative values to a large number of solutions. The whole purpose has 

 been to determine the general magnitudes involved and to explain 

 something of the nature of the course of absorption by the plant, with 

 the hope of gaining further knowledge of the principles involved. It 



