^ 12 



Determinations of sodium and potassium in the ash from sap obtained 

 on January 9, 19 15, from plants treated with potassium sulfate, were made 

 in order to show the increased intake of potassium. Similarly, determina- 

 tions of phosphorus were made upon the sap from plants fertilized with 

 disodium phosphate. The results, calculated to milligrams per cc. of 

 sap, are given in Table VIII . 



Table VIII. — Mineral Content of Sap. 



Effect of Overfeeding on Osmotic Pressure of Sap. — Sap was expressed 

 from the stems of shoots after freezing them with an ice-salt* mixture, 

 and the lowering of the freezing point determined by the method of Harris 

 and Gortner** of allowing supercooling until the solution froze and cor- 

 recting the value of A' obtained by the formula 



A = A' — 0.0125 mA' 

 where A' is the maximum temperature attained in the system and u the 

 difference between this value and the minimum temperature. The 

 relation between A and the osmotic pressure given by Lewis^^ in the 

 approximate equation 



■K = I 2 . 06 A 



was used in calculating the value for t. 



Description of Experimental Method. — Choosing a time when for two 

 or more hours previous no appreciable draft had been stirring the air in 

 the greenhouse, from four to eight shoots at the same stage of growth 

 were removed from each of the sections of plants and quickly taken to 



* Care was taken to select samples from the check and affected plants at the same 

 time of day and shoots in the same stage of growth were taken, to insure freedom from 

 variations in osmotic pressure due to differences in location and illumination, while 

 the fact that the sections studied were usually adjacent obviated the difficulty that 

 differences in temperature change the osmotic pressure of plants. See Dixon and 

 Atkins,^' Atkins,^ Ewart,'^ Drabble and Drabble, ''' Cavara.' 



** The method in genera! was an adaptation of that recommended by Gortner 

 and Harris. ^^"^^ Andre 1 and also Dixon and Atkins'^ have shown that successive por- 

 tions of sap expressed from unfrozen tissue become more concentrated, while the latter 

 have shown that the sap from frozen tissue always has a lower freezing point than that 

 from unfrozen, and that successive portions gave nearly identical lowerings, leading 

 to the conclusion that sap so expressed is representative of that originally within the 

 tissue. 



