Hoagland — 50 — Plant Nutrition 



sap largely uncontaminated by other components of 

 the cell can be recovered (Hoagland and Davis, 1929). 

 Thus it is possible to make definite comparisons of 

 internal and external fluids separated by a cell wall 

 and layer of protoplasm, to reduce this system to the 

 simplest terms. 



A few semi-quantitative analyses of sap had been 

 made on large marine cells — the Valonia cells, which 

 subsequently were to receive so much attention — but 

 the Nitella cells were of special interest for our pur- 

 pose because the pond water in which they lived and 

 from which they absorbed salt was after all not funda- 

 mentally different from some soil solutions in total 

 concentration of electrolytes. The analyses of Nitella 

 sap and pond water indicated quite clearly and un- 

 ambiguously that the cells must have absorbed all the 

 principal ions they contained against concentration 

 and activity gradients. The cells obviously had ab- 

 sorbed salt during their development, but from one 

 point of view they were highly impermeable to salt, 

 since they could be placed in distilled water and unless 

 they were injured or their metabolism damaged, prac- 

 tically no salt was lost to the water from the vacuoles. 

 {See plate 16). 



The vacuolar sap was found to be primarily a salt 

 solution and the ions were neither held to an ap- 

 preciable extent in an adsorbed state nor precipitated 

 out in the form of insoluble compounds. The electrical 

 conductivity of the sap was about what would be 

 expected on the basis of the total salt in the sap if 

 this was present in ionized form. The distribution 

 of ions could not be explained in its major aspects in 

 terms of the Donnan equilibrium, which comes into 

 play when an ion of one sign of charge cannot pass 

 through a membrane. After the general relation of 

 the ionic composition of the vacuolar sap of the cells 

 to that of the pond water medium became established 

 it was decided to try experimental procedures to deter- 

 mine whether or not additional amounts of ions could 



