No. 2, December, 1921] PHYSIOLOGY 115 



tion of work previously reported by the author (Staz. Sper. Agrarie Ital. 53: 233-243. 1920) 

 and is a contribution in respect to the principle of the apparatus used in the determinations. 

 [See also Bot. Absts. 10, Entry 739.] — A. Bonazzi. 



73S. Cas.\le, Luigi. Studio fisico chimico sul potere assorbente delle terra e sul mode 

 con cui le piante assorbono i materiali nutritivi dal terreno. [A physico-chemical study of the 

 absorptive power of soils and of the method whereby plants absorb nutritive materials from the 

 soil.] Staz. Sper. Agrarie Ital. 54: 65-113. 1921. — The soil colloidal particle is considered 

 as if coated by a membrane developed by a process analogous to the one that leads to the for- 

 mation of CuoFe (CN)6 membranes. The particles bear a positive or negative charge accord- 

 ing to whether they have yielded to the surrounding solution their anions or cations. Dif- 

 ferences of potential are thus established between the particles and a zone of ionic concentra- 

 tion surrounding them. The coagulating power of an electrolyte upon, and the degree of 

 absorption of its ions by, a negative colloid is directly proportional to its ionic concentration 

 and to the relative velocity of its cations, and it is inversely proportional to the solution ten- 

 sion of these same cations. Since the zone of ionic concentration surrounding a particle con- 

 tains also negatively charged ions, these will also be entrained and, if capable of forming in- 

 soluble precipitates, retained on the surface of the particle. Since the relative velocity of the 

 cations present in the zone of concentration regulates the position each one will hold in the shell 

 surrounding the particle, their order will be in a centrifugal direction K, NH3, Ca, Mg, and Na, 

 and the last ones to reach this shell will also be the ones most easily yielded to a new solvent 

 or electrolyte solution. Causes which vary the difference in potential at the particle-solution 

 surface will also affect absorption; thus basic silicates and humates, treated with boiling 

 HCl and washed free of the products formed therefrom, lose their absorptive powers because 

 of the few cations capable of being yielded to the solution. Organic and other positively 

 charged colloids act by virtue of the ion they yield, and can therefore act within certain limits 

 as protective agents in the coagulation of negative colloids by electrolytes, beyond which 

 limits they facilitate the phenomenon. They absorb both positively and negatively charged 

 ions, but when treated with salts, the metal of which has a lower solution tension than H, 

 the}' behave similarly to the negative colloids. — Absorption of soil constituents by plant 

 roots takes place by a process analogous to the above. The ectoplasm yields to the soil solu- 

 tion H ions, and establishes thereby a difference of potential between the plant and the soil 

 particles which, in a process of equalization of the unequal charges thus established, adhere 

 to the root cell and yield to it some cations. A continuity is thus established between the 

 soil and the plant, the more distant particles yielding cations to the nearest ones and these 

 in turn yielding them to the ectoplasm. Thence, equilibrium being continually disturbed by 

 the transfer of these materials to the interior of the cell and by their utilization therein, 

 currents are established which, by a play of osmotic forces, regulates the water intake and the 

 concentration of the zone of ionic concentration surrounding the cell. Absorption is enhanced 

 by the transfer of the water from the soil to the plant. When a plant is grown in a nutrient 

 solution the medium is found to increase in acidity, whereas if the solution is made to contain 

 a colloid the H ions are neutralized by the negative charge of the colloid particles and the plant 

 can endure far greater dilutions of nutrient salts. Besides, the removal by the plant of the 

 cations from the suspended particles, increases the negative charge on these and hence also 

 the degree of dispersion, with the result that the solution in the immediate vicinity of the roots 

 becomes clear while it remains cloudy at greater distances. That this phenomenon is not due 

 to a flocculation of the colloid is shown by the fact that equal quantities of solution taken 

 from the 2 zones yielded the following quantities of dry matter : After G and 15 days respectivelj* 

 there were in the clear portion 28 and 48 mg., while in the turbid part there were 8 and 4 mg. 

 The passage of ions from cell to cell leads to a partial utilization, the non-utilized portion 

 being poured, together with water (when the osmotic pressure of the cell contents is lower 

 than that of the bundles), into the vascular bundles. The latter, then, are not specific organs 

 for the transportation of water but only regulating receptacles. Fertilizers act by changing 

 the difference of potential between the plant root and the soil particle. — A. Bonazzi. 



