N. M. CoMBKH 367 



water content of plants '^. Its bearing on the mineral nutrition of plants 

 still awaits consideration. 



Now it is a well-known Ijotanical fact that when the root hair comes 

 into contact with a soil particle the outer la)'er of the root hair is trans- 

 formed into a mucilage and an indissoluble attachment is made with the 

 soil particle. One relation of the soil colloids to the plant is here apparent. 

 The soil particle is coated with iiydrophilous colloid. The root hair be- 

 comes coated with hydrophilous colloid. By the union of these colloids 

 the plant and the soil particles become cemented together. Tlie particles 

 so attached to the plant cannot be removed without damage to the root 

 hairs. 



Underlying the present teaching is the conceptiou of the root hair 

 "dipping into" the soil solution, and taking up its nutrient material in 

 exactly the same way as it does from experimental water culture solutions. 

 It seems, however, important to acknowledge the fact that by the union 

 of their respective colloids the plant and the soil form one si/stcm and not 

 I wo si/steni.? in mere contact, and to admit the possibility that the mechanism 

 of the nutrition of plants grown in soil is not necessarily and entirely the 

 same as the mechanism of the nutrition of plants grown in water solution. 

 The physical possibilities of the union of the plant and the soil. The 

 migration of ions in the colloidal complex by which the plant and soil are 

 united has been discussed by Casale^. He argues that positive and negative 

 colloids exist in the soil and that the charge on them is due to their 

 throwing off anions and cations respectively. The negative colloids are 

 dominant and absorb the positive colloids. The colloids of the plant, 

 according to Casale, throw off hydrogen ions and are negatively charged, 

 but less so than the soil colloids. Hence there is a potential difference 

 between soil and plant and a migration of cations from the soil to the 

 plant accordingly. Equilibrium is never reached, because the cations 

 pass on into the plant by a similar electrical mechanism. 



Sufficient seems to be known about the different electrical charges on 

 various parts of the plant cells to indicate that a migration of ions under 

 difference of electrical potential probably plays an important part in 

 plant nutrition and cannot be left out of account in a consideration of 

 the availabihty problem. 



The chemical possibilities of the union of the plant and the soil. When 

 the root hair and the soil particle are cemented together there is an 

 obvious possibility of the cell sap of the root hair dissolving material 



1 See Sludl, rmiix. Faraday Soc. 1922, 17. 



= Slaz. spa: agr. ital. 1921, 54; J.C.S. Abs. (i), 1922, 509. 



