SOLVENT ACTION OF ROOTS 223 



the larger proportion are adsorbed, that is, retained in the undissolved 

 condition by virtue of the molecular forces resident in the minute 

 soil-particles. The adsorbed nutrient materials include, for example, 

 salts of potassium and ammonium, as well as phosphates and iron- 

 compounds. We need not concern ourselves here with the chemical 

 and physical forces that underlie this phenomenon of adsorption. It 

 will suffice if we imagine the adsorbed food-materials to exist in the 

 form of a very fine precipitate which envelopes each tiny soil-particle, 

 being in its turn surrounded by films of water of varying thickness ; 

 for the water in the soil is also, in part at least, attached to the soil- 

 particles by molecular attraction. 



In the light of these facts we can now fully comprehend the 

 significance of the close connection amounting almost to fusion 

 which exists between root-hairs and soil-particles. It is only through 

 this intimate contact that the root-hairs are enabled to withdraw 

 water and adsorbed nutrient material from the soil by which they are 

 so firmly retained. 



The absorption of mineral salts is further greatly facilitated by 

 the circumstance that roots secrete acid substances and thus exert 

 a solvent action upon the surrounding medium. This fact is easily 

 demonstrated by a simple experiment, which was first performed by 

 Sachs. 119 A slab of polished marble is covered with a moderate 

 thickness of soil, in which a plant is allowed to develop its root- 

 system ; the roots that come in contact with the slab corrode its 

 surface, and after a time produce a very distinct etched pattern. The 

 lines of such root-prints are very sharply defined, a fact which 

 indicates that the acids secreted by the absorbing tissue do not 

 distribute themselves throughout the soil-water, but are on the con- 

 trary largely retained in the water of imbibition of the cell-walls 

 and in the innermost of the water-films that adhere to the outside 

 of the roots. This circumstance furnishes an additional reason for 

 the exposure of the greatest possible surface on the part of the 

 roots. 



Czapek has shown that root-prints are produced, mainly if not 

 entirely, owing to the excretion of carbon dioxide. According to 

 Stoklasa and Ernest, organic acids {acetic acid and formic acid) are 

 only excreted when the roots are insufficiently supplied with oxygen. 

 As soon as any solids have been brought into solution, they can readily 

 penetrate as far as the ectoplast which regulates the diosmotic inter- 

 changes of the root-hair since the wall of the hair is, with few 

 exceptions, not more than "00 0G to '001 mm. in thickness. At the 

 rounded tip of the hair the wall is often twice or even thrice as thick 

 as in the other portions, an arrangement which evidently assists the 



