ABSORPTION OF NUTRITIVE SALTS 411 



tissue of the roots. If the plant depended wholly on the 

 prepared solution in the soil water, a similar structure 

 would doubtless suffice. The special modification by 

 which the root-hairs come in intimate contact with the 

 soil particle and almost surround it, indicates a direct 

 relation between the soil particles and the plant,' and 

 not merely between the soil solution and the plant. 



New root-hairs are constantly being formed, and the 

 old ones become inactive and disappear. The contact 

 of a root-hair with a soil particle is not long-continued. 

 Whether the period of contact is determined by the 

 ability of the root to absorb nutriment from the particle 

 is not known. Certain it is that only a small portion of 

 the particle is removed. 



327. Possible root action on colloidal complexes. — 

 It has already been stated that there is some evidence 

 to lead to the belief that the surfaces of soil particles are 

 covered to a large extent with colloidal complexes, com- 

 posed of both organic and inorganic matter having vigor- 

 ous absorptive properties and holding the bases and phos- 

 phorus in an absorbed condition. Roots of growing 

 plants have been found to cause coagulation of at least 

 some colloids, possibly by leaving an acid residue in the 

 nutrient solution by reason of the selective absorption of 

 bases and rejection of the acids of the dissolved salts. 

 It is conceivable that the root-hair, by removing bases 

 from the solution existing between the cell wall and the 

 colloidal covering of the soil particle, may cause coagula- 

 tion of the colloidal matter and thus liberate the plant- 

 food materials held by absorption. The liberated ma- 

 terial, being of a readily soluble nature, would be taken 

 up by the solution between the rootlet and the soil particle, 

 from which the root-hair could readily absorb it. Such 



