204 ABSORPTION OF FOOD-MATERIALS 



insoluble in pure water, dissolve appreciably in water containing 

 carbon dioxide. 



It must also be noted that carbon dioxide, potassium hydrogen 

 phosphate and other substances possessing an acid reaction 

 permeate the cell-walls of the root-hairs, and enable the latter 

 to corrode and dissolve certain mineral compounds such as 

 calcium phosphate and the carbonates of calcium and magnesium 

 with which they come into contact. 



3. When the roots of a plant are immersed in a vessel of water 

 containing a substance in solution, the dissolved substance may 

 not be able to pass through the cell-wall or the cytoplasm of the 

 root-hairs in which case none enters the plant. If, however, the 

 substance can diffuse through both cell-membranes, it will pass 

 into the root-hairs and from there into the rest of the cells of 

 the plant until the cell-sap contains the same proportion of it 

 as the water outside the plant; when this condition is reached, 

 equilibrium is established and no more of the dissolved material 

 is absorbed. Should the substance after entering the plant be 

 used up in the processes of nutrition, or changed into an insoluble 

 or non-diosmosing compound, the osmotic equilibrium in regard to 

 this particular material is destroyed, and more of it can then enter. 

 In this manner a plant is able to completely extract the whole 

 of a substance dissolved in water to which its roots have access, 

 and can accumulate within itself large amounts of certain elements 

 from solutions containing the merest traces of them. For example, 

 sea-water contains not more than one part of iodine in 100 millions 

 of water, and yet certain sea-weeds accumulate such quantities 

 that from i too 3 per cent, of their ash consists of this element. 



The total amount of any particular element occurring in the 

 ash of a plant is dependent (i) upon the amount of the soluble 

 material containing it present in the soil upon which the plant 

 is growing ; {2) upon the peculiar specific permeability of the 

 protoplasm of the root-hairs ; and (3) also upon the question 

 of whether the plant utilises, transforms or removes the par- 



