vi.] ENTRY OF PLANT FOODS BY OSMOSIS 179 



soil water, because they are constantly being withdrawn 

 from solution by the living protoplasm of the cells. 



It has been supposed that solvent action of the soil 

 water is also assisted by the cell sap of the root-hairs, which 

 is always distinctly acid in its reaction ; these root-hairs 

 are always very closely in contact with soil particles, 

 and some of the acid has been supposed to diffuse out- 

 wards through the cell-wall. Sachs has shown that a 

 polished slab of marble is etched wherever the fine 

 roots of a plant came in contact with it, and on the 

 strength of this and similar experiments, the cell sap 

 has been regarded as a factor in bringing the minerals 

 of the soil into solution for the plant. All the solvent 

 actions, however attributed to the cell sap, can be 

 brought about by the carbon dioxide which is always 

 being excreted by the root, and more critical experiments 

 seem to negative the opinion that any fixed acids pass 

 outwards through the cell-wall of a living plant, at any- 

 rate after it has passed the seedling stage. 



The soil solution existing in the films of water 

 surrounding the soil particles is thus the nutrient 

 medium upon which the plant feeds, and if it could 

 be readily extracted from the soil it would probably 

 provide the best analysis of the soil as a means 

 of determining its richness or otherwise in available 

 plant food. But it is difficult to obtain by pressure 

 or other mechanical means the true soil solution 

 from the soil, and any extraction by water or otherwise 

 will at once upset the equilibrium existing between 

 the normal soil solution and the absorptive soil colloids 

 with which it is in contact. Still, an extract of the soil 

 with pure water does show such a variation in its 

 content in phosphoric acid and potash as would be 

 expected from the known character of the soil. For 

 example, a series of extracts was made from certain 



