62 THE BIOLOGY OF FLOWERING PLANTS 



obtain further supplies. At still lower water- contents, after 

 the root hairs are plasmolysed, and especially after the death 

 of the plant, water is no longer absorbed osmotically ; the 

 imbibition forces of the colloids of the cell walls and of the 

 protoplasm come into play. The osmotic absorption of 

 water by normally functioning root hairs must go on much 

 longer in such desert plants as those studied by Fitting than 

 in ordinary mesophytes. Taking the soil used by Shull, 

 one might expect a mesophyte to absorb water osmotically 

 down to a moisture content of about 17 per cent., while a 

 desert plant might do so down to 12 per cent. 



Bouyoucos (1921) identifies his " unfree water " with the 

 wilting coefficient. It seems more probable that it lies 

 lower in the scale of soil moisture, and that the plant wilts 

 while there is still " free water " in the soil, though 

 Bouyoucos refers to this fraction as " physiologically very 

 available." The capillary-absorbed fraction of the unfree 

 water (*' physiologically slightly available ") which freezes 

 below 4° C. must be held with a force of over 50 atmo- 

 spheres and can hardly be absorbed osmotically. It is 

 difficult, however, to give Bouyoucos' fractions a physio- 

 logical meaning at present. 



§ 5. The Absorption of Salts from the Soil 



SoU Solution and Salt Supply. — The root hair absorbs 

 not only water but also the necessary mineral nutrients. 

 Something has already been said of the constitution of the 

 soil solution. It is a weak solution of mineral salts momen- 

 tarily in equilibrium with the soil colloids ; its concentration 

 varies primarily with the soil constitution, and also, and 

 very markedly, with the degree of moisture ; its constitu- 

 tion depends on the nature of the soil and, in cultivated 

 ground, on the manuring ; its reaction depends on its 

 salt content, on the nature of the soil colloids, on the acids 

 of the humus, and on the lime- content of the soil. 



Hall, Brenchley and Underwood (1914) state that the 

 concentration of the soil solution has an important effect 



