N. M. Comber 3<i9 



root hair to make its natural attachment. Tiie importance of such pro- 

 perties has already received some recognition commercially: a process 

 has recently been patented^ whereby "insoluble phosphates... are con- 

 verted to a colloidal form and rendered suitable for use as manures by 

 treatment with a large quantity of water and about 0-1 to 0-3 per cent, 

 of a mineral acid or alkali in a high speed disintegrator... substances 

 which act as protective colloids, e.g. tannin, or salts of lysalbinic acid or 

 humic acid, or the like, may also be added."' 



According to the differences in the composition of the cell sap and of 

 the mucilage developed by the root hair, so plants will vary in their 

 power of attachment to mineral particles and in their power to dissolve 

 material from the particles to which they are attached. There is no 

 measure of availability apart from the plants concerned and the con- 

 ditions of their growth. 



Summary. 



The assumption that plants feed in the soil j ust as they feed in water 

 culture solution is unjustified and contrary to the facts. In modification 

 of the usual hypothesis two possibihties are discussed. 



1. The absorption of colloids by the plant. 



2. The union of the root hair with soil and other mineral particles 

 (so that the plant and the soil form one system) and the dissolution of 

 the particle by the organic matter of the root hair so attached. 



1 See J. Hoc. Chan. hid. I'.lL'i', 41, Nu. 10, 385 a. 



{Received August 11///. 1922. 



