42 DISEASE IN PLANTS. 



rigid, and the thin protoplasmic fihn close pressed 

 against the cellulose cell-wall. 



Nothing whatever can pass into the cell-sap, or 

 out from it, without traversing both the lining of 

 living protoplasm and the cell-wall. 



If we gently pull a living root, of wheat, pea, 

 mustard, etc., from a normal soil, we find particles 

 of soil so closely adherent to the root-hairs that 

 they cannot all be washed off without tearing the 

 hairs : the root-hairs establish relations of contact 

 with these particles, so close that they are cemented 

 to the solid surfaces by means of the gelatinous 

 layer already referred to. This peculiarity has 

 the following consequences. In the first place, the 

 enormous holdfast, ensured by the millions of 

 points of adherence, enables the plant to withstand 

 even powerful lever actions from above, and pro- 

 vides fixed points against which the root-tips can 

 work as they drive deeper into the soil. In the 

 second place, the intimate contact of the root-hairs 

 and particles of soil, ensures that the films of 

 water held by surface-action on the soil-particles 

 and root-hairs shall be in continuity with the 

 water saturating the cell-walls of the latter, and 

 therefore with the protoplasm and cell-sap in their 

 interior. The importance of this at periods when 

 the soil is " dry " will be obvious, when we reflect 

 that no soil is ever naturally so dry that surface- 

 films of water are absent from the particles. 



The fact that the root-hair contains living- 

 protoplasm, enables us to understand to a certain 

 extent the results of the following experiments. 



