52 DISEASE IN PLANTS. 



of cells possess osmotically more powerful con- 

 tents at periods coincident with the escape of 

 the water from the now osmotically weaker root- 

 hairs. 



A little reflection will show that where we have 

 a group of such cells as the above, all capable 

 of absorbing water and dilute solutions and of 

 becoming turgid, movements of the absorbed 

 water must go on until all the cells are in 

 equilibrium, as regards their osmotic pressures. 



Now the living rootlet is just such a system, 

 the various cells of which are in different con- 

 ditions of osmotic pressure at any given time : 

 some of these cells are old, and their protoplasm is 

 allowing sap to filter out under pressure: others are 

 in the height of their vigour, and their protoplasm 

 extremely impervious to the highly osmotic sap- 

 constituents which it itself is forming actively : 

 others are too young to have attained their full 

 turgescence : while others again are in stages 

 intermediate between the above. 



There Js another point of importance, however, 

 to explain some peculiarities in the absorption of 

 these dilute solutions of salts, etc., by the root- 

 hairs from the soil, and by cells lying deeper in 

 the plant from these root-hairs. 



It is easy to understand that if a root-hair 

 absorbs a given substance say calcium sulphate, 

 for illustration -and hands it over to other cells 

 unchanged, a time must be supposed to arrive 

 when, the sap of all the cells being equally 

 charged with calcium sulphate, no more could be 



