50 DISEASE IN PLANTS. 



in the sap-vacuole, but which are not permitted 

 free egress through the protoplasm (and the 

 formation of such bodies will occur if the pro- 

 toplasm is actively respiring), the conditions for 

 absorption of water, with or without any dissolved 

 salts, which the protoplasm allows to traverse it, 

 are set up. 



But the above supposed case is realised, as 

 Pfeffer showed in 1886, when he found by a series 

 of beautiful experiments that certain aniline dyes 

 can accumulate in living root-hairs, and other 

 living cells, whereas others cannot pass the living 

 protoplasm. After accumulating for some time, 

 the dye may either remain stored there, or may 

 eventually diffuse out. 



Pfeffer made another discovery, of equal im- 

 portance, namely, that under the influence of 

 dilute organic acids, such as citric acid, the perme- 

 ability of the living protoplasm may be altered, so 

 that it allows substances to pass which could not 

 otherwise have traversed it. De Vries had also 

 shown that the condition of the protoplasm affects 

 its power of retaining the colouring matter in the 

 sap of the Beet : so long as the protoplasm is 

 alive, the crimson sap is retained, even when the 

 cell is plasmolysed, but immediately it begins to 

 die the colour escapes through it. A similar 

 case exists when the chlorophyll-corpuscles 

 retain their colour in living cells known to be 

 charged with acids : so long as the protoplasm 

 is alive and normally active the green bodies 

 are protected. 



