48 DISEASE IN PLANTS. 



elastic reaction of the cell-wall and the distending 

 power of the absorbed water. When this limit is 

 reached, water begins to filter back again through 

 the cell-wall. Numerous researches during the 

 last fifteen years have shown that the sap of such 

 a living cell as the root-hair is charged with sub- 

 stances of various degrees of osmotic power ; 

 bodies like sugars, amides, vegetable acids and 

 their salts, being formed by the metabolic activity 

 of the protoplasm and accumulated there. More- 

 over, we now know that the salts of the vegetable 

 acids in particular are effective, and the researches 

 of Warburg and Palladin in 1886 have placed it 

 beyond reasonable doubt that these acids are con- 

 tinually being developed and destroyed in the 

 living cell during normal growth and respiration, 

 and that great variations as to quantity may be 

 brought about by alterations in the conditions 

 of the environment e.g. temperature, oxygen, etc. 



If, now, we bring a solution of some salt, such 

 as potassium nitrate, which has a powerful attrac- 

 tion for water, on the outside of the living root- 

 hair, the question whether the water remains in 

 the cell, or passes out of it, merely depends on 

 whether the substances inside or that outside have 

 the most powerful attraction on the water in the 

 sap, since the protoplasm allows water to pass 

 freely. 



But the protoplasmic lining may affect the 

 whole matter in another way ; for it may allow 

 the dissolved salt, or other substance, in the 

 solution outside or inside the cell to pass through 



