Relation to Deleterious Chemical Agents 437 



of minor importance in plant work. General poisons are 

 usually either strong (such as salts of mercury), or weak 

 (alcohol) for all organisms. Nevertheless, plants may 

 show some specific adjustment to poisons, and diversity 

 in effect may be due to one of the following causes : - 



(1) A certain selective absorption may be shown, as in 

 the case of the nutrients, so that penetration will be rapid 

 in one case and practically prevented in another. 



(2) Upon penetration the deleterious substance may be 

 converted into a relatively insoluble and nontoxic form, 

 before effecting serious injury to the protoplasmic organi- 

 zation. 



(3) There may be specific differences in the effects upon 

 protoplasm, - - peculiarities which it is at present impos- 

 sible to explain definitely. 



One parasitic fungus may be killed by a dilute solution 

 of a copper compound, and another may germinate in a 

 relatively concentrated fungicide. Again, alkaloidal or 

 other toxic organic bodies may be produced within living 

 tissues, where they seem to set up no particular disturb- 

 ance ; whereas they may serve as strong toxic agents 

 when placed in contact with other cells or organisms. In 

 the fermentation of fruit sugar the common yeast plant 

 produces alcohol, which soon prevents the growth of other 

 micro-organisms. Brown has demonstrated a marked 

 selective permeability in the coverings of seeds of a variety 

 of barley. These seeds take up water from a fairly strong 

 solution of sulphuric acid, and remain uninjured; but 

 mercuric bichlorid penetrates them with comparative ease. 



264. Comparative resistance. - The fungi and bacteria 

 are commonly much more resistant to toxic agents than are 



