64 DISEASE IN PLANTS. 



about by the presence or absence, relative pro- 

 portions and vigour, and specific nature of the 

 organisms in the soil at its roots, and it is easy to 

 see that many cases of disease may be due to the 

 absence of advantageous bacteria or fungi, or to 

 circumstances which disfavour their life, as well as 

 to the predominance of competing organisms. 



It will now be evident that the old points of 

 view must be abandoned, and with them, especially, 

 the widely prevalent notion that chemical analyses 

 of the plant and soil can explain the real problems 

 of agriculture. 



It was of course an enormous advance in the 

 science when, thanks to the splendid labours of the 

 chemists, at the end of the last century and the 

 beginning of this, we obtained that preliminary 

 knowledge of the constitution of the air, and of 

 the composition of the water, acids and salts, 

 etc., which plants require for their food-materials 

 and life-processes. Much was gained by De 

 Saussure's establishment of the fact of oxygen 

 respiration, though we now understand by the 

 term something very different from, and much 

 more complex than, what he understood by it, 

 as, also, much had been gained by the previously 

 acquired knowledge of the gas-exchanges in carbon- 

 assimilation : nor must we forget the services of 

 those who proved, by laborious analyses, continued 

 for long periods, what chemical compounds are 

 found in the tissues of plants, and in the soils at 

 their roots and the atmosphere which surrounded 

 them. We must also remember man)^ other con- 



