4 DISEASE IN PLANTS. 



research and suggest better ideas, but which 

 gained a hold over men's powers of reasoning 

 to an extent which now appears monstrous and 

 phenomenal. 



Many errors crept in during this reign of terror, 

 one of the most fatal of which was De Candolle's 

 revival of the idea of " spongioles " ; and another^ 

 equally disastrous in many of its effects, was the 

 conception of a sort of vegetable food-extract,, 

 humus, existing in the soil in a form peculiarly 

 suitable for direct use by plants. It was during 

 this period that the confusion between the pro- 

 cesses of respiration and carbon-dioxide assimila- 

 tion arose, and exerted its effects for evil into our 

 own day. 



The now astounding statement that oxygen - 

 respiration in plants did not occur, laid the founda- 

 tion of many subsequent difficulties, and so did 

 the positive and authoritative views on the uses of 

 minerals to the plant. Liebig, in fact, stood in 

 the invidious position of being a high authority on 

 purely chemical questions, who was impelled to 

 give opinions on matters which can only be solved 

 by physiological experiments : his great service 

 was to clear up mistakes as regards the chemistry 

 of soils and of plants his great mistakes were 

 due to his pronouncing on physiological matters ; 

 and it may be doubted whether his great services 

 to the purely chemical side of subjects connected 

 with agricultural matters are the more to be 

 admired, or the disastrous influence of his state- 

 ments on subjects which do not belong to the 



