46 EXPERDIEXT STATION. [Jan. 



gen j)t'r(.xi(le or some other residuary 1)_v-])ro(luct of })lanl me- 

 tabolism which has been allowed to remain nnassimilated in the 

 cells of the leaves throngh the excessive development and re- 

 tai'dation of certain enzymes. We have shown conclusively 

 in previous experiments that catalase in the diseased leaves was 

 present in far less amounts than in healthy leaves, and as it 

 has be*en found and proved by Loow (4) and other investigators, 

 j)rinci])any Jensen (5), that the chief function of catalase seems 

 to be its power of destroying free hydrogen peroxide, breaking 

 it up into water and oxygen, and this in so powerful a manner 

 as to be observable in the presence of the least amount of 

 catalase, nnd also as there was an excessive amount of other 

 enzymes present, of which we will speak later, we arc led to 

 conclude from the observations and experiments carried on by 

 us that the disease is not caused by a specialized virus. There 

 is no doubt whatever that the disease is infectious, but it may 

 be stated with equal certainty that the trouble is not con- 

 tagious. 



The disease being caused primarily by the excessive action of 

 certain enzymes and a corresponding reduction of function of 

 others, it is easily seen that any disruption of the normal 

 metabolism of a plant "will tend to produce the disease, and also 

 A. r. Woods has pointed out that the excess enzymes in de- 

 caying roots, etc., are able to start up these changes and so 

 produce the disease. The more technical experiments which 

 w^e have carried on supporting these views will be given in 

 another paper. 



The secondary causes are by far the most important to the 

 grower, as in many instances he can by careful attention re- 

 move or control the conditions, especialh^ in the seed bed. 

 Among these causes may be mentioned too high a temperature 

 in the seed bed, accompanied by excessive moisture or too little. 

 Soil moisture conditions alone do not seem to be of any effect 

 as a causative agent. This has been shown by various investi- 

 gators, and was not ]iarticularly studied by us. Where the 

 trouble shows in the seed bed and is not brought on by too high 

 a temperature it may be due to the following causes, as has 

 been dcfluccd from our observations and ex])(»riments: — 



The seed bed may have been used for several successive years 



