MOSAIC DISEASE OF TOBACCO. 105 



an extended explanation. The results obtained by this method in every 

 case showed the diseased leaves to contain much more oxidases than the 

 healthy ones of the same age; this was also true for peroxidases, but here, 

 of course, the reaction with guaiac was somewhat masked owing to the 

 presence of the oxidases and their reaction. 



In examinations of the roots of healthy and diseased plants the same 

 condition was observable; there was always an excessive activity of the 

 oxidizing enz3''me to be noted. 



In going over the results of the experiments with the enzymes in ques- 

 tion, the main point brought to the attention is that there is in all diseased 

 plants an excessive activity of the oxidizing enzymes, and a corresponding 

 decrease in the activity of the diastatie enz3rmes and catalase. This at 

 least indicates a very much disturbed equiUbrium and a consequent 

 derangement of normal function on the part of the cells. Naturally the 

 ones most affected by this disturbance are the dividing or meristematic 

 cells, as these are the cells upon which the plant is dependent for its sub- 

 sequent growth, and any deviation from the normal is more Ukely to be 

 indicated in the development of these ceUs than in those of the other 

 parts of the plant. Any change in function induced here will leave its 

 imprint to a greater or less extent on the cell during its subsequent exist- 

 ence, hence the peculiar manifestations of the disease in the leaves. 



It is true that plants attacked by parasites sometimes show an exces- 

 sive activity on the part of certain enzymes, but, as a rule, the disturb- 

 ance is more local in its nature. It is also a fact that malnutrition, such 

 as partial starvation, drought, etc., will bring about an excessive produc- 

 tion or activity of the oxidizing enzymes in particular, as has been pointed 

 out by Bunzel, of general distribution throughout the plant; but tliis, 

 except in cases of maturing plants, changes upon restoration of normal 

 conditions, and tends to become normal. 



Reaction of Mosaic Sap with Various Substances. 



We have seen that the enzymatic activities of the plant are very much 

 disturbed in disease; also that it has been impossible to demonstrate the 

 presence of any forms of bacteria or fungi either in the tissues themselves 

 or in the expressed juice. 



It is a fact, as shown by practically aU investigations, that the disease 

 is very infectious. This fact alone in the minds of many is sufficient to 

 place the causative agent among the parasitic organisms. The field, 

 however, is hmited to that class of organisms designated as "ultramicro- 

 scopic" organisms, about which very little is known, and in the case of 

 plant diseases not even a semblance of the demonstration of the activities 

 of such organisms has been made. 



Owing to the fact that the enzyme activities are much changed, as has 

 been demonstrated in the preceding pages, and also to the fact that not 

 only the activities of the oxidizing enzymes are changed, but also the 



