112 MASS. EXPERIMENT STATION BULLETIN 175. 



place, but so far as the writer's observations go the virulence of the 

 causal agent is not lessened appreciably. This property is one of the 

 strongest points advanced by those favoring the theory of the presence 

 of a definitely organized parasite as the causal agent of the disease. It 

 is, however, entirely possible that enzymes or similar substances intro- 

 duced into a plant even in extremely small quantities, are capable of 

 regeneration of a certain kind, and indeed it is held by some that enzymes 

 do grow and even reproduce themselves under certain conditions. The 

 difficulties encountered in the study of this phase of enzyme work are 

 very great, however, and it is questionable if such statements can be as 

 yet definitely accepted. 



Organisms, even of the ultramicroscopic class, in their reactions would 

 not follow the law of proportionaUty, but in the case of mosaic sap and 

 its reactions we find, by measuring the relative activities and reactions 

 of the enzjTnes present that apparently a proportionality of reaction for 

 any one lot of sap does hold. The writer has very often found in the 

 measurement of the activities of the catalase and oxidase particularly 

 that not only a fairly definite relation exists between the various enzjTnes, 

 but that reaction of any one is dependent on the amount of sap used. 

 Of course, here we are deaUng with a mixture, and it may be open to 

 question if the measurement of the enzjone activities is a true measure 

 of the activities of the causal agent. 



The whole subject of the differential diagnosis of enzymes, toxins and 

 ultramicroscopic organisms is an extremely difficult one, and no sharpl}'' 

 dividing lines can properly be drawn between them. It would appear to 

 the writer that in some cases, at least, it is entirely dependent on the view- 

 point and interpretation of the investigator as to the class to which certain 

 diseases should properly be ascribed. 



The factors of reproduction and infection, as ordinarily understood, 

 have proved a stumbling block to the acceptance of the idea that there 

 may be other forms of matter aside from organisms capable of reproducing 

 a disease, but there is in reality very little real ground for taking this 

 attitude. In the case of the mosaic disease there are certainly many reac- 

 tions which will not allow of placing the causal agent in the class of ultra- 

 microscopic organisms. The general distribution of the causal agent in 

 a diseased plant, its exceedingly localized action on the meristematic 

 tissues, this action being apparently confined to the nascent chlorophyll, 

 the non-uniformity of response to apparently favorable conditions during 

 any one season even on one field, and also its individualism as shown by 

 plants growing together (one often diseased and the other not) are to the 

 wi'iter indicative of something of a different character. 



It is also possible that in the search after the infinitesimal the fact 

 that a highly organized plant as a whole may react in the same manner 

 as some of the simpler organisms has been overlooked. It is as a rule not 

 the presence of an organism alone which is responsible for the manifesta- 

 tions of disease, but the products of the metabolism of the organism. 



