tree is not only in a position to develop the next set of leaves, but also to set 

 numerous fruit buds, which would have remained suppressed had a full crop 

 exhausted the store. When late frosts injure the blossoms and young fruit, 

 the individual organs are certainly severely sickened and fall off later ; but 

 the tree itself has the advantage of saving a quantity of food material. As 

 often happens, the cultural purpose can also profit in this case, because the 

 blossoms developing after the action of the frost yield more perfect fruit 

 and thus an increased revenue. 



This defines clearly the difference between pure and applied science. 

 Pure science studies the process of disease in itself and can be only cellular 

 pathology, while applied science takes into consideration the effect on the 

 diseased individual and its agricultural significance. We must unite both 

 forms of science since we take the purely scientific studies as the basis of 

 our consideration and explanation of the economic effects of the attack of 

 sickness. 



The consideration of the cultural needs forces us to the following 

 division of our subject; first of all, we will have to consider all cases which 

 threaten the individual aim of the organism, i. e. its longest possible life ; — 

 these are absolute diseases. Then we must discuss the disturbances which 

 the momentary cultural aim experiences and which we term relative diseases. 

 I'hese relative diseases may vary since what cultivation considers worth striv- 

 ing for to-day may be neglected to-morrow. For example, with savoy, every 

 reversion of the plant to Brussels sprouts is a disturbance of the cultural 

 aim to be avoided by changing the seed. If we intend growing Brussels 

 sprouts, however, each variation of these plants toward the savoy form is a 

 deterioration, undesirable in cultivation. Finally, malformations are usually 

 unimportant agriculturally but must be considered. Such malformations 

 may be a maturing of organs in a manner differing from the usual process 

 of development. These natural occurrences, which, we believe, may often 

 be traced back to changes in pressure conditions and other mechanical in- 

 fluences due to the formation of the organs, constitute a special branch of 

 knowledge, — Teratology. This is, however, to be considered as one branch 

 of pathology and we will have to draw into our discussion these phenomena 

 so far as their causes are known or may be surmised with some certainty. 



The method of treating the material which falls under the province of 

 the study of plant diseases or Phytopathology, will have to be according to 

 the following scheme:— 



I. Pathography or symptomatics, i. e., the description of the 

 disease according to its individual signs or symptoms. 



II. Pathogeny or etiology, namely, investigation as to the cause 

 of the disease. Only after the causes are known is it possible to 

 bring into use 



III. Therapy or the study of healing methods and to draw 

 into the discussion 



IV. Prophylaxis or some method of prevention. 



