SPECIFIC DISEASES. 41 



great variety of plants. The best example we have is the recently discovered crown-gall 

 bacterium. So far as we yet know, Bact. tumefaciens is the only organism capable of pro- 

 ducing the crown-gall, but it can do this in plants belonging to an astonishingly large num- 

 ber of families, i. e., in not less than 18 widely separated ones. In other words, it is an 

 organism, or a closely related group of organisms, with a very generalized and simple set 

 of requirements such as many plants are able to offer. Diseases superficially resembling 

 crown-gall may, however, be produced by a number of organisms, c. g. on sugar beet by 

 Bacterium beticolum, on olive by Bacterium savastarioi. 



In the matter of the experimental production of parasites, or better, let us say, in the 

 testing of all sorts of bacteria in all sorts of plants to learn their behavior, we are only at the 

 beginning, and some statements already made must be taken with a grain of salt. 



Some useful knowledge would undoubtedly result from systematic experiments of 

 this sort, but much time and labor would be necessary to go over the field even in a cursory 

 way. During such inquiries it is not unlikely that one might stumble upon certain active 

 parasites, cell-wall destroyers, etc., but the waste of time in testing a miscellaneous lot of 

 organisms would be very considerable, and after all it might be questioned very properly 

 whether this is really the best way to approach the problem. A better way would be to 

 begin with organisms known or suspected to have particular actions, and first determine 

 the nature and extent of these actions. With this end in view, the writer has been in the 

 habit of taking organisms known to be pathogenic to certain plants and testing them in a 

 variety of other plants to determine their behavior. He has also made some experiments 

 with saprophytic forms. Nothing has been observed, however, which favors the view that 

 non-parasitic forms can be induced readily to adopt a parasitic life. To be or to become a 

 parasite an organism must be endowed with certain peculiarities adapting it to conditions 

 as they occur in particular plants or animals. The environment must be congenial. An 

 organism may never have functioned as a parasite, but if it possess these necessary pecu- 

 liarities it is capable of becoming one when introduced into the plant or animal and then 

 we shall have a new disease. Such an organism is from the beginning a parasite in posse, 

 if not in esse, and it is only our ignorance of such facts that would ever lead us to suppose 

 that we can easily convert all sorts of saprophytes into parasites. 



The most important papers are those of Laurent. Lepoutre, van Hall, and Jensen. 

 (See also individual and varietal resistance.) The subject is so vital that I have abstracted 

 papers by the above named writers at some length. 



THE EXPERIMENTAL PRODUCTION OF PARASITES. 



In 1899, Laurent published an account of his experimental researches upon diseases of plants, 

 dealing especially with the influence of foods upon the resistance of plants to parasites. 

 His field of experiment was in good clayey soil, containing, according to an analysis: 



Organic matter 54-4 per cent. 



Total nitrogen 1.70 per cent. 



Lime 15.60 per cent. 



Potash 0.96 per cent. 



Phosphoric acid 3.03 per cent. 



Four equal plots were laid out, and received the following doses of fertilizer per hectare : 



Plot I. 1 100 kg. of sulphate of ammonia. 



Plot II. 2200 kg. of kainite, containing 13 per cent of anhydrous potash. 



Plot III. 2200 kg. of superphosphate of lime, containing 15 per cent of anhydrous phosphoric acid. 



Plot IV. 15,500 kg. of quick lime (chaux grasse). 



Upon each plot potatoes (var. Simson) and carrots (var. Nantaise), and other species, were 

 planted early in April. The tubers and roots harvested in October were used for experiment the 

 following February. 



Slices of both potatoes and carrots from the four plots were placed under a moist bell jar and 

 were sowed with conidia of Bolrytis cinerca which had been cultivated upon gelatinized must of 



