METHODS OF INVESTIGATING PLANT DISEASES. 175 



opened up a hidden world to eager investigators, but it alone 

 was entirely inadequate to the solution of the manifold problems 

 that presented themselves. Second, prior to 1880 there was no 

 accurate and at the same time convenient method of making 

 pure cultures of bacteria and fungi. It is the application of the 

 perfected methods of making cultures, to the study of plant 

 diseases that has been the most productive of results in recent 

 years. The plant pathologist now has besides his ordinary 

 microscope, a modern stand fitted with immersion lens and 

 condenser; he has cupboards full of glass-ware and analine 

 stains; his laboratory is equipped with imbedding baths and 

 microtomes, hot-air sterilizers, steam sterilizers for streaming 

 steam and steam under pressure, incubators, test-tubes, Petri- 

 dishes, damp chambers, and many other appliances that I need 

 not mention. 



I must tell you about one term that I have used, pwre culture. 

 If the commercial grower of flowers had to grow his roses and 

 carnations all jumbled together in one bed, and possibly over- 

 run with weeds of various kinds, his difficulties would not be 

 greater than that of the botanist who wished to study bacteria 

 and fungi a few years ago. The horticulturist segregates his roses 

 and carnations and keeps out the weeds. The plant pathologist 

 at the present time can for example separate his blight bac- 

 teria from the common bacteria of decay and moulds that might 

 overrun them Hke the gardner's weeds, and grow them under 

 conditions to which they are suited on some sort of an artificial 

 medium. The medium is to the bacteria and fungi, what the 

 soil is to the green-house plant. Just as the gardner has to use 

 different kinds of soil for different purposes, so the pathologist 

 has to use different kinds of media for his bacteria and fungi, 

 only he deals with creatures that are far more delicate in their 

 requirements than any green-house plants, and consequently 

 must prepare many different kinds of soils to suit the varying 

 needs. 



One single kind of an organism, a disease germ for example 

 growing on some medium, with all others excluded, is a pure cul- 

 ture. If you should grow your carnations in a bed in such a way 

 that it was impossible for any weeds to get into the soil, you 

 would be doing only similar to what the pathologist must do 

 for his disease organisms. 



