294 MANUAL OF MICROBIOLOGICAL METHODS 



which a cut is made into the vascular system, so that the hquid is taken 

 by the plant directly into the transpiration stream. The stem can be 

 opened to form a small cavity which is kept filled by means of a capil- 

 lary tube and funnel. If an enzyme hke pectinase is being tested, thin 

 sections of tissue need merely be immersed in a few drops of the liquid. 



So many substances appearing in cultures influence plants in one way 

 or another that rigid controls are necessary in searching for the prod- 

 ucts responsible for pathogenicity. Whenever feasible, an attenuated 

 culture of the same organism or a closely related nonpathogenic culture 

 is carried in a parallel series of trials. 



The methods of testing for plant ''hormones" and ''vitamins" are 

 being improved so rapidly that one should consult an active investiga- 

 tor for the latest procedure. 



ANTIBODY PRODUCTION 



Questions on the development of antibodies in plants following inocula- 

 tion or natural infection are discussed in a considerable literature reviewed 

 by Chester (1933, 1935). A number of controversial points are involved. 



The injection of plant bacteria into an experimental animal com- 

 monly results in the production of antibodies useful for various investi- 

 gations. Suitable methods appear in Chap. IX. 



COGNATE CONSIDERATIONS 



Strain variations. When studies involving strain variations are 

 made, it is well to consider Frobisher's (1933) comment, "Plating and 

 fishing of colonies, while generally useful, is not a suflaciently reliable 

 method of purifying cultures in work involving bacterial variations. 

 It is sometimes extremely difficult, if not impossible, to separate bac- 

 terial species by this means. Single-cell methods are much more reliable 

 and, it would seem, furnish the only satisfactory means of solving our 

 problems, but even such procedures as are at our disposal require very 

 expert manipulation and may lead to error." The relative unreliability 

 of the poured-plate technique for such studies has been discussed by 

 Riker and Baldwin (1939). The need for cultures with a known origin 

 from a single cell has stimulated much work on methods for securing 

 them. Literature on this work has been reviewed by several writers, 

 e.g., Hildebrand (1950). Unfortunately, some reports on bacterial 

 variations have appeared in which the cultures were purified merely 

 by several successive dilution plates, and such purified cultures were 



