6l2 



LABORATORY EXERCISES 



the whole is sterilized. To each flask 2 c.c. of a 2 per cent, solution of ammonium 

 sulphate is subsequently added. The temperature for incubation is 3o°C (Fig. 

 216), This organism can be successfully grown on silicate jelly. As silicate jelly 

 is difficult to make it is optional for the students to attempt its manufacture. For 

 reference the method is given. ^ 



Pot Experiments with Nitrogen Fixation.-— ?>\nct the experiments of Hellriegel 

 and Wilfarth and other experimenters, it has been known that certain bacteria 



Fig. 216. — Double-walled copper incubator constructed with non-conducting 

 materials, with water gauge and openings for insertion of thermometer and thermo- 

 stat. Padded outer door of copper, inner door of glass. (Fig. 22, p. 46, Schneider', 

 Pharmaceutical Bacteriology, 191 2.) 



{Bacillus radicicola, etc.) have the power of fixing free atmospheric nitrogen, when 

 they enter the roots of leguminous plants with the formation of root nodules. The 

 formation of these nodules can be followed in a series of experiments. 



^ It is optional of course for the teacher to omit these rather difficult exercises 

 entirely. If followed by the student or class, a useful work to consult in connection 

 with Lesson 17 is Smith, Erwin F.: Bacteria in Relation to Plant Diseases, I : 36-39. 



2 An important paper on the culture and isolation of Bacillus radicicola is by 

 Harrison, F. C. and Barlow, B.: The Nodule Organism of the Leguminosae— Its 

 Isolation, Cultivation and Commercial Application. Centralblatt fiir Bakteriologie, 

 Parasitenkunde und Infektionskrankheiten, 19, Abt. 2, 1907: 264-272, 426-440, 

 pis. 9. Consult for other details Lipman, J. G. and Brown, P. E.: A Laboratory 

 Guide in Soil Bacteriology, 191 1. 



