LABORATORY AND TEACHING METHODS 63 1 



Second Exercise. — The members of the class can raise material for such morpho- 

 logic study after the first exercise has been completed by partially filling test-tubes 

 with such materials as chopped hay, prunes, lima beans, split peas, cracked oats and 

 cabbage leaves, adding water, and treating, as follows: 



One set of tubes should be plugged and thoroughly sterilized by differential 

 sterilization. This experiment, after examination of the material under the micro- 

 scope, demonstrates that bacterial growth in the tubes does not take place. 



A second set of test-tubes can be left open to the air after the water and the 

 culture material have been completely sterilized. This gives the organisms that 

 come from the air. 



A third set of tubes can be partially filled with water, plugged and then sterilized, 

 and after sterilization unsterilized material can be added. This gives the organisms 

 that enter through the vegetable substance. 



A fourth set of tubes can be filled with the culture material, plugged and steril- 

 ized. Unsterilized water can be then added to each of these tubes. This gives the 

 microbes that come in through the water. These are rough methods adapted to 

 general class work, and in each case the organisms which appear should be mounted 

 and systematically studied to determine the different generic forms which are present, 

 as far as that can be done by staining methods and the microscope. 



Third Exercise. — The teacher can distribute material of diseased plants in which 

 the disease is directly traceable to some bacterial organism. For this exercise, the 

 professor should have a stock of at least a half dozen diseased plants properly fixed 

 and preserved in 50 per cent, alcohol. The material, which. has been distributed, 

 should be cut free-hand by the student and the sections mounted as directed, or the 

 student can imbed the material in celloidin, or in paraffin, to secure thinner serial 

 sections by the use of a sliding, or rotary microtome. To carry on this exercise, the 

 student should have an acquaintance with celloidin and paraffin technique. 



Fourth Exercise. — Where the student has plenty of time and expects to specialize 

 in the study of the bacteria] diseases of plants, then he, or she, should follow the 

 following scheme suggested by Chester in his "Manual of Determinative Bacterio- 

 logy,'' the descriptions and keys of which can be used in a detailed systematic 

 study of bacterial organisms. This exercise can be pursued only after the student 

 has learned cultural and isolation methods and not at the beginning of a course in 

 mycology and its technique. 



LESSON 27 



Scheme for the Study of Bacteria. — The Society of American Bacteriologstis has 

 adopted a numeric system of recording the salient characters of an organism (group 

 number). 



100 Endospores produced. 



200 Endospores not produced. 



10 Aerobic (strict). 



20 Facultative anaerobic. 



30 Anaerobic (strict). 



