12 S. E. A. McCallan 



necessary, the spore suspension is filtered through cheesecloth or cotton to 

 remove bunched spores and pieces of mycelium or of substrate. The spore 

 concentration of the suspension is regulated by dilution until, as ascertained 

 by examination of representative drops under the microscope, the desired 

 concentration has been attained. Test tubes have been found most suit- 

 able for holding the suspension, since with these it may be readily agitated 

 and kept uniform. By means of a pipette that will deliver about twenty 

 drops to a cubic centimeter, the spore suspension is pipetted as drops on the 

 slides. The optimum number of drops per slide is four. A greater num- 

 ber tend to run together, and a less number results in a decrease in manual 

 efficiency and loss of replications. There will thus be sixteen drops to a 

 chamber. To insure that the drops contain as representative a number 

 of spores as possible, it is well to draw a fresh pipette-full of spore sus- 

 pension for at least every two slides. Only one fungicide should be placed 

 in a chamber; experience has taught the necessity of this. The moist- 

 chamber cover is put in place, and water is poured around it into the 

 bottom. This seals the chamber and preserves an atmosphere approach- 

 ing saturation. Drops may be kept in this manner for a number of days 

 before complete evaporation results. A moist chamber completely equipped 

 and with the drops of spore suspension present, but just prior to the final 

 act of sealing with water, is illustrated in figure 1, b. Controls are of 

 course maintained, drops of spore suspension being placed on clean blank 

 slides. One moist chamber containing four slides will usually constitute 

 a satisfactory control. Thus the spores are allowed to germinate undis- 

 turbed at their optimum temperature. When the time has elapsed 

 (determined for the particular fungus) in which a maximum number of 

 spores in the control have germinated, producing vigorous germ tubes, 

 the results may be taken by recording the percentage of germination and 

 the general vigor of the germ tubes. 



IMPORTANT FACTORS * 



In conducting these tests there are a number of factors that must be 

 taken into consideration. For convenience these may be discussed in 

 the order in which they appear, and may be classified under the following 

 general headings: clean glassware, standardization of the fungus and of 

 the conditions for spore germination, and application of the fungicide. 

 Disregard of any of these factors gives unsatisfactory results. 



CLEAN GLASSWARE 



It is very essential that all the glassware be clean. The free use of 

 chromic-acid cleaning mixture is necessary, for the slides must be chem- 

 ically clean. The cleaned slides should be stored and protected from 

 dust. At the end of each experiment, the moist chambers and the slide 



