44 S. E. A. McCallan 



described has been followed throughout, with the simple modification 

 that the aqueous suspension of spores was added to the solution or 

 suspension to be tested. The resulting spore suspension was then pipetted 

 as drops on the slides, and the germination tests were thus made. In order 

 to prevent undue dilution, a concentrated spore suspension usually was 

 added to the solution or suspension before the latter was placed on the 

 slides. Commonly one volume of spore suspension was added to four 

 volumes of solution or suspension; thus the latter was diluted to four-fifths 

 of its original strength. 



In all cases, for a specific test the percentage germination is based on 

 the counts from at least sixteen drops. Usually, however, this number 

 has been doubled, and not uncommonly it is the average of three or four 

 tests. The results are recorded as explained on page 21; that is, the per- 

 centage and the type (A, B, C, or D) of germination are recorded. The 

 A type of germination is the most vigorous, having germ tubes of approxi- 

 mately 200/x in length or longer; in B type, the germ tubes are 100 to 200m 

 long; in C type, they are 25 to lOO/i long; and in D type, they include 

 those just produced and those up to 25/* in length. 



It might be well to state here that the writer agrees with Clark 

 (1902:29), that it is more desirable to consider the point of inhibition of 

 germination rather than the death point, for, " if the germination of the 

 spores be inhibited by a concentration of a copper salt, it was merely a 

 question of time when they would be killed." 



After experimenting with various fungi, conidia of the fungus Sclerotinia 

 americana were finally selected as being the most suitable for these investi- 

 gations. Sclerotinia americana (Wormald) Norton and Ezekiel is the 

 common brown-rot fungus of stone fruits in North America (Ezekiel 1924, 

 Wormald 1928a, 1928 b). In culture it produces an abundance of fairly 

 large conidia. These spores germinate readily in water and with a high 

 percentage of germination. Standardized cultures (page 13) giving from 

 about 97 to 98 per cent germination were used in all tests. The spores 

 were from cultures five to twelve days old; distilled water was used for 

 spore suspensions; concentrations of spore suspensions showed from 

 fifteen to forty spores per microscopic field of 1575 p diameter; incubation 

 was at a temperature of 21° to 26° C; and spore-germination counts 

 were taken from 24 to 36 hours after planting. In certain cases, tests 

 with other fungus spores supplemented those made with S. americana. 



THE TOXICITY OF COPPER 



On the selection of Sclerotinia americana conidia for these investigations, 

 it seemed desirable to obtain more nearly accurate information relative 

 to the toxicity of copper toward these spores. For this purpose, dilute 

 solutions of copper sulfate in distilled water were made up. At infinite 



