62 LON A. HAWKINS 



the presence of the lighter metals in various concentrations might or might 

 not decrease the toxic effect of the heavy ones. It was also considered 

 worth while, when such a decrease was found to occur, to determine as 

 far as possible whether this influence might be related to a direct effect of 

 salts on each other in the solution or to some modification of the organism 

 itself. Furthermore the results obtained in these experiments should throw 

 some light on the problem of the comparative toxicity of the various sub- 

 stances here employed, when used alone, and thus upon the general physio- 

 logical problem of toxicity. 



The investigation was carried out at the Laboratory of Plant Physiology 

 of the Johns Hopkins University, and the writer's sincere thanks are due 

 to Professor Burton E. Livingston for his many helpful suggestions and 

 valued assistance throughout the progress of the work. 



ORGANISM. 



The fungus spores used in this research were of the Gloeosporium or 

 conidial stage of Glomerella cingulata (Stonem.) S. and v. S., the fungus 

 causing the disease of the apple known as " bitter rot." Not only is the 

 fungus parasitic upon the apple, but according to Shear and Wood, 13 it 

 is also the cause of disease on other plants. On the apple fruit it produces 

 brown, sunken areas, usually nearly circular in shape, which may be covered 

 with the fruiting bodies of the fungus, the conidia being borne in acervuli. 

 In mass, the spores appear orange-colored but have a hyaline appearance 

 under the microscope. They are usually ovate or oblong in shape, the two 

 diameters being 12-16 p and 4-6 p. Shear and Wood have shown that cer- 

 tain strains of this fungus, when grown on proper artificial media, may 

 produce conidia for generation after generation, without the interpolation 

 of the ascogenous form at any time. Cultures of such a strain were secured 

 from Dr. Shear for these experiments, and conidia only were produced 

 throughout the investigation, which lasted; about eighteen months, though 

 forty or more generations must have passed. 



Corn meal agar was used as a medium for the stock cultures. This 

 was prepared by adding 4 teaspoonsful of white corn meal to one litre of 

 distilled water, which was then allowed to stand at about 58 C. for one 

 hour. After filtration, 12.5 g. of agar flour was added to the infusion, 

 and the mixture was steamed for one and one-half hours. It was then 

 re-filtered and was ready to tube and sterilize. The tubes were slanted 

 and the stock cultures were inoculated in streaks. On this medium the 

 fungus produces but little mycelium, at or beneath the surface, and the 

 conidia are borne in relatively large, orange-colored masses on the surface 



13 Shear, C. L., and Wood, Anna K., Studies of fungus parasites belonging to the genus Glomerella. 

 U.S. Dept. Agric. Bur. Plant Ind. Bulletin 252. 1913. 



