436 PHYSIOLOGY OF THE FUNGI 



B. Effect of acidity. This experiment may be carried out in liquid 

 media 72 to 76, using drops of spore suspension as described above in 

 part A, or the media may be solidified with agar and Petri dishes used. 

 Observe the results at intervals up to 48 hr. Your records should include 

 the approximate time required for germination of the first spores and the 

 percentage of germination at each examination. The first appearance of 

 a germ tube may be considered as germination. 



C. Effect of relative humidity. Place drops of spore suspension in dis- 

 tilled water on sterile glass slides to serve as controls. On three other 

 dry glass slides, place dry spores. Place these in desiccators as follows: 

 (1) spores in water and in desiccator which will maintain saturated atmos- 

 phere; (2) spores on dry slide in saturated atmosphere; (3) spores on dry 

 slide in desiccator with relative humidity at approximately 98 per cent; 

 (4) spores on dry slide in desiccator with relative humidity at approxi- 

 mately 92 per cent. To maintain 98 per cent humidity, use a 1.00 molal 

 solution of sucrose; for 92 per cent humidity use a saturated solution 

 of K2HPO4 (see Clayton, 1942). Open the desiccators after 24 to 30 hr. 

 and examine the spores for germination. 



D. Effect of temperature. Use four Petri dishes with medium 74 or 75. 

 On each place three or four drops of spore suspension and mark these 

 spots on the bottom of the dish. Incubate these as follows: (1) in an 

 incubator at 30°C.; (2) at 25°C.; (3) in a refrigerator at 18°C.; (4) in a 

 refrigerator at 10°C. 



Examination of spores. The time required for spores of the various 

 species to germinate under the usual conditions varies from 2 to 24 hr. or 

 more. For some fungi all the experiments in the exercise may be exam- 

 ined and compared after 12, 18, or 24 hr. One examination should be 

 made after 48 hr. For most species the experiments may be concluded 

 at this time. 



List of test fungi: Alternaria sp., Cephaloihecium roseum, Choanephora 

 cucurhitarum, Glomerella cingulata, Guignardia hidwellii, Helminiho- 

 sporium sativum, Monilinia fructicola, Penicillium sp., Phytophthora 

 infestans. 



EXERCISE 12 

 The Associative Effects among Fungi 



Pure cultures of a single organism seldom exist in nature. Instead, 

 each organism is constantly exposed to a biotic as well as a physical 

 environment. As a result, there is usually competition between different 

 fungi and between fungi and other organisms in the same substrate, 

 particularly the soil. On the other hand, many organisms are benefited 

 by their association with others. Often the metabolic products of one 

 favor the growth of another. 



