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bined and dried at 100°C. to constant weight. Thus each figure given in the following tables, unless otherwise stated, represents the composite weight of five cultures. The actual weight of mycelium obtained under controlled conditions has been made the criterion for the evaluation of the comparative effectiveness of various factors. However, it should be emphasized that biological phenomena cannot be reduced to mathematical figures without making a generous allowance for the frequent and often disconcerting fluctuations that characterize the behavior of living things. No matter how rigidly one may control the environmental conditions and repeat a given experiment, one will always find some differences in the weight of the crop. Even the different cultures of the same experiment made at the same time and under identical conditions will continue to vary. We have found that such variations may be from 5 to 10 per cent; 'however, this is not great enough to be significant. P ART I. THE EFFECT OF THE AGAR FACTOR.-Allyn and Baldwin (1932) found that agar-agar exerted a decisive influence on the growth of certain bacteria. They ascribe this beneficial effect to the fact that agar is a mild reducing agent. The present writers (1938) observed that a number of fungi made better growth on solid media than on nutrient solutions and assumed that some chemical factor was the cause.The following experiments were made to test the effect of small amounts of agar upon the yield of mycelium of several organisms. The medium used consisted of 2.5 per cent dextrose, 0.3 per cent amino acids, the inorganic salts, and three concentrations of thiamin: 1/20th, 1/100th, and 1/500th ppm. One set of each solution received 0.05 per cent agar, while the other was without agar (table 1). Table 1 shows that the increase was greatest in the presence of 1/20th ppm. thiamin and that it disappeared in all cases except one when the vitamin was reduced to 1/500thppm. Thus, the yield of Ph,ycomyces blckesleeanus was increased by 84 per cent when thiamin was added at the rate of 1/20th TABLE 1. The effect of agar on the yield of mycelium in mg. b!/ five fungi.THE NECESSITY of an extraneous supply of thiamin for the growth of many fungi has been definitely established, but we know comparatively little concerning the influence of a number of contributory factors upon the growth of such organisms. The present investigation is an attempt to extend our knowledge concerning these factors. Part I of this paper concerns itself with the agar factor; Part II with the effect of organic acids upon nitrogen utilization; and Part III with nitrogen-dextrose-thiamin ratio.TECHNIQuE.-Unless otherwise stated, all cultures were grown on 25 ml. of nutrient solution in Erlenmeyer flasks of 250. ml. capacity. The basic medium consisted of 0.5 gram each of ammonium nitrate, potassium dihydrogen phosphate, magnesium sulfate, 5 grams of Bacto-Dextrose of highest purity, 0.5 gram of a mixture of amino acids (d-arginine, d-glutamic acid, l-aspartic acid, 2 parts each, and 1 pa...

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