8 BULLETIN" 227, U. S. DEPARTMENT OF AGRICULTURE. 



Lenzites sepiaria, 28° to 32° C; and for Lenzites ihermophila, 35° C. 

 Below these temperatures growth ' becomes greatly lessened as the 

 minimum is approached, while a rise of 4 to 8 degrees above the opti- 

 mum often causes a total inhibition of growth or even death in the 

 case of very sensitive species. 



Different stages of the same fungus may also offer a different re- 

 sistance to temperature changes, this being much less under moist than 

 under dry conditions. For instance, Falck (7, p. 339) found that 

 fresh fruit bodies of Merulius domesticus were killed in 30 minutes at 

 40° to 42° C, and in 15 minutes at 46° C, while from 12 to 16 hours 

 were required to kill dry spores at 42°. 



As compared with this fungus, the same author shows that agar 

 cultures of Lenzites sepiaria can survive more than three hours at 

 60° C. 



The resistance of a fungus to toxic substances is greatest under 

 temperature conditions most favorable to its development. After 

 conducting a series of tests on several molds to determine the germi- 

 native capacity of the spores in varying concentrations of nitric and 

 sulphuric acids and copper sulphate at different temperatures, either 

 directly in the solutions or after removal to nutrient media following 

 immersion for 24 hours, Brooks (2) states that "in most cases the 

 deleterious action increased very rapidly with rise in temperature," 

 but that "in all instances the injurious effects were least at the opti- 

 mum for the fungus." 



RELATION OF LIGHT TO FUNGOUS GROWTH. 



Light also exerts an appreciable effect on the development of wood- 

 destroying fungi. This is evidenced in two ways : (1) By its influence 

 on the growth of the mycelium, and (2) by the role it plays in the 

 production of normal fruiting bodies. In most instances at least, 

 partial illumination is essential to normal fruiting. The effect on the 

 rate of growth of the mycelium, however, is less marked, but still quite 

 appreciable. Of seven species of wood-destroying fungi studied, Hoff- 

 mann (13) reports that growth in the dark was from 4.1 to 17.8 per 

 cent (average 9.9 per cent) better than in sunlight. In carrying his 

 experiments still further and examining the effect of the red and blue 

 ends of the spectrum, respectively, he found that the former gave 2.6 

 per cent better growth in the case of Paxillus acluruntius and 59.3 

 per cent in the case of Polyporus vaporarius (average for nine fungi 

 14.6 per cent). 



WHAT DETERMINES TOXICITY? 



An adequate discussion of the subject of what determines toxicity 

 would lead us into one of the most difficult fields of biological chem- 

 istry and physiology, hence, for the purposes of this publication, the 

 writers omit reference to a great mass of literature covering the 



