[Vol. 2 



308 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



many of those described must have been found occasionally, 

 and have had no serious influence upon the cultivation of 

 plants. 



Not only among the cultivated plants do we find little fun- 

 gous growth, but also in the natural vegetation. In the virgin 

 woods the trees have few enemies among the fungi, and even 

 the flora of mushrooms on the ground, so characteristic of 

 our woods, is absent. Everything seems to point to the con- 

 clusion that conditions are unfavorable to fungous growth. 



Why is this so? As has already been said, there are two 

 conditions which characterize a tropical climate: (1) a high 

 temperature which is about equal through all seasons, and 

 (2) a high humidity, the latter varying somewhat in the dif- 

 ferent monsoons, but being altogether much higher than in 

 our climates. 



It seems to me that the tropical temperature is too high for 

 many fungi. I cultivate in my laboratory over 600 fungi, 

 and this collection shows clearly that the temperature of opti- 

 mum growth of the greater part of the fungi lies beneath 30° 

 C, often under 25 U C. An exposure to high temperature 

 prevents many parasites from forming their spores or fruit- 

 ing bodies, whereas others require a change of temperature 

 for normal growth. The Polypor<iceae f for instance, bear 

 exposure to frost very well, but many of them scarcely develop 

 at 30° C. High temperature very often gives rise to an abnor- 

 mally abundant mycelial growth, combined with an absence of 

 spores. On the other hand, the high moisture content of the 

 air must be favorable to fungous development. 



But every fungous disease of plants involves two organisms, 

 the parasite and the host, and the same conditions may in- 

 fluence these two in a very different way. The heavy rain- 

 falls, combined with the abundant transpiration — owing to 

 the intense heat, must cause a high water-content and a small 

 air-content, of the wood-vessels of the trees, thereby making 

 a substratum poor in air. We know that this is an important 

 factor in fungous growth. This fact, combined with the high 

 temperature, would explain the rare occurrences of Hymeno- 

 mycetae and other wood-destroying fungi in the tropics. 



