60 



Mycologia 



content and a small air-content, of the wood-vessels of the trees, 

 thereby making a substratum poor in air. . . 



The assumption that an abundant transpiration may cause a 

 high water content in the tissues is in direct contradiction to the 

 latest research on the subject. Dixon- demonstrated that the 

 transpiration pull during the growing season tends to reduce the 

 water content of the plant, hence the gas content is increased. 

 That the gas content in the wood is an important factor influenc- 

 ing the entrance of the mycelium and its subsequent spread has 

 been experimentally determined by Miinch.^ More recently 

 Zeller^ showed that any factor influencing the proportion of water 

 and air in the substratum is of great importance. 



A factor which would apparently retard the development of 

 wood-destroying fungi in the wood-vessels of trees growing in 

 the tropics is the absence of any marked periodicity in wood 

 formation. Naturally the vital processes in tropical vegetation 

 have a rhythmical alternation of periods of rest and activity, de- 

 termined usually by a wet and dry season, but the latter period is 

 short and no such contrast between large and small vessels is pro- 

 duced as in temperate zones. This condition of almost continu- 

 ous growth in contrast to the alternating periods of rest and 

 growth in colder climates may tend to retard the spread of 

 mycelium in the wood. This is reasonable to suppose, in view of 

 the fact that in the temperate zone wood-destroying fungi are 

 more active during the dormant period of their hosts. Continu- 

 ous growth, or unimportant temperate changes, may either one 

 conceivably result in decreased air content, and the mycelium 

 would have difficulty in finding the proper balance between air and 

 moisture in the wood. The great density of most tropical woods 

 would also be a factor in this respect. Again, it is conceivable 

 that the soil solutions in the tropics, being warmer and therefore 

 less able to hold gas in solution, contain less oxygen than in tern- 



2 Dixon, H. H. Transpiration and the ascent of sap in plants. Mac- 

 millan & Co. 1914. 



3 Miinch, E. Untersuchungen iiber Immunitat und Krankheitsempfanglich- 

 keit der Holspflanzen. Naturwiss. Zeitschr. f. Forst. u. Landw. 7: 54-75. 

 87-114, 129-160. 1909. 



4 Zeller, S. M. Studies in the physiology of the fungi. II. Lenzites 

 saepiaria Fr., with special reference to enzyme activity. Ann. of Mo. Bot. 

 Card. 3: 448-449. 1916. 



