DISEASES OF TKEES LIKEEY TO FOLLOW ENJLTIEES. 149 



branches and cause them to rot. It is a very common experience 

 that the rotting which began in a wound gradually extends to the 

 main trunk, so that although the bark, except where the wound 

 exists, appears to be perfectly sound, on cutting the tree down, 

 the whole trunk is found to be rotten or hollow. 



What happens, except in very small wounds which heal at once, 

 is as follows : The porous wood takes up moisture from the air in 

 greater or less amount according to the season, but in almost all 

 cases enough to cause the outer exposed part to decay in the 

 course of from a few weeks to a few months. Not onl}^ is water 

 absorbed from rains and mists but dust and other organic sub- 

 stances gradually collect on the surface and there is thus formed a 

 sort of soil, in a thin layer to be sure, but enough to support, at 

 first, the growth of bacteria, which help on the decay of the solid 

 parts, and, later, offer a favorable field for the germination of the 

 spores of toadstools. A ver}' small amount of damp soil is suffi- 

 cient to start the growth of these toadstool- fungi. Their spores, 

 when they germinate, give out a series of branching threads, the 

 mycelium. The threads gain sti'eugth as they grow, and, in a 

 good many species, they at length acquire the power of dissolving^ 

 the walls of the sound plant cells, even if they do not in the 

 beginning possess this power. Aided by the increased moisture, 

 which is favored by the presence of a damp, earthy layer on the 

 surface of the wound, the destructive threads make their way 

 slowly along the interior of the trunk, the process of destruction 

 causing an increase of moisture and sliminess, which only makes 

 the ultimate destruction of the hitherto sound wood the more 

 certain. While all this is going on within the trunk there ma}' be 

 no definite indication on the outside of the harm done. The toad- 

 stools, as we call them, are the fructification of the fungi, of Avhich 

 the threads are the organs of A'egetatiou, and it is not until after 

 the latter have attained a somewhat advanced development that the 

 toadstools themselves appear on the surface. Their appearance 

 almost invariably indicates not that trouble ma}' be expected, but 

 that the disease has already made considerable progress. 



The toadstools and punk-fungi, of which I have spoken, are 

 usually not limited in their growth to any one species of tree, but 

 may grow on a good many different kinds. There are some species, 

 however, which attack only particular kinds of trees, and among 

 the number are forms which are more virulent and rapid in their 



