146 RESEARCHES ON FUNGI 



small thin living branches, it is probable that all the trees in a forest 

 would become infected with wood-destroying fungi very early in 

 life. But this we know is not usually so and we often find trees 

 in old forests which have lived for scores or even hundreds of years, 

 which have shed tens of thousands of small twigs, and which yet 

 have perfectly sound trunks and main branches. Let us assume 

 therefore that our Fomes only enters a tree via large wounds made 

 by the removal of bark from the trunk and thick limbs and by 

 the breaking away of stout branches. 



What chance have the spores of a fruit-body of Fomes applanatus 

 to settle upon tree- wounds that can be infected ? In a square mile 

 of forest the number of trees with infectible wounds must be strictly 

 limited ; -and the total area of these wounds must indeed be very 

 small relatively to the square mile of forest. Let us assume that 

 this total area of infectible wound-surface is 100 square feet. Then 

 the barren area relatively to the infectible area in the square mile 

 of forest would be roughly : 



28,000,000 



-^oo - 280 ' 000 - 



Supposing therefore that the spores of the fruit-body of Fomes 

 applanatus were dispersed by the wind so that they settled fairly 

 evenly over the square mile of forest, the chances that any given 

 spore liberated by the fruit-body would settle on an infectible 

 wound-surface would be 280,000 to 1 against. But our assumptions 

 have been too favourable to the chances of spore-dissemination, 

 for the wounds would often be more or less vertical or even looking 

 downwards rather than strictly horizontal and looking upwards, 

 and it is probable that, owing to periodic rainfall and stillness of 

 the atmosphere and to leaf-screens, etc., the odds are considerably 

 against any particular spore being carried even half a mile from its 

 parent fruit-body before reaching the earth. The mathematical 

 calculation given above is based on assumptions which can be only 

 more or less true, but I think that it is sufficiently accurate to 

 indicate that the chances of any particular spore settling upon an 

 infectible wound-surface in a forest are millions to one against. 

 This being granted, it is clear that, if the spores of our Fomes 



