INTRODUCTION 19 



It would, be a mistake to attempt to answer the question 

 whether a fungus is really a parasite or not after the failure of 

 one or a few attempts at infection. Let us only consider how 

 numerous are the factors regulating success even in the 

 sowing or planting of our forest trees, with whose conditions 

 of life we are to some extent familiar. Of the fungus to be 

 investigated, however, we, as a rule, know almost nothing ; we 

 do not know the external conditions of germination, we often 

 scarcely know whether the spores are really ripe, whether they 

 are in too damp or too dry a medium, whether sufficient oxygen 

 is admitted to them, or whether the season of the year is the 

 right one for sowing for spores, like the seeds of forest-trees, 

 require to rest for different periods after ripening, before they will 

 germinate. What has already been said about the numerous 

 conditions predisposing plants to disease will show sufficiently 

 how, even with the best material for infection, experiments may 

 give only negative results. Since even trained mycologists and 

 pathologists often succeed only after innumerable abortive 

 experiments in making themselves acquainted with the con- 

 ditions under which a plant becomes infected, it will be clear 

 that it may be regarded as simply an accident when an amateur 

 succeeds at all with an infection-experiment. 



When the infection has succeeded, it is not enough merely to 

 follow the course of the disease through its various stages in 

 doing which, moreover, it is of the utmost importance to compare 

 cases of disease met with in the forest but it is also necessary 

 to discover the external influences which restrict or further the 

 development of the disease. 



This part of the investigation is the most difficult. It 

 demands, very specially, the power of accurate observation ; 

 apparently unimportant accessory circumstances must be noticed 

 and compared ; and, above all things, excursions to the forest 

 must be made as often as possible. Investigations of the 

 diseases of our forest trees will seldom lead to any definite 

 result unless we make careful and extensive observations and 

 comparisons in the forest itself. At the same time, still less 

 prospect of success will attend observations of diseases in the 

 forest if they are not accompanied and supported by exact 

 scientific investigation. 



