INTRODUCTION 17 



the further investigation offers comparatively little difficulty. It 

 is more difficult, as a rule, to determine the true cause of disease 

 and death in the case of plants already dead, although the skilled 

 plant-pathologist will seldom fail to recognize with certainty the 

 true character of a disease. 



If we are dealing with injuries caused by animals or plants, 

 we shall discover and recognize them, or at least their traces, 

 with most certainty in the preliminary stages of the disease. In 

 very many cases it is not sufficient, where we are dealing with 

 injuries due to animals, including insects, that we catch the 

 creature at work and seek to observe it and its mode of life in 

 nature, as has hitherto generally been done ; but, and particularly 

 in the case of insect-injuries, we must determine whether the 

 injured plants did not already possess some predisposition to 

 disease before they were attacked by the insects, &c. Especially 

 does this hold good for the great family of the bark beetles, 

 which often only appear in the train of other prejudicial agencies, 

 and especially of injuries caused by parasitic fungi. In the case 

 of parasitic plants, again, it is not to be concluded forthwith from 

 the presence of a fungus in the dead tissues that death has been 

 caused by that fungus. True, where we find the mycelia of fungi 

 vegetating in the apparently unaltered living tissues of a plant, 

 there is practically no room for doubt that we have to deal with 

 a parasite ; but even in the latter case the attempt must next 

 be made, by means of suitable infection-experiments, to induce 

 arbitrarily, and in a somewhat artificial manner, the disease that 

 we are seeking to investigate. 



If spores or gonidia of the suspected fungus are to be had, we 

 make use of these in carrying out the investigation, after having 

 first proved that they are capable of germinating. Should no 

 material capable of germinating be at our disposal, we must, if 

 possible, undertake artificial cultures in a damp chamber, and 

 await the ripening of spores, or even the production of sporo- 

 phores.* According to the character of the disease, infection is 

 secured by scattering the spores on the leaves, or by placing 

 them in a wound artificially made in the host-plant. In the 



* [Spores may be of several kinds, and the term is used as a general one. 

 Gonidia are a-sexual spores. Sporophore is a general term to denote any of 

 the various kinds of spore-bearing structures met with among Fungi. ED.] 



C 



