4 DISEASES OF TREES 



the forest and of giving their attention to the diseases of trees. 

 The credit of having first stimulated interest in this direction 

 undoubtedly belongs to M. Willkomm. 1 Hallier's attempt to 

 collate the scattered materials in the form of a text-book 2 was 

 subsequently repeated with happier results by P. Sorauer 3 and 

 Frank, 4 whose handbooks are useful compilations, in which the 

 matter diffused through numerous periodicals and works is 

 collected and systematically arranged. My own investigations 

 have been published partly in periodicals and partly as 

 independent works. 5 



THE CAUSES OF DISEASE 



In the present state of science it is scarcely possible to draw 

 a sharp line of distinction between those conditions of the . 

 plant known, on the one hand, as healthy, and, on the other, as 

 diseased. The development of any plant depends upon a series 

 of external factors of nutrition, and these, such as light, heat, 

 the kind and proportion of the nutritive materials, and of the 

 water and oxygen contained in the soil, of the carbonic acid 

 present in the atmosphere, &c., are available for the plant in 

 very different quantities. When all these external factors 

 influence the development of the plant in the most favourable 

 manner, it is vigorously nourished and flourishes well. But 

 probably the case is never realised when all these factors of 

 life act simultaneously and concurrently in the most favour- 

 able manner possible : on the contrary, one or more is sure 

 to be deficient or superabundant, and this causes interference 

 to a greater or less extent with the development of the 

 plant. We cannot as yet say, however, that such plants are 



1 M. Willkomm, Die Mikroscopischen Feinde des Waldes. Dresden, 1866, 

 1868. 



2 E. Hallier, Phytopathologie. Die Krankheiten der Culturgewachse. 

 Leipzig, 1868. 



3 P. Sorauer, Handbuch der Pflanzenkrankheiten. Berlin, 1874. 2nd 

 Edition, 1886. 



4 B. Frank, Die Krankheiten der Pflanzen. Breslau, 1880. 



5 R. Hartig, Wichtige Krankheiten der Waldbaume. Berlin, 1874. Die 

 Zersetzungserscheinungen des Holzes der Nadelholzbdume und der Eiche. 

 Berlin, 1878. Untersuchungen aus dem forstbotanischen Institut zu 

 Miinchen. I. Berlin, 1880. III. Berlin, 1883." Die echte Hausschwamm, 

 Merulius lacrymans. Berlin, 1885. 



