671 



CHAPTER I. 



GENERAL ACCOUNT OP THE DISEASES OF PLANTS. 



1. Definition. 



A FOEEST plant is said to be diseased when, owing to 

 disturbances in the functions of its organs and in the 

 chemical or physical processes going on within them, it 

 assumes such a condition that it is hindered from further 

 useful development and may consequently die, either wholly 

 or in part. Disease therefore causes blanks in woods of all 

 ages, and also loss of wood-increment and consequent 

 reduction in their value. 



2. Causes of Disease. 



Many different causes of disease in forest trees may occur, 

 for instance, old age, injuries by men and animals, injuries 

 by parasitic plants (weeds and fungi), by atmospheric agencies. 

 Disease is also due to certain local circumstances, such as soils 

 too poor in the chemical compounds necessary for plant-life, 

 soils too dry or very wet, too compact or too loose, etc. 



Although much progress has been made during the last 

 twenty years in the study of the diseases of forest trees, a 

 wide field is still open for discovery in this respect. 



3. Classification of Diseases. 



The diseases of forest plants may be grouped according to 

 their origin, the nature of the organs which are attacked, the 



* A capital account of the conditions of environment which encourage disease 

 in a plant is given in the Proceedings of the Royal Soc., vol. 47, " The Croonian 

 Lecture," by H. Marshall "Ward. 



Hartig, Dr. Robert, " Lehrbuch der Baumkrankheiten." Berlin, 188!>. 



Frank, Dr. B., " Die Krankheiten der Pflanzen," 3 vols., 1896. 



Soraner, Dr. Paul, " Zeitschrift fiir Pflanzerkraukhciten." S uttgart. This 

 perio^lical commenced in 1891 



