672 PROTECTION AGAINST DISEASES. 



progress of the disease and its importance in forestry. These 

 four headings have been considered in the following list : 



1. Diseases arising from physical agency (frost-crack, bark 

 blister, etc.) and those from physiological causes, such as red 

 and white rot. 



2. Local diseases, such as of the roots, or of the stem, bark, 

 buds, leaves or shoots, or of the inflorescence and fruits of the 

 trees. 



3. Acute or rapidly developing diseases, or chronic diseases 

 which develop slowly. 



4. Diseases which merely cause loss of increment, and 

 others which affect the economic value of the wood, the latter 

 consisting either in an abnormal growth of otherwise healthy 

 woody tissue, such as burrs, twisted fibre, etc., or in an 

 unhealthy state*of the tissues, as in red or white rot. 



The worst kinds of damage to forest plants by men, 

 animals, plants, and atmospheric agencies have been already 

 dealt with in the preceding chapters of this book. For the 

 study of abnormal growth in healthy wood-tissues, the reader 

 is referred to treatises on Forest Utilisation. In the following 

 pages will be described certain diseased conditions which 

 could not well be classified under any of the foregoing heads 

 and are limited to the following: red rot, white rot, stag- 

 headedness, abnormal needle- shedding, and damage by factory 

 fumes. 



