ADMINISTRATION 97 



of monographs. Many of these insects are diffi- 

 cult to identify and concerning others very little is 

 known. Investigations on tree diseases have not 

 made such good progress, because tree diseases are 

 much more difficult to control. Tree diseases, like 

 human diseases, must be prevented instead of con- 

 trolled. A general survey of the tree diseases 

 prevalent in the National Forests has been made, 

 especially in California. Further studies have 

 brought to light little known or even unknown dis- 

 eases. In California, studies have shown that a 

 certain relation exists between old age and disease. 

 Incense cedar, for example, seems to become in- 

 fested after it reaches maturity at an age of about 

 150 years. 



Tree Studies. Commercial tree studies are 

 made of important tree species. The results are 

 published in the form of monographs dealing with 

 the range, silvicultural characteristics, growth, 

 yield and management of each tree. These studies 

 bring together all the important facts known about 

 the tree described, such as: the industrial uses of 

 the wood, the conditions under which the tree suc- 

 ceeds, the rate of growth in different situations, 

 and the most suitable methods of management to 



