38 MAINi; AGRICULTURAL EXPERIMENT STATION. 191I. 



conditions, and Groth's experiments have been along this Hne. 

 In addition, as Crandall again points out, the physical condition 

 oi" the leaves as related to injury demands experiments to de- 

 termine this point. 



Clark* considers that the amount of injury done to a species 

 is dependent upon the following conditions : 



The specific susceptibility of the protoplasm of the plant to 

 the toxic action of copper. 



The solvent properties of the cell sap on the copper hydroxide. 



The permeability of the epidermis or cuticle to the cell con- 

 tents under favorable exosmotic conditions. 



Weather conditions following spraying which provide con- 

 ditions for exosmosis of a portion of the cell contents. 



Obviously, if these are the necessary conditions, there is an- 

 other important physiological factor to be considered. This 

 is the nature of the cell content of the leaves, which will regu- 

 late the degree of permeability. Such cell contents will depend 

 largely upon the conditions that affect the metabolism of the 

 leaf. Nothing makes this appear more evident than the strik- 

 ing differences in the amount of injury produced under identi- 

 cal conditions on trees of varying degrees of thriftiness. 



Choice oe Insecticides. 



The use of insecticides has not to date given rise to such 

 serious problems. Insoluble arsenicals have been the common 

 agents employed, usually combined with the fungicide solution. 

 Paris green was the form of arsenic originally used and is still 

 adhered to by some. Although effective, it has been very large- 

 ly superseded by arsenate of lead which combines equal effi- 

 ciency with increased adhesiveness and greater safety to foliage. 



The tendency of arsenate of lead to undergo chemical change 

 when combined with the fungicide, and the nature of such 

 change when it does occur, are questions now before station 

 chemists. Of late arsenite of lime has been recommended by 



*Clark, J. F. "On the Toxic Properties of Some Copper Compounds 

 with Special Reference to Bordeaux Mixture." Botanical Gazette 23: 

 pp. 26-48. (1902.) 



