92 BOOKS AND CURRENT LITERATURE 



external factors such as temperature, humidity, water content of the 

 soil, mechanical injuries due to shaking and wounding, toxic gases, and 

 light, and to such other factors as the time required for separation to 

 occur and the mechanism and method or manner of abscission. As a 

 result of this study of more than thirty species of plants it is concluded 

 that chemical changes (perhaps enzymotic) are always present in the 

 abscission layer to some extent whether growth and turgor changes in- 

 tervene or not. Weisner, Kubart, and Loewi regard turgor as the more 

 important factor in effecting separation and believe that chemical 

 changes may in some cases be absent. In Impatiens and Ampelopsis, 

 at least, Lloyd finds that the abscission cells show no higher osmotic 

 equivalent at the time of abscission than adjacent cells. By the use of 

 5% KOH the walls of the abscission cells in Cheiranthus could be made 

 to simulate autolysis. — Frederick A. Wolf. 



