1914] The Ottawa Naturalist. 65 



that abscission may occur under water (poplar, maple, horse 

 chestnut, etc.) is, of course, to be accounted for in some other 

 wav than reduction of water. 



In general, therefore, we must conclude, in view of the effects 

 of drought upon trees and shrubs, that there is a relation between 

 lack of water and defoliation, but it is not possible to attribute 

 abscission directly to a reduction of water content, such as may 

 be meastired. It may, however, result indirectly by the dis- 

 turbance of some other relation. As we shall see, very slight 

 departures from the normal condition of the environment in 

 other regards are sufficient to cause or to hasten abscission. 

 Mechanical Causes of Abscission. 



This apparently indirect effect is further exemplified in the 

 abscission response to mechanical stimuli. For convenience we 

 regard as mechanical, stimuli such as shaking and wounding, 

 though we cannot consider these as working directly. 



Shaking. It is to Darwin (3 5) that we owe the observation 

 that, if the flower stalk of the mullein (Verbascum) is sharply 

 jarred the corollas will presently fall off. Fitting, having satisfied 

 himself that the falling of the corolla was not due to accident or 

 to the movement of the calyx, as held by Devaux, found by 

 repeating Darwin's experiment, that separation was consum- 

 mated in from 45 seconds to 5 minutes, but for the most part in 

 from 1 to 3 minutes. This is equally true of young and older 

 flowers, so that it is not due to their age. He found a similar 

 behaviour in Geranium pyrenaicum, with a reaction time of from 

 30 seconds to 6 minutes. This phenomenon has been little 

 studied, and only few plants are known to show it. 



Wounding. The importance of the effect of wounding on 

 abscission will at once be realized in view of the great economic 

 losses occurring each year from the dropping of buds, flowers and 

 fruits from the plants of our orchards and gardens, as a result 

 of insect and other injury in the form of wounds. Young peaches, 

 when wounded by curculio, drop. The great loss to cotton 

 growers in x^labama, Louisiana and Texas, and the great 

 financial disturbances accompanying it, caused by the boll- 

 weevil, to which must be added the immense expenditures 

 involved in scientific research in finding a way out of the 

 difficulty, have caused a highly dramatic interest to attach to 

 the problem. This case will serve, therefore, as a good example 

 for the present discussion. 



The boll-weevil lays its egg in a young "square" or flower 

 bud. In from 1 to 22 days'^ the square falls to the ground, the 



**I am indebted to the United States Bureau of Entomology, through 

 the courtesy of Dr. L. O. Howard and Dr. W. D. Hunter, for the use of 

 valuable data on the shedding of bolls after weevil injury. 



