1918] Kendall: Abscission of Flotvers and Fruits in Solanaceae 397 



tioned began to fall. It was found that signs of abscission hardly 

 ever appeared until thirty to forty minutes before actual fall occurred. 

 This indicates that the actual process of cell separation in F^ H179 

 takes place in from thirty to forty minutes. Experiments carried on 

 in the same manner with X. Tahacum "Maryland" indicate that 

 abscission here takes place in from forty-five to sixty minutes. 



Both the reaction time of abscission and the actual abscission time 

 are profoundly influenced by temperature and by humidity. Varia- 

 tion in the intensity of the illumination, however, seems to have no 

 direct influence upon abscission. In comparing the effect of changes 

 in temperature and humidity it was found that the results of experi- 

 ments intended to show the time of abscission are far more dependent 

 upon temperature than upon humidity. This is not because changes 

 in humidity have little influence upon abscission but because such 

 changes have to be very great indeed before bringing about any appre- 

 ciable effect. Very slight changes in temperature, on the other hand, 

 often influence abscission to a marked degree. Abscission goes on 

 very actively under high temperatures and conversely very slowly 

 under low temperatures. It starts in the case of F^ H179 about seven 

 hours after insertion in 1.5 per cent ilhiminating gas at a temperature 

 of 19° C. If the same experiment be repeated in a temperature of 

 approximateh' 9° C. abscission may not occur for fifteen to twenty- 

 four hours. 



Drought has to be quite severe before retarding abscission. There 

 is no doubt, however, that wilted shoots will not drop flowers as 

 quickh^ as fresh ones and if the wilting proceeds far enough no abscis- 

 sion will occur. This effect is all the more noticeable if the air around 

 the wilted shoot is kept free from moisture. 



EXPERIMENTAL INDUCTION OF ABSCISSION 



1. Induction by Illuminating Gas 



The first subject to be considered under tliis heading is the com- 

 parative effect of illuminating gas in causing abscission in several 

 species of the Solanaceae. The method of determining this consisted 

 largely in placing flowering shoots of the different species in water 

 under bell-jars and introducing enough illuminating gas under the 

 jars to make the percentage of narcotic vapors in the air around the 

 plant 1.5. The temperature during the experiments was eompara- 



