1918] Ketidall: Jhscission of Flowers and Fruits in Solanaceae 349 



1. Abscission is the detaching of an organ by the separation of 

 actively living cells at or near its base. 



2. The separation layer (Mohl's Trennungschichte) is the layer of 

 cells the components of which will separate from one another at 

 abscission. 



3. The separation cells or ahsciss cells are the cells that make up 

 the separation layer. 



4. The separation zone is the general region through which abscis-' 

 sion takes place and usually is largely proximal to the separation layer. 



A preliminary account of abscission in F^ species hybrids of Nico- 

 tiana has already appeared (Goodspeed and Kendall, 1916). The 

 present study represents an amplification of this investigation and its 

 extension to other species of the Solanaceae. It is particularly con- 

 cerned with the following : ( 1 ) the position of the separation layer ; 

 (2) the origin of the separation layer; (3) the cytology of the separa- 

 tion layer; (4) the process of abscission, including (a) a description 

 of the appearance of the separation layer in consecutive stages of the 

 process and (&) the method of cell separation; (5) the time occupied 

 by abscission, including (a) the time between the application of the 

 stimulus and fall (reaction period) and (h) the time involved in the 

 actual process of cell separation (abscission period) ; (6) experimental 

 induction of abscission. 



Although the investigation reported here is largely a morpholog 

 ical one, the results of the experiments on the method of cell separa- 

 tion, the time of abscission and the induction of abscission seem to 

 have a distinct physiological significance as well. 



