298 University of California Puhlications in Botany [Vol. 5 



the only couclusions that could be drawn from the considerable mass 

 of data at present available. 



7. At the completion of the process of abscission the flower or fruit 

 is often retained for a short time or until mechanical agencies break 

 the epidermis and the few tracheal elements still intact. For this 

 reason a slight tapping of the flowers or fruits is necessarj^ to deter- 

 mine exactly the time of complete separation in collecting data such 

 as are presented in the above table. Longitudinal sections of the de- 

 tached pedicel after abscission show a distinctly convex outline at the 

 abscission surface with a slight notch at the tip. The surface itself 

 is composed of the protruding rounded ends of cells, with here and 

 there a spherical, completely isolated cell mechanically adhering, and 

 broken ends of tracheal elements. The surface cells, both those in 

 place and those isolated, are normal in every respect and apparently 

 do not differ from the cells of the abscission layer prior to separation 

 nor from the remainder of the cells of corresponding tissues so far 

 as thickness of wall and size, shape, and character of cell inclusions 

 are concerned. The exposed, basal, attached portion of the pedicel 

 corresponds in appearance, except that the abscission surface is flat, 

 not convex. Soon after separation the cells on this exposed surface 

 collapse and form a comparatively heavy protective layer. 



8. The number of cells actually concerned in the process of ab- 

 scission is greater in the hybrids mentioned above than in the parental 

 species. The same is true in the case of ''automatic" as contrasted 

 with "spontaneous" abscission. 



The preliminary nature of this report was indicated at the start. 

 A full discussion of the literature dealing with and bearing upon ab- 

 scission, together with the results of more extensive experiments at 

 present in progress, will be the subject of a more extended commun- 

 ication. 



We take pleasure here in mentioning the obligation under which 

 Professor F. E. Lloyd has placed us for his many helpful suggestions 

 in connection with the work reported upon above. 



Transmitted September 8, 1916. 



