1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 413 



THE PROCESS OF ABSCISSION 



1. The process of abscission conforms to the usual type, which 

 involves the separation of cells along the plane of the middle lamella 

 of the cell wall separating them. 



2. No cell divisions or elongations were observed to accompany 

 abscission. 



3. All the cells across the pedicel in the region of the separation 

 layer take part in separation except the tracheae and cuticle, which 

 must be broken mechanically. The total number of cells which may 

 be involved is greater in some species than in others. This number 

 may also vary in the same species because of changes in the external 

 conditions. 



4. Cell separation is brought about by the hydrolysis and conse- 

 quent dissolution of the middle lamella (primary cell membrane) or 

 perhaps both the primary and, in part, secondary cell membranes. 

 The agency active in the hydrolysis of the cell membranes is probably 

 an enzyme. 



5. An increase in cell turgor frequently occurs during abscission, 

 but probably serves merely to hasten and facilitate the process. Most 

 of the frequently observed expansion and the turgid appearance of 

 the separation cells during abscission are probably due to the natural 

 release of pressure caused by the dissolution of the middle lamellae. 



6 : Abscission of the style and corolla in Nicotiana and Datura 

 resembles, to a large extent, abscission of the flower. 



TIME OF ABSCISSION 



1. The length of time between anthesis and normal flower-fall due 

 to lack of fertilization differs among the varieties of Nicotiana. This 

 variation was found to range between an average of five to eighteen 

 days in some fifteen species and varieties of Nicotiana. A much 

 smaller range of variation (0.7 to four days, with the largest fre- 

 quency in the three day group) was noted for the time between an- 

 thesis and fall of the corolla after pollination. 



2. The stimulation of the stylar tissues by the growth of the pollen 

 tubes tends to shorten the time between anthesis and fall of the 

 corolla, this effect being independent of fertilization. Such stimula- 

 tion of the stylar tissues has no appreciable effect upon floral ab- 

 scission. 



3. Floral abscission occurs in F x H179 seven hours after subjecting 

 shoots of the plant to 1.5 per cent illuminating gas at a temperature 



