1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 353 



of two or three tiers of cells but in N. Tabacum the layer is made up 

 of ten to fifteen tiers. 



Hannig (1913), by means of various microchemical tests, can 

 detect no chemical difference between the cell walls of the separation 

 layer and those of the cells on either side. Lloyd (1914a), however, 

 claims that the cell walls of the separation cells break down more 

 quickly when treated with caustic potash than do the walls of normal 

 cells. Starch grains are frequently noted by Hannig and Lloyd 

 (1916a) as occurring in the separation cells, especially in the abscis- 

 sion of internodes by Mirabilis Jalapa. 



An examination of the literature thus makes it evident that there 

 has been a great difference noted in the various species in regard to 

 the character of the separation cells. The one characteristic of these 

 cells, however, to which there is no exception is that they are in an 

 actively living condition. 



2. THE PROCESS OF ABSCISSION 



a. METHODS OF ABSCISSION 



It has been found that in practically all cases of abscission the 

 detaching of the organ is brought about by the separation of cells 

 along the plane of the middle lamella. It is the method noted by 

 Mohl (1860), Wiesner (1871), and Kubart (1906), who call it a pro- 

 cess of maceration. Correns (1899) calls it a process of ' ' schizolysis. " 

 Correns, however, in the same work describes a new and different 

 method of abscission (rhexolysis) which he finds in mosses. In this 

 latter method, separation is accomplished by a seemingly passive 

 break of tissues irrespective of the position of cell walls. This may 

 be the case in the style of cotton (cf. Lloyd, 1914&). This same 

 method has been reported by Tison (1900) in the leaf of Aristolochia 

 Sipho, although the evidence has been called in question by Lloyd 

 and Loewi (1907). Still another type of abscission has been described 

 by Hannig (1913) as a result of experiments on Mirabilis and Oxy- 

 baphus. In these plants he finds separation being brought about by 

 a disorganization and dissolving away of a complete tissue. Lloyd 

 (1916a), on the other hand, states that separation in these species is 

 accomplished by cell separation and is thus true schizolysis. Hannig 

 was doubtless confused in this case by the cell elongations which 

 Lloyd observes and by which the membranes surrounding the proto- 

 plasts are drawn out exceedingly thin. Loewi (1907), working on 



