376 University of California Puhlications in Botany [Vol. 5 



in the fact that in the former, with the exception of Solanum tuber- 

 osum, separation occurs in the stem at the very base of the pedicel, 

 whereas in the latter three it occurs through the pedicel a varying 

 distance from the base. 



2. Method of Cell Separation 



a. GENERAL EEMAEKS 



It will be remembered that two theories have been proposed to 

 account for the cell separation that is responsible for abscission. 

 First, it is conceivable that cell separation may be caused by an 

 increase in cell turgor, which causes the cells to round up and pull 

 apart without any change taking place in the chemical nature of the 

 middle lamella. Second, cell separation may be caused by a chemical 

 dissolution of the middle lamella with or without an increase in cell 

 turgor. The main difference between the two theories is that the 

 second, in contrast with the first, maintains that chemical alteration 

 of the middle lamella is always necessary before abscission can occur. 

 The first theory gains support from the work of Fitting and the second 

 from thg work of Hannig. Lee, Strasburger, and Lloyd. Wiesner, 

 Kubart, and Loewi ])elieve that cell separation takes place by the 

 action of both factors hut that either factor may at times be the more 

 important. 



b. CYTOLOGICAL CHANGES ACCOMPANYING ABSCISSION 



It was stated in a preliminary discussion (Goodspeed and Ken- 

 dall, 1916) first, that no indication of cell divisions or elongations 

 were observed accompanying abscission, and, second, that no evidence 

 of the dissolution of the middle lamella had at that time been obtained. 

 The first statement has been corroborated in that, during all the later 

 experiments, no divisions or elongations have been observed in any of 

 the described species. The dissolution of the primary cell membrane, 

 however, because of more exact knowledge of the proper time to take 

 sections and of more successful staining methods, has been fairly well 

 established. 



The main problem here was to determine by the use of various 

 stains whether or not tlie primary and secondary cell membranes of 

 the separation cells stain differently in the early stages of abscission 

 than under normal conditions. This was a point which was found 

 very difficult to determine, principally because of the fact that the 



