422 University of California Publications in Botany [Vol. 6 



show a marked inability to hold the stain, appearing much lighter in 

 color than the cell walls of adjacent tissues (cf. Jones, 1909). 



The optical quality of these cell walls is markedly altered prior to 

 abscission since quite different results are obtained using the open 

 diaphragm on the abscission zone and on adjacent cells. The walls of 

 the abscission cells are less highly refractive and appear lighter and 

 more indefinite. 



However, the most notable change lies in the fact that during 

 abscission the walls of the abscission cells show a marked swelling and 

 gelatinization which often reaches a magnitude of two to three times 



Fig. 1 — Normal cortical cell from the abscission zone at the base of the 

 petiole. Shows the relatively thick walls and simple pits. Taken prior to the 

 application of the stimulus leading to abscission. Washington Navel orange. 



the original thickness. This is particularly noticeable in the thick cell 

 walls of the cortex although it occurs in all the living tissues. These 

 cortical cells possess simple pits in considerable numbers (fig. 1). The 

 portion of the wall constituting the pit membrane seems to be indur- 

 ated or impregnated with some substance resisting this gelatinization, 

 thus giving the cell wall a decidedly beaded appearance (fig. 2). This 

 swelling occurs throughout the abscission zone and results in consid- 

 erable pressure being applied to the protoplasts causing them to be 



