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University of California Puhlications in Botany [Vol. 6 



to show that the swelling and ultimate dissolution of the cell walls in 

 the separation layer is brought about by the chemical process of hydro- 

 lysis initiated by enzymes secreted in the abscission cells. In this sepa- 

 ration process it is necessary to distinguish clearly two distinct stages. 

 The first consists in the marked swelling and gelatinization noted above. 

 The second stage is the dissolution of this gelatinized wall which 

 results in the freeing of the cells concerned. These two clearly defined 

 stages may be taken to indicate that there may be more than one 

 reaction involved and more than one enzyme concerned. 



In this connection it seems well to review briefly some of the more 



Fig. .3 — A later stage in the abscission jiroeess showing separation of the 

 tertiary lamellae from the pit membranes which remain free in the gelatinous 

 matrix until subsequently hydrolyzed. The marked swelling of the walls, producing 

 considerable pressure, results in the crushing of the protoplast. Taken from 

 the abscission zone at the base of the terminal leaflet. Washington Navel Orange. 



All of these drawings are semidiagrammatic. 



recent literature on the origin and composition of middle and inner 

 lamellae of the cell wall and their reactions with various enzymes. 

 Until a few years ago it was generally understood that the walls of 

 parenchymatous tissue of the kind under discussion were composed of 

 cellulose. However, the last fifteen or twenty years have seen the 

 development of the chemistry of cellulose and cellulose compounds and 

 it is now accepted that "cellulose" includes a group of closely related 



