418 University of California Publicatioyis in Botany [Vol. 6 



The material used in this study consisted for the most part of 

 leaves taken from trees of the Washington Navel variety Citrus sinensis 

 and Eureka lemon variety Citrus Limonia grown in the greenhouses 

 of the University of California. The mode of foliar abscission was 

 found to be similar in both species. Practically all the histological 

 and cytological work was done with the aid of fresh sections cut either 

 with the table microtome or free-hand. Abscission was induced by 

 placing cut shoots in large moist chambers kept in the laboratory at 

 room temperature and was found to be consummated within 24 to 96 

 hours. Sections were cut at intervals from the time of insertion into 

 the chambers until actual separation had occurred. After being placed 

 in the moist chamber the most important factors in determining the 

 time at which abscission was complete, named in the order of their 

 importance, were found to be temperature and age of the material. 



Series were also run in which illuminating gas and carbon dioxide 

 were introduced in varying concentrations to ascertain their effect 

 upon the time at which abscission was complete. This time was found 

 to be substantially the same, no matter which of these gases was used 

 or whether none was used. This is taken to indicate that in the case 

 of citrus shoots placed in moist chambers or exposed to illuminating 

 gas, carbon dioxide, or ordinary laboratory air the stimulus leading 

 to abscission is probably not of an external nature, at least directly. 

 Indeed, it seems more likely that it is internal and of the nature of a 

 disturbance in the physiological equilibrium existing within the shoot. 

 Preliminary studies on the relation between the time at which abscis- 

 sion occurs and the area of tissue distal to the abscission zone con- 

 cerned have furnished some evidence that something of the nature of 

 mass action is involved in the stimulus. Leaves which had been re- 

 duced in area by removal of a portion of the terminal leaflet were 

 found to remain on the shoot longer than leaves not so treated and a 

 general relation between the area removed and the time reaction was 

 noted. This phase of the general abscission problem in Citrus as well 

 as several others are reserved for discussion at a later time. 



The Abscission Zone 



The ordinary citrus leaf consists of a main terminal leaflet behind 



which is a somewhat elongated petiole bearing a narrow extension on 



either side. There is little doubt but that the whole represents a 



modified compound leaf, the two basal leaflets of which have been 



