416 University of California Publications in Bota-ny [VOL. 5 



abscission. Experiments described in the present paper have indi- 

 cated that this course may be direct as well as indirect. Assuming for 

 the present that some of the factors bringing about abscission always 

 act directly while others act indirectly, we might classify the general 

 factors operative in the case of the Solanaceae as follows : 



DIRECT 1. Narcotic vapors. 



2. Injury to floral organs. 



3. Sudden rise in temperature. 



4. Lack of fertilization. 

 INDIRECT 5. Changes in soil conditions. 



6. Factors evident in normal physiological development. 



The direct factors act directly on the cells at the base of the pedicel 

 and consequently the reaction time must be comparatively rapid. The 

 indirect factors act indirectly through the general physiological con- 

 dition, which in turn furnishes the direct stimulus for cell separation. 

 In the latter case the reaction time must, as a general rule, be slow. 

 The nature of factors under 6 are most difficult to understand. An 

 example of the action of these factors would be given in those cases 

 where most of the flowers of an inflorescence are normally abscissed 

 leaving only one or two to continue development, and in those species 

 which absciss male flowers after anthesis. 



A further analysis of the course of the abscission reaction intro- 

 duces another unsettled problem the nature of the agency which is 

 directly responsible for the dissolution of the middle lamella. It has 

 been pointed out before that an enzymatic body of some kind is prob- 

 ably involved. The following discussion brings out certain facts 

 which it is necessary to take into consideration when speculating as 

 to the nature of this supposed enzyme. The activity of the enzymatic 

 body must be subject to both internal and external conditions. The 

 enzymatic material must also be extremely sensitive to slight changes 

 in the normal environment. It must be continually present in the 

 cells of the separation zone and ready at any moment to react to such 

 changes in the environment. A comparison of several species in 

 regard to their abscission reactions to the factors listed above indicates 

 that this supposed enzyme must be more sensitive in some species 

 than in others. Indeed, in certain species in which no abscission 

 occurs the enzyme must be absent from the region of the separation 

 zone or entirely inactive. Finally, it seems necessary to assume that 

 in certain species the action of the enzyme is suddenly inhibited at 

 about the time of the opening of the corolla. 



