1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 415 



CONCLUSION 



It is proposed in what follows to take up consideration of such 

 phenomena in connection with abscission as are still but slightly 

 understood. One of the most perplexing of these is undoubtedly the 

 definitely predetermined location of the separation layer _when no 

 morphological and sometimes no physiological (Datura) difference can 

 be detected between the cells that separate and those that do not. 

 There need be no doubt, however, that such a difference does exist 

 and that a sufficient refinement of technique will serve to detect it. 



In considering this matter further it may be recalled that the 

 separation layer in axial abscission is located at or near the base of an 

 internode. There is undoubtedly some connection between this fact 

 and the fact that the cells most active physiologically are often found 

 in this region. The growth of an internode may be brought about by 

 the action of an intercalary meristem located at the base of the organ 

 and a meristem so located in some cases retains its original activity 

 in the mature internode. Now it is well known that the walls of young 

 active cells are more readily subject to hydrolysis than the walls of 

 older cells, because of the fact that the former contain more water. 

 If we assume, then, that the internode is a metabolic gradient with 

 the most active cells at the base, it would be expected that the walls 

 of these cells would be more subject to hydrolysis than any other cells 

 of the internode. If some hydrolysing agency becomes active 

 throughout the pedicel, it might be expected that the walls of the 

 cells at the base of the internode would react first, causing their sep- 

 aration and thus cutting off the flower or internode. By assuming 

 in this way that separation always takes place through the most 

 active cells of the internode it seems possible to explain the predeter- 

 mined location of the separation layer. 



There is undoubtedly some connection between the above problem 

 and the fact that some plants must perfect a separation layer before 

 detachment can take place. In such cases the tissues at the base of 

 the organ are too old for separation. The same stimulus which causes 

 abscission in some species causes a renewal of activity at the basal 

 region of an organ, resulting in cell divisions and new cells. These 

 new cells may, under a continuation of the stimulus, separate one 

 from another. 



Another perplexing problem, which also includes many subsidiary 

 problems, relates to the exact course taken by the stimuli in causing 



