1914] The Ottawa Naturalist. 69 



There can be no doubt, furthermore, that the age of an organ 

 has some influence. In the small cotton boll (8 mm. diam.) no 

 evidence of separation (following injury) can be seen at the end of 

 16 hours, but at 20 hours the process is practically complete, 

 as shown by the ease of removal of the peduncle. In larger ones 

 cell-divisions in the separation layer can be seen for a day 

 previous. However, the act of cell-division is not necessarily 

 a precttrsor of separation, since in the young bolls above men- 

 tioned no cell divisions are to be seen at the moment of. separa- 

 tion of the cells concerned, there being only a slight elongation 

 of them, accompanied by a chemical alteration of the cell walls, 

 causing the loosening. The essential phase of the process of 

 abscission may, even in older as well as younger cotton bolls, 

 be of much shorter duration than above indicated, less, namely, 

 than four hours. 



With reference to petals, Fitting found that in the dark, 

 at temperatures of 31 ''-32'', they are shed earlier than normally, 

 and the older the more quickly. On the other hand, petals of 

 younger flowers were found by Fitting to be less sensitive than 

 older, an apparent reversal of things which may be regarded as a 

 "phenomenon of interference," between increasing adaptability 

 and shortening of reaction time.^'' In petals, however, there is 

 no development of mechanical elements, such as quickly appear 

 in many leaves, to increase the amount of preparation before 

 separation may become effective. Early in the season I have 

 observed that abscission (in a moist chamber) will overtake 

 older leaves frequently more rapidly than younger. The slow- 

 ness of separation in indurated organs may simply mean inter- 

 ference by tissues or mechanical elements in which separation 

 takes place only passively or not at all, as, e.g., in the non-living 

 or moribund pith in older cotton peduncles. 



The Mechanisms of Abscission. 

 By "mechanism" is meant that histological behaviour 

 resulting in the separation of one organ, or a part of it, from 

 another organ or part. To the best of our present understanding 

 it may be purely mechanical, either by a break (a) passing 

 directly across the tissue irrespective of the position of the cell 

 walls (rhexolysis, according to Correns 1, 3 72, e.g., Dicranum 

 scopariuni), or (b) passing along the middle lamella, causing 

 separation of entire cells (schizolysis, Correns). In the latter 

 alternative the separation is believed b}^ some to result from a 

 marked increase in turgor, the pressures causing the cells to 



^'The cfiaracter of the behaviotxr of petals under various stimulants 

 has led Fitting to insist on a conception of abscission as an active separa- 

 tion of an entirely living organ in response to stimulation, quite analogous 

 to movements, etc. He proposes the term " chorism " for this phenomenon. 



