[LLOYD] ABSCISSION OF FRUITS 21 



marked "July" in Fig. 2. This is essentially like the graph for the nut 

 whose behaviour has already been described. The record is short, and 

 embraces only two days on which growth occurred, in much reduced 

 amount on the second day when abscission was beginning to make 

 itself apparent. On the third and fourth days shrinkage was con- 

 tinuous, being, as would be expected, more marked during the daytime. 

 Abscission was found to have drawn to completion at 8 a.m. when the 

 record was removed. 



The method of abscission is of the type exemplified in Mirabilis 

 Jalapa^ differing only in unimportant secondary details connected 

 with the thicker cell walls such as the persistence, in a living condition, 

 of the cells which have separated by the hydrolysis of their cell walls. 

 The process begins by the elongation of the cells (Fig. 5) in a more or 

 less irregular zone (Fig. 4), following on the alteration of portions of 

 their walls, including both primary and secondary material, the 

 innermost layers only persisting to keep the protoplasts enclothed. 



Fig. 4. Diagram indicating the position of the abscission zone at the base of 

 the fruit. 



The zone of cells involved is irregular, and lies very slightly above 

 the base of the fruit, the earliest abscission activity occurring in the 

 pith. If the fruits are allowed to be exposed to high humidity for some 

 time at the close of abscission, considerable proliferation and loosening 

 of abscission cells occurs, and this condition is, therefore, more pro- 

 nounced in the inner regions of the abscission zone (c, Fig. 5). 



It may be concluded that, in the species herein discussed, abscis- 

 sion is brought about by the repeated too great water deficit in the 

 tissues of the growing fruits resulting from competition between them, 

 the stem being inadequate for the transmission of sufficient water. 

 The process occupies, in all, three to five days, the major portion of 



^ Lloyd, F. E. Abscission in Mirabilis J alapa. Bot. Gaz. 61: 213-230. March, 

 1916. 



