1914] The Ottawa Naturalist. 51 



that one niav describe the process as a fragmentation of the 

 lower part of the style. In the course of a short time, the raw 

 edges of the fissures becom.e brown by oxidation, thus discover- 

 ing them to the eye. The separation of the corolla proceeds at 

 the same time, although the actual cadence is evident only on 

 the following morning. The abscission plane in the tobacco 

 corolla is irregular and lies a millimeter above the base of the 

 tube (Kubart, 19), while in the epigynous forms it is found above 

 the ovar}'-, and so not at all near the morphological base of the 

 elements of the corolla. 



It is only rarely that in leaves there is any externally visible 

 strtictural indication of the position in which abscission will 

 ensue, and it frequently happens that grooves which are some- 

 times taken for such indications bear no relation at all to the 

 process. It is probable that in the case of the corolla, or of the 

 individual petals, it is more frequently the case^that a more 

 constricted region is made use of. Fitting points this out 

 especially in Geranium, Erodium, etc., describing it as an 

 extremely narrow, isthmus-like reduction of the blade — a very 

 usual condition. Nevertheless, such a reduced region is entirely 

 wanting in manv other species, so that it seems hardh' probable 

 that there is any necessary connection between the two 

 phenomena. 



Periodicity in Abscission. 



We consider, under the head of periodicity, more especially 

 that of leaf abscission. It is true that external conditions affect 

 also that of floral organs, and, to the careful studies of Fitting 

 (20) and Hannig (12) more especially, reference will be made 

 beyond. The apparent synchrony between the fall of the 

 leaf and the end of the growing season being the most widely 

 recognized, it is convenient to discuss this especially. 



I say apparent, since, in a large measure, we are deceived 

 in temperate and boreal regions, as well as in the tropics, by the 

 continuity of verdure in its entirety. The constant dying-off 

 of leaves escapes attention, albeit a little careful observation 

 will discover the fact. During, or soon after, the unfolding of 

 the buds in spring, the bud-scales suffer abscission, and as the 

 new shoots advance in age the earlier formed leaves in their 

 turn drop off. Nevertheless, the majority of the leaves produced 

 during the season are, under normal seasonal conditions, shed 

 within a rather short space in the autumn, the exceptions to 

 this rule being the so-called evergreens, in which the life of the 

 leaf is extended over a longer period, namely, two seasons or 

 more. Obviously, in these the longevity of the leaf is pre- 

 dominant, seasonal responses being seen in the growth of stem 



