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but it may occur quite independently. I have seen a half-dozen instances, but only 

 in plants kept in a moist chamber. In all instance the buds remained undeveloped, 

 though in otherwise normal condition. No evidence can be clearer than this to 

 show that the position of the abscission layer is determined physiologically. 



Gortner and Harris observe that injury of an internode by insects is an effic- 

 ient cause, as no doubt it may be. But to what extent, and what the nature of this 

 injury must, as I have said, still be inquired into. Hannig (9) found it impossible 

 to cause shedding (of peduncles or internodes) by any kind or degree of wound- 

 ing so long as enough vascular tissue was left to insure the necessary quantum 

 of water and nutrition. The lack of the latter is doubtfully the direct cause in any 

 event, since he observed an abundance of starch and a healthy appearance when 

 abscission did intervene. 



These results are generally true also of Impaticns sultani. I did the fallow- 

 ing experiments. 



1. Longitudinal cuts were made from one end to the other of long internodes. 

 In one case the cuts were made so deeply that, as the result of callus formation, 

 the wounds gaped widely enough to allow one to see through the stem. Some- 

 times sectors extending throughout the internode were removed. In no case did 

 abscission follow. 



2. Transverse cuts were made so as to trans-sect all the vascular tissue of 

 the internode at one point or another, without result. 



3. Very oblique cuts were made, passing almost parallel to the petiole and 

 through it (figure 4). So long as the leaf did not wither, no abscission took 

 place. 



4. A slender internode was punctured by a needle, this being allowed to 

 remain in situ. No result beyond the drying up of one side of the stem near the 

 needle resulted. 



injury itself is therefore not a direct stimulus to abscission, for it is only 

 when the physiological corres'pondence between the proximal and distal parts is 

 incomplete that it intervenes. 



The position of the abscission layer. In the case of internodes. whether 

 floral or vegetative, the abscis.sion layer lies a short distance above the base, but 

 is not fixed strictly in any particular position. The significance of the occurrence 

 of two superimposed layers has been pointed out above. In the floral axis it is 

 transverse while in the vegetative it is usually oblique, often enough so as to form 

 an angle of about 30 degrees with the axis of the stem. It begins 3 mm. or less 

 above the axillary bud and passes downward. 



When two leaves are placed directly opposite to each other, (the leaves being 

 normally alternate) a separate abscission layer will be formed for each leaf, and 

 w'ill meet in the middle to form a single V-shaped one when complete, the sulcus 

 of the V lying at right angles to the plane of the two leaves (figure 5). If, in- 

 stead of being absent, there is a short internode between neighboring leaves, the 

 abscission layers of them may still become confluent by running longitudinally to 

 meet each other. I have observed this when the internode was as many as 7 mm. 

 long (Figure 6). 



The abscission layer is furthermore not always symmetrical with reference 

 to the longitudinal mid-plane of the stem passing through the leaf, but may be 

 twisted (figure 7). It is difficult to explain the varying obliquity and other de- 

 partures from a plane transverse position except on the theory of displacement of 



