— 76 — 



the tissues, a view which must be called into service to e>q)lain the peculiarities 

 seen in the abscission of the cotton boll, which I have described elsewhere (1. c.)- 



The position of the abscission layer which removes the leaf-base and accom- 

 panying bud and which I have called lateral axial abscission, is, above the bud, 

 identical with abscission in the oblique position, if that were taking place. This 

 appears to be so from the fact that both oblique and lateral abscission may take 

 place at the same time, the single layer separating into two which swerve away 

 from each other in the middle of the stem (figure 3c). The lateral abscission 

 layer then passes downwards and out on the same side of the stem that it entered, 

 either below, or, it may sometimes be through, the leaf-base. 



The course of the abscission layer through the tissues varies but it is not 

 "almost as smooth as a knife cut", if I place on their phrase the meaning which 

 Gortner and Harris intended. It may indeed appear so after dessiccation, but 

 when examined at or soon after the completion of abscission, it is as follows: 



In the floral axes, the abscission layer passes transversely through the cortex 

 but moves distally when it reaches the vascular tissues. The disruption of the 

 traces is a little irregular. Through the pith the layer again runs transversely, 

 but the plane is placed further away from the base of the internode, so that the 

 central portion of the stem stands up like a peg on the stump which is left (fig. 8). 



In the vegetative internodes it is quite otherwise. Here it runs downwardly 

 at a sharp angle through the cortex and pith toward the centre, forming at the 

 top of the stump which is left a crater-like depression (figure 9) from which the 

 ends of the vascular strands project upwards. Rarely the concavity does not 

 occur in the pith, but when I have observed this to be the case, the layer passes 

 downwards at some point, (figure 10). There results from the steep angle of the 

 abscission layer through the cortex a very thin frill-like margin on the stump. 

 The first evidence of actual separation becomes apparent when, because of the 

 lifting effect of the pith, the already separated surfaces lying between the frill 

 and the apposed cortical cells of the distal part are thereby moved apart, and 

 so allowing the entrance of air. The total reflection of the intercellular external 

 cell-surfaces makes a shining band indicating the abscission layer. After this the 

 cuticle is torn. 



The method of abscission. There is no special abscission tissue. The im- 

 mediate cause of abscission in internodes, leaves and floral parts is the hydrolysis 

 of the middle lamella, and no disintegration of tissues in any other sense takes 

 place. This is equally true of all the living element of the vascular tissues. In 

 suitable preparations, the free ends of sieve tubes and companion cells can be per- 

 fectly seen. There is no tearing or distortion except such as is due to absorption 

 of water. The loosening of the cells is not confined to a single intercellular 

 plane, as is shown by the fact that if a drop of water be placed on a fresh surface 

 just after abscission, separated individual cells will float out. Increased turgor 

 which has been supposed by Loewi to be capable of effecting separation in a 

 mechanical way (Ampclopsis) does not enter in since the abscission cells and 

 those of nearby tissues are isotonic and since there is no swelling of the tissues 

 previous to abscission- Furthermore I have found that abscission is not hindered 

 by a slight but evident amount of wilting. However, after the digestion of the 

 middle lamella has proceeded in the cortex, thus releasing the pressure on the 

 pith, the abscission surfaces may be pulled apart by the extending pith. In sup- 

 port of this view that turgor itself is insufficient to cause abscission, I may cite the 



