﻿LLOYD— ABSCISSION 



passage of water, since they are not sufficient either in size or 

 numbers, and are not present at all in many vessels. The forma- 

 tion of tyloses appears to be rather a wound response, more or 

 less incomplete at the time of rupture of the leaf , since the greater 

 development, if incomplete, is found below the abscission plane, 

 that is, the tyloses appear in the general region and at the same 

 time as the general wound response. 



The younger phloem elements, those quite near the cambium 

 cells and with difficulty distinguishable from them, appear to 

 behave the same as the latter. The older, on the other hand, 

 which show a development of callus (though whether this is syn- 

 chronous with the development of tyloses, as Tison holds, I cannot 

 at present say) show indications that they behave in a quite passive 

 manner. One can detect no definite zone of thinning in the wall, 

 but, if stretched at all, they act merely as a soft yielding material, 

 becoming thin throughout their whole length, quite as a soft india- 

 rubber band behaves, and finally break. The very small size of 

 the elements makes it difficult to be quite sure of this conclusion, 

 but I have seen no evidence which would lead to any other. 



The abscission cells of the epidermis behave as do the paren- 

 chyma cells, except for a certain asymmetry due to the unyielding 

 cuticle, which merely breaks at last, after being loosened by alter- 

 ation of the adjacent cellulose membrane. 



The final condition of the active abscission cells is then as 

 follows. The walls are locally, and it may be very irregularly, 

 much extended and extremely thin. The thin membrane of each 

 cell is distinct from that of any neighboring cell, and also from the 

 proximal end walls of the cells above. The cytoplasm consists 

 of a correspondingly delicate membrane, but shows no alteration 

 in the direction of degeneration. The same is equally true of the 

 nucleus, which is in appearance quite the same as evidently normal 

 nuclei elsewhere, but that it is frequently much larger. When this 

 condition has been reached, abscission may be regarded as complete. 

 After the cuticle is broken, the abscission cells near by quickly 

 collapse, while the shrinkage of the parts above, due to curtail- 

 ment of water caused by local evaporation and by the severance 

 of the vascular elements, produces a shearing and rupture of the 

 abscission cells. At a slight touch, the whole layer of weakened 



