﻿BOTANICAL GAZETTE 



occur where there is the greatest heterogeneity of structure or 

 maximum mechanical resistance, namely, in the oldest stems and in 

 the vascular tissues, so markedly indeed that I have frequently ob- 

 served in some plants an entire misdirection of the plane of abscis- 

 sion, in that it comes to lie parallel with the vascular bundles for 

 some distance. Conversely, the greatest regularity is seen in the 

 pith and medullary rays, where, however, in spite of the homo- 

 geneity of the tissues, minor local departures from the ideal path 

 may be observed (pi. figs. 1-4). 



CYTOLOGICAL BEHAVIOR OF ABSCISSION CELLS 



a) The role of starch. — Previous to abscission, there is an 

 abundance of starch in the general region below the separation 

 layer, while it is withdrawn from above it except from the starch 

 sheath. As abscission progresses, the starch disappears below, 

 from the middle of the pith last. From this point it has dis- 

 appeared usually by the time abscission is well advanced through- 

 out its whole extent. In the early stages of abscission starch is to 

 be found within all the separation cells themselves, but it largely 

 or entirely disappears as abscission is completed. The place where 

 the absence of starch is first to be noted is in the prosenchyma 

 sheath next to the starch sheath. The latter also loses starch in 

 the vicinity of the abscission tissues. When abscission is com- 

 plete, I have observed in some cases various but always small 

 amounts of starch, a few large grains in the middle of the pith and 

 a very few scattered grains elsewhere or none. The fact therefore 

 appears to be that, as abscission progresses, the starch in the 

 abscission cells decreases until it is very materially reduced in 

 amount or entirely disappears. The inference is that it is used by 

 them as a source of energy, expended in the active growth leading 

 to separation. 



Hannig described the abscission layer as being marked by 

 a transverse band of starch grains, but adds that those cells which 

 are reduced to extremely thin membranes and are more or less 

 collapsed still contain starch. He suggests that this is due to the 

 rapid autolysis of these cells; but the purpose of this essay is to 



