98 BOTANICAL GAZETTE [AUGUST 
characteristics. Young axes in good physiological condition always 
show a very definite susceptibility gradient from the apical end 
basally, that is, death with the disappearance of the neutral red 
* begins at the tip of the stem or branch and proceeds basipetally 
with a high degree of regularity. The progress of the decoloration 
and death from cell to cell along the axis can be followed under the 
microscope. Occasionally a cell here and there or a small group of 
cells may die earlier or later than the other cells of their level, but 
in general the progress of death in the basal direction from the 
apical end is very regular in the younger axes. 
In the large tubular axes, however, such a uniform death gradi- 
ent does not appear. The first cells to die are scattered more or 
less over the whole length of the axis, or in many cases the death 
gradient is more or less distinctly acropetal in the more basal por- 
tions instead of basipetal as in the young axes. 
These differences between the young and the large tubular 
axes are undoubtedly connected with the differences in the regions 
of growth and cell division in these different stages. In the newly 
formed developing axis where there is a more or less definite growing 
apical region, this is the region of greatest metabolic activity, and 
therefore a basipetal susceptibility gradient appears. Later, how- 
ever, the activity of the apical region decreases and is apparently 
replaced by cell division and growth in the basal region or more or 
less along the whole axis. This change is associated perhaps with a 
greater or less degree of physiological isolation resulting from the 
decrease in activity of the apical region, which no longer dominates 
other parts to the same extent as earlier, and the earlier orderly 
behavior disappears. The original basipetal susceptibility gradient 
disappears, therefore, and is replaced by an acropetal gradient, 
at least in the basal region, or by a susceptibility mosaic. This 
method then makes possible an optical demonstration of the differ- 
ent localization of the highest metabolic activity at different stages. 
With a little care axes in all stages of this change can be found. - In 
some the apical region is still active to some extent and a basipetal 
gradient extends for a short distance from it, while farther down the 
axis a susceptibility mosaic may appear, or the basal region may 
show a high susceptibility. 
