106 BOTANICAL GAZETTE [AUGUST 
branched thalli, the susceptibility gradient is basipetal. Irregular- 
ities in the course of death and acropetal gradients observed in 
some cases probably result from injury or bad environmental con- 
ditions which affect the more active regions to a greater extent 
than the less active regions, or they may indicate perhaps the 
completion of apical growth in an axis. In general the physio- 
logical ages of different branches on a single axis are indicated 
by their susceptibility, the smaller less advanced branches being 
the more susceptible. 
In some of the plants examined cystocarps were present. In 
the sterile wall of the cystocarp the gradient is basipetal, as in the 
vegetative portions of the plant. Differences in susceptibility in 
the cystocarp contents, indicating differences in physiological age, 
were also observed. No attempt was made to distinguish the 
various stages in the development of the carpogonium and carpo- 
spores, but it was merely noted that the susceptibility of the fully © 
developed carpospores was much less than that of the cystocarp 
contents in the earlier stages. This difference indicates that the 
carpospore has the low metabolic rate characteristic of physiologi- 
cally old cells. 
The transparent hairs which arise in tufts at the apical ends of 
the thallus branches are beautiful objects for the observation of 
the susceptibility gradient. Each hair is monosiphonous and 
dichotomously branched, and the susceptibility gradient in each 
hair as a whole is very distinctly and uniformly basipetal, the apical 
cell of each branch dying first and death progressing basipetally 
- from cell to cell in regular order. 
In each single cell of the hair, however, the susceptibility 
gradient is almost invariably acropetal. The disintegration of the 
protoplast and the appearance of the dark masses resulting from 
coagulation begin at the basal end and proceed very uniformly 
to the apical end of the cell, the progress of death over the whole 
length of the cell often requiring 2-3 minutes. With rare exceptions 
the death of each cell is completed before that of the next cell basal 
to it begins. 
My observations were limited to hairs which were apparently 
full grown or. nearly so, and only further work can determine 
