A) ee CHILD—GRADIENTS IN ALGAE 99 
When the plant is killed by a high concentration of neutral 
red alone, the same axial differences in susceptibility appear as in 
death from KCN, although of course death begins later and pro- 
gresses much less rapidly. The simple experiment of gradually 
killing the plants by overcrowding in standing water was also per- 
formed, and the changes in susceptibility and the progress of death 
were followed. After two days under these conditions, staining 
with neutral red showed that longer or shorter portions of the 
apical regions of the small short branches were dead and would 
no longer take the stain, while other parts still stained as living 
cells. In the large tubular thalli practically all cells were still 
alive at this time. After 5 days in standing water most of the 
short young branches were wholly dead, but in some a short basal 
region was still alive. The large thalli at this time showed a 
mosaic or patches of dead cells, often more marked in the basal 
region. These crude experiments are sufficient to show that 
differences in the susceptibility of different regions of the plant to 
overcrowding in standing water are essentially the same as to 
KCN. Death progresses in the same way in both cases. 
One other point of interest in relation to the physiological age 
of parts may be mentioned. I have found it possible to use the 
susceptibility method very widely among animals to distinguish 
differences in physiological age, the young animals being more sus- 
ceptible than the old because of their higher metabolic rate (11). 
In Enteromorpha the short branches, evidently in early stages of 
development, are more susceptible than the larger older branches. 
Where such short branches arise laterally on a larger stem, death 
proceeds basipetally in the short branch to the point where it joins 
the stem, but here it stops for the time being and the adjoining 
cells of the stem may remain alive for several hours after the branch 
is dead. The branch is evidently much younger than the stem at 
its level, and consists dynamically of a metabolic gradient of much 
higher rate than that of the stem, and this gradient ends rather 
sharply at its base. The gradient in the branch itself is also in a 
sense an age gradient, the cells of the apical region being youngest 
physiologically because of their continued reproduction, while 
basipetally the physiological age of the cells increases. In the large 
