1916] CHILD—GRADIENTS IN ALGAE rit 
rate and the gradient is basipetal. Under these conditions each 
level lives, so to speak, at the expense of the level or levels basal 
toit. At this stage the gradient is probably basipetal throughout, 
at least in some cases; but as senescence progresses and the activity 
of the apical region decreasés, the nutritive factor may become more 
important in determining the metabolic rate at different_levels. 
Since the direction of movement of nutritive substance must be 
acropetal in each cell, the basal portions of the cell are most favor- 
ably situated and perhaps therefore show a higher metabolic activity 
with an acropetal gradient, provided relations to more apical cells 
do not obliterate or reverse it. In this way a gradient in the cell 
opposite in direction to the gradient in the hair as a whole may 
arise, at least in the later stages of development. It may be that 
in some of the multicellular hairs this condition is present from the 
early stages of development, but the fact that in the unicellular 
- hairs of Ceramium and Champia the gradient is basipetal in what 
appear to be the earlier stages of development, and acropetal in 
the fully developed hairs, indicates that such change may occur. 
It remains, however, for future investigation to determine whether 
these suggestions are correct. 
Attention has been called incidentally to the differences in 
susceptibility which apparently indicate differences in physiological 
age, such, for example, as the lower susceptibility of fully developed 
as compared with small developing axes, and the low susceptibility 
of the carpospores. The basipetal axial gradient itself is in a sense 
an age gradient, for the physiological age of the cells increases from 
apex to base, the apical cell or cells remaining physiologically young 
because of continued division and cell reproduction, and the other 
cells differentiating and growing old more or less rapidly according 
as they divide less or more frequently. We may say in fact that 
the axial gradient is a more or less definite gradient in cell behavior, 
_and it is this which must be accounted for. It is here, as I believe, 
that the physiological dominance of the apical region plays a réle. 
This region is the first part of the axis to appear, and it originates 
as a region of high metabolic activity, either in consequence of 
physiological isolation -from other apical regions or by the local 
