1916} CHILD—GRADIENTS IN ALGAE IOI 
in a regular basipetal gradient. These irregularities undoubtedly 
indicate the disappearance of the regular gradient in the later vege- 
tative stages, with intercalary growth and cell division in the more 
basal regions. Here, as in Enteromorpha, elongation of the axis 
beyond a certain length, or decreasing activity of the apical region 
in consequence of advancing physiological age or for other reasons, 
may lead to a greater or less degree of physiological isolation of the 
more basal cells, which then again become active and therefore 
show a higher susceptibility. 
Castagnea tuberculosa.—The complex structure and opacity of 
the thallus of this form interfered with the use of the susceptibility 
method, but it was found that the apical regions were most suscep- 
tible, and here in many cases the cells actually disintegrated in 
KCN as they do soon after death in many of the lower animals. 
Over most of the length of the thallus, however, no definite and 
constant gradient was observed. 
The delicate transparent hairs which arise on the surface of the 
thallus afford interesting objects for susceptibility experiments. 
They consist of a single series of elongated cells and are unbranched. 
These hairs are very highly susceptible to KCN, and in m/100 they 
begin to die almost at once. Most hairs show in general a basipetal 
susceptibility gradient, but there are frequent irregularities, one 
or a few cells dying out of the order proper to such a gradient. 
Many if not all of these irregularities are undoubtedly due to 
injuries, for the hairs are so delicate that even the most careful 
handling of the plant is likely to break or bend them, but it may 
be that other factors are also concerned. The usual susceptibility 
gradient in these hairs indicates that their growth is primarily 
apical like that of most plant axes, but it is not improbable that in 
later stages secondary growth and division may occur in other cells 
as the activity of the apical region decreases. 
Callithamnion (roseum?).—This monosiphonous form is very 
favorable for demonstration of axial differences in susceptibility. 
Both young plants consisting of a single stem with bipinnately 
arranged branches and more advanced stages in which branching 
had proceeded much farther were examined with similar results 
in all cases. In each branch death begins at the apex and proceeds 
