108 BOTANICAL GAZETTE [AUGUST 
cells in collecting and handling. The great difference in suscepti- 
bility between the two species is evident from the fact that in P. 
fibrillosa death begins almost at once and is usually completed within 
a half hour or less:in KCN m/t1oo, while in P. variegata even the 
more apical regions live in the same concentration from half an 
hour to an hour, and the less susceptible parts die only after several 
hours. 
Dasya elegans.—In both the polysiphonous axes of the thallus 
and the very numerous monosiphonous, dichotomously branched 
secondary branches, the susceptibility gradient is in general 
distinctly basipetal. In the secondary branches, however, irregu- 
larities appear, chiefly near the base. As in Callithamnion, the 
basal cell or the two most basal cells of the older branches very 
often die before those above them. The suggestion made in the 
case of Callithamnion may apply here also, namely, that these 
cells are more stimulated mechanically than those above them by 
the movements of the branch with the water currents. It may be, 
however, that some degree of physiological isolation of the basal 
region has occurred in the older branches. 
A basipetal gradient also appears in the system of secondary 
branches on an axis as a whole, the most apical branches being most 
susceptible, those lower down less so, and so on. This is true even 
for the apical regions of the secondary branches, and indicates -heir 
differences in physiological age, corresponding in general to their 
level on the axis, the most apical being the youngest, the most 
basal being the oldest. 
The elongated stichidia present on some of the plants examined 
also show a basipetal susceptibility gradient, and the gradient in 
development of the tetrasporangia in the stichidia is paralleled 
by a susceptibility gradient, the most basal and so most advanced 
tetraspores being the least susceptible parts of the plant on which 
they occur. 
Discussion 
The observations on these 13 species indicate, so far as they go, 
that in each definite vegetative axis of these forms, at least in plants 
that are in good condition and not too old, a more or less definite 
