1916] CHILD—GRADIENTS IN ALGAE 97 
After testing various concentrations of KCN, I found that 
although different species showed considerable differences in suscep- 
tibility, a concentration of m/100 served the purpose for the forms 
examined, the survival time in this concentration ranging from a 
few minutes to several hours. 
Neutral red itself, particularly in high concentrations, is some- 
what toxic to living cells, and in the course of my experiments I 
found that a strong solution of neutral red in sea water could be 
used in the same way as cyanide to determine differential suscepti- 
bility, the only difference observed being that death occurs much 
less rapidly in the neutral red alone than in cyanide. 
The work was done at the Marine Biological Laboratory at 
Woods Hole during August and September 1915. From among the 
forms available at that time, species with definitely axiate, and in 
most cases with more or less highly branched thallus and with a 
more or less definite growing tip (at least during the earlier stages of 
growth), were selected, because it was desired first of all to deter- 
mine whether characteristic differences in susceptibility were asso- 
ciated with the visible axes of stems and branches. Most of the 
forms examined were Rhodophyceae, and it might be supposed that 
the natural color of these forms would interfere with the use of 
neutral red as an indicator. Asa matter of fact, however, the color . 
of the protoplasm stained with neutral red is distinctly different 
from the natural color, and there was no serious difficulty in observ- 
ing the change and disappearance of color in any case, and the 
protoplasmic changes afforded additional means of determining 
the time of death in many cases. 
_ For the identification of a number of the species I am indebted 
to Mr. Gustavsen of the M.B.L. Supply Department. Others 
were identified with the aid of FAarLow’s Marine Algae of New Eng- 
land, supplemented by RABENHORST’s Die Meeresalgen. Thus 
far the growing thalli or parts of them have been examined in 14 
species. 
The susceptibility gradients 
Enteromor pha sp.—Many plants of this species were examined, 
ranging from stems and branches in very early stages of develop- 
ment to large old thalli which had taken on the ‘‘intestinalis”’ 
