1. The thalli of all axiate algæ examined show an axial gradient in susceptibility to various agents, KCN, alcohol, ether, HCl, HgCl2, CuSO4, neutral red, high temperature, etc. To concentrations or intensities sufficient to kill rapidly without acclimation the apical region is most susceptible, and the susceptibility decreases basipetally in each axis. This susceptibility gradient may undergo more or less complete reversal under various conditions. Certain concentrations of certain agents may even reverse the gradient in susceptibility to themselves. 2. As in animals the susceptibility gradient is in general an indicator of the vitality, metabolic rate, or physiological condition at different levels of the axis. The gradient may be altered or more or less completely reversed by change in external conditions, by advancing age, by physiological isolation of parts, etc., and the readiness with which alterations occur in altered environment is in some degree a measure of the sensitiveness of the species.