1919 Child; on Susceptibility of Algae 265 



certain relation to the rate of the fundamental vital processes. So far as 

 oxidation is such a fundamental process a relation between susceptibility 

 and oxidation rate must exist. While in general the condition of the cell 

 surface undoubtedly varies with conditions inside the cell, it does not follow 

 that all changes within the cell must produce changes at the surface and so 

 changes in susceptibility. It is probable that certain functional processes 

 in the interior of the cell, such, for example, as digestion in the entoplasm 

 of a protozoan do not affect the surface to any marked degree, though they 

 may increase the oxidation of the cell as a whole. 



Susceptibility, then, is measured by the time required under given con- 

 ditions to kill or destroy the physiological and morphological characteristics 

 of the superficial region of the cell. So far as the condition of this region 

 is related to conditions in the interior of the cell, susceptibility is a measure 

 of the condition of the cell as a whole. 



The similarity of susceptibility relations to different agents and the 

 general relation between susceptibility and rate of metabolism or oxidation 

 undoubtedly result from the fact that living protoplasm is not a mosaic of 

 independent reactions and conditions, but a system of correlated and inter- 

 dependent states and changes, and that life consists in the maintenance of 

 this system within a certain range of variation or oscillation about a condi- 

 tion of dynamic equilibrium. But whenever any essential factor in the 

 system is altered beyond a certain limit by conditions external to the system, 

 the whole system is affected, and if the change in the particular factor is 

 sufficient, maintenance of the system and of life becomes impossible. Grant- 

 ing that the various agents and conditions used in determining susceptibility 

 act upon the protoplasmic system in different ways, i. e., primarily or chiefly 

 upon different constituent factors of the system, their general effect in the 

 concentrations and intensities used is essentially the same. Whether a parti- 

 cular agent affects primarily colloid aggregation, lipoid condition, ion con- 

 tent or distribution, enzyme activity, the oxidative reactions or any other 

 essential feature of the living system, this action, if sufficient in degree, will 

 retard or prevent the continued activity of the system and alter the proto- 

 plasmic structure associated with this activity. Moreover, we find, as might 

 be expected, that a relation exists between the sensitiveness of the system 

 to such action and the rate or intensity of its own activities. To grosser 

 disturbances, i. e., killing concentrations or intensities, the more intensely 

 active system is more susceptible, while, on the other hand, adjustment or 

 acclimation to less extreme disturbances, i. e., lower concentrations or inten- 

 sities, occurs more rapidly or more completely in the more active than in 

 the less active svstcni. 



