1919 Child; on Susceptibility of Algae 263 



The relation between susceptibility and growth activity in the algae 

 indicates that in these forms also susceptibility is in a general way a measure 

 of certain aspects of physiological or metabolic condition. The process of 

 photosynthesis in plants has no parallel in animal metabolism, but photo- 

 sj'nthesis is dependent upon external conditions and varies in rate quite 

 apart from the other metabolic activities of the plant. Apparently in the 

 complex of these other activities, oxidations constitute for the plant, as for 

 the animal, a fundamental factor. In general rapidly growing cells respire 

 more rapidly than those growing slowly, and, since susceptibility shows a 

 very definite relation to growth activity, we may expect to find a general 

 relation between it and respiration or oxidation. 



This, of course, does not imply that all agents used to determine sus- 

 ceptibility enter chemically into or directly influence the oxidative reactions. 

 It means only that in general protoplasm which has a high rate of oxida- 

 tion is in a condition that renders it more susceptible to the toxic action of 

 at least many external agents. Different agents undoubtedly act upon pro- 

 toplasm in different ways. The "vital" dye neutral red, for example, which 

 enters readily all cells along an axis in most plants, though its rate of pene- 

 tration decreases to some extent basipetally, does not kill the cell-surface by 

 its entrance, but death results from its accumulation within the cell. To 

 acids, alkalies and many other substances, on the other hand, the living 

 plasma membrane in its normal condition is only slightly permeable, and 

 these substances do not enter the cell, except to a slight extent, until they 

 have killed its surface or produced in it irreversible changes which lead to 

 death. Nevertheless, the differences in susceptibility in different regions 

 of the plant are the same for neutral red as for acids, alkalies and many 

 other substances. 



It is evident, then, that these differences in susceptibility cannot depend 

 solely upon differences in permeability of the living plasma membrane. 

 Even with neutrqj red the differences in susceptibility are very much 

 greater than the differences in permeability. The susceptibility of a cell, 

 as determined by this method, really depends primarily upon the rapidity 

 with which the surface, the jilasma membrane, is killed or undergoes certain 

 changes through the action of an external agent. The data on susceptibility 

 do not give ver}^ definite information concerning the interior of the cell. It 

 is probable that in neutral red phj'siological conditions inside the cell are 

 greatly altered before the surface dies or undergoes irreversible changes, 

 while in acids and alkalies the interior of the cell undergoes little or no 

 change in condition until tlic surface is killed or irreversibly altered. In 

 other words, neutral red and various other vital dyes kill from within the 

 cell, while it is ])raotic.illy true that acids, alkalies and many other agents 

 kill or irreversibly alter tlie surface before they can act on the interior. 



