SUSCEPTIBILITY GRADIENTS. 89 



agent may reverse the susceptibility to itself, as in the case of 

 neutral red, and it is also possible in many cases to determine the 

 effect on susceptibility of one agent by using another for killing 

 after exposing to the first. The whole complex problem of the 

 nature of the action of external agents of different kinds on pro- 

 toplasm is involved in these data on susceptibility, but even 

 though we are far from the solution of that problem we can 

 establish certain general laws of susceptibility as characteristic 

 features of a physiological axis, i. e., as an expression of polarity. 



When the gradient is obliterated or reversed multicellular hairs 

 often separate more or less completely into individual cells. 

 The persistence of the hair as a multicellular order seems to be 

 associated with the existence of the gradient. It will be shown 

 elsewhere that this is true, not only for these hairs, but for the 

 vegetative axes of certain species of algae. Tobler ('02, '04) has 

 observed in various species of the red algse this tendency to 

 separate into cells under unfavorable conditions and has used it 

 as a method for inducing experimental reproduction. In the 

 isolated cells of the hairs reproduction can scarcely be expected, 

 but the relation between morphological order and physiological 

 correlation is evident. After obliteration or reversal of the dy- 

 namic axial gradient the morphological order soon ceases to 

 exist except where the structural stability is high. 



A question of some interest concerning what may be called the 

 death-level arises particularly in connection with the Chondria 

 hairs. The young highly susceptible hairs of Chondria die with- 

 out perceptible reversal of the gradient under conditions which 

 bring about complete reversal in the hairs of medium age and 

 some reversal of intracellular gradients with perhaps partial 

 reversal of the hair-gradient in the oldest hairs. To reverse the 

 gradient before death in the youngest hairs less extreme condi- 

 tions must be used. These and various other facts suggest the 

 possibility that the death-point, or more properly speaking, the 

 death-level is not necessarily the same quantitatively for more 

 active and less active protoplasm of the same kind and for more 

 and less rapid killing, in other words the death-level may be 

 relative rather than absolute. With our present lack of knowl- 

 edge this remains merely a possibility. 



