AXIAL SUSCEPTIBILITY GRADIENTS IN ALG^E. 437 



we should expect such action to be more marked with high 

 rather than with low concentrations. 



The more or less complete reversal of the gradient observed in 

 Griffithsia with the higher concentrations of HgCU and CuSC>4 is, 

 evidently not identical with the preceding cases of reversal, but 

 is probably due to a decrease in permeability to the killing 

 agent resulting from the action of the high concentrations on the 

 surface of the protoplast. The fact that under such conditions 

 a more or less complete reversal of the susceptibility gradient 

 results means that in the most active protoplasm the permea- 

 bility is decreased to a very much greater extent than in the 

 less active cells, so that even the agent which has produced the 

 surface change is more completely excluded from those cells 

 where the change is greatest. 



These data concerning reversal of the gradient are fragmentary 

 because attention has been directed chiefly to the demonstration 

 of the primary or normal gradient. Further investigation of the 

 changes and the conditions under which they occur will un- 

 doubtedly throw more light on the problems involved. 



Changes in the axial gradient may also occur in the life of the 

 plant and may be brought about in other ways than those already 

 described. Some of these are merely the result of local action, 

 for example a wound may reverse the gradient, at least tem- 

 porarily, in regions apical to it. Other changes are due to the 

 action of general external factors as in the case of more or less 

 complete reversal in Griffithsia by exposure to high temperature. 

 In this case the high temperature acts like the various toxic 

 agents in high concentration, i. e., the susceptibility gradient to 

 high temperature is basipetal. 



Plants which are found detached in shallow water along the 

 shore after storms often show more or less irregularity or reversal 

 of the gradient, undoubtedly in consequence of depressing en- 

 vironmental conditions, such as exposure to high temperature, 

 intense light or drying at low tide. It is quite unsafe to base 

 conclusions on such plants alone. In Griffithsia for example, a 

 rather sensitive form, all plants collected along shore after de- 

 tachment, so far as examined, show more or less reversal in the 

 apical regions, i. e., these regions have been depressed or injured 



