266 Pub. Puget Sound Biol. Sta. Vol, 2, No. 48 



Summary , 



1. In the algae with apical growth a gradient in susceptibility exists 

 along each axis, the susceptibility decreasing from the apical region basi- 

 petally. 



2. In branching thalli, in which a distinction in growth-form between 

 main axis and lateral branches exists, the apical region of the main axis 

 shows the highest susceptibility, and in the lateral branches the suscepti- 

 bility decreases from the most apical branches basipetally. In such forms, 

 then, a susceptibility gradient exists, not only in each single axis, but in 

 each system of main and subordinate axes, and usually in the thallus as a 

 whole, though irregularities are likely to appear in the older portions. 



S. In Nereocystis luetheana the susceptibility is highest near the 

 apical end of the stipe and decreases basipetally, while in the frond it is 

 highest in the basal region and decreases toward the tip. Here, as in other 

 forms, the regions of most rapid growth are most susceptible. 



4. The pseudothallus of the diatom Navicula shows a distinct gradient 

 in susceptibility corresponding to the growth-form, the apical region of each 

 axis being most susceptible. 



5. With neutral red and methylene blue a gradient in penetration cor- 

 responding in axial relations to the susceptibility gradient usually appears 

 during at least the early stages of staining, but the differences in suscepti- 

 bilitj' are much greater than the apparent differences in permeability. 

 Methylene blue in the concentrations used is apparently not toxic to some 

 algae and passes out of the cells only somewhat less readily than it passes 

 in. The passage outward, like the passage inward, occurs most rapidly in 

 the apical regions with a basipetal decrease in rapidity. 



6. While permeability may perhaps be a factor in determining sus- 

 ceptibility, it alone will not account for the observed facts. In order to be 

 toxic a substance must affect some constituent or reaction of the protoplasm, 

 and it may produce its effect first at the surface and enter the cell only as 

 it alters or kills the surface, or it may enter the cell readily without killing 

 or appreciably injuring the surface and produce its toxic effect by accumu- 

 lating within the cell. The axial susceptibility relations are essentially the 

 same in both cases. 



7. The general relation between susceptibility to a wide range of 

 agents and conditions and rate of metabolism or oxidation results from the 

 fact that living protoplasm is a system, and whatever the point of attack 

 of a toxic agent or injurious condition, the general effect is essentially the 

 same as regards the system as a whole, if the action is sufficient in degree. 



