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



general laws. The data thus far at hand indicate that in concentrations 

 or intensities of an agent which are too high to permit the organism to 

 acquire tolerance or become acclimated, the susceptibility varies directly, 

 though by no means necessarily proportionally, with the rate of general 

 metabolism, probably with rate of oxidation. On the other hand, capacity 

 to acclimate or acquire tolerance to low concentrations or intensities of the 

 same agents also varies directly with rate of metabolism or oxidation. 



It is impossible to consider at this time the evidence on which these 

 conclusions are based. A part of it has been published in the writer's 

 earlier papers and in those of Dr. L. H. Hyman,^ but some of it is still 

 unpublished, and work is still going on to make the evidence more complete. 

 If these conclusions are correct it Is evident that differences in suscepti- 

 bdity may be used to some extent as a means of determining quantitative 

 differences in physiological condition. In the present paper the method 

 is used in this way with killing concentrations. It has been found, more- 

 over, that the differences in rate of staining in permanganate parallel the 

 differences in susceptibility, so that this method also serves to show some- 

 thing of differences in physiological condition. With these facts and con- 

 clusions in mind, the significance of the observed differences in suscepti- 

 bility with reference to physiological condition will be clear. 



The susceptibility relations in forms with apical growth 



Since the susceptibility gradients of various species have been de- 

 scribed in earlier papers (Child, 1916a, 1916c, 1917a) and since most of 

 the Piiget Sound species examined show similar gradients, the general 

 results may be briefly stated, but certain species and certain methods not 

 previously used require more extended consideration. In all forms exam- 

 ined except Nereocystis and Callymenia, which are discussed below, each 

 axis of the vegetative thallus shows with all agents used a distinct gradient 

 in susceptibility decreasing basipetally. Such a gradient is called for 

 convenience a basipetal gradient. In all forms showing such a gradient the 

 axes are of the usual type with apical growing region, consequently the 

 susceptibility gradient indicates a graded difference of some sort in physio- 

 logical condition from the apical cell or cells basipetally. This suscepti- 

 bility gradient is most distinct and regular in the younger portions of the 

 thallus, while in older parts irregularities sometimes appear, a cell or 

 region showing greater or less susceptibility than would be expected from 

 its level in the axis. Some of these irregularities are evidently due to 

 injury, while others are doubtless due to what may be called chance or 

 incidental conditions. For example, when the stems or branches of a 



1 See for example Child 1913, 1915a, 1915b chapter IX, 1916a, 1916b, 1916c, 1917a, 1917b, 

 1919a ; Hynian 1916, 1917 and references given in these papers. 



