AXIAL SUSCEPTIBILITY GRADIENTS IN ALG/E. 435 



shallow water after a storm showed in most axes the usual basi- 

 petal gradient, but in some cases the progress of death was ir- 

 regular or even acropetal. In most species examined plants 

 detached and washed in by the waves show reversals and 

 irregularities much more frequently than those collected in situ, 

 and in Lomentaria the irregularities observed are doubtless due 

 to bad condition, but since this species was not found in situ 

 there was no opportunity for checking the results. 



GENERAL DISCUSSION. 



From the data recorded here and in the preceding paper it is 

 evident that a gradient in susceptibility is a characteristic feature 

 of the thalli of axiate forms among algae. In the cases described 

 in the present paper the apical region is primarily the region of 

 highest susceptibility and the decrease is basipetal in each axis. 

 Under unfavorable conditions and in many cases with advancing 

 age, this primary gradient may be altered by local alterations in 

 metabolic activity, by physiological isolation of certain regions, 

 and in many other ways. 



In some plant axes the growing tip and the region of highest 

 susceptibility are at the attached or morphologically speaking 

 the basal end and the susceptibility decreases toward the free 

 "apical" end. Some cases of this sort are found in the hairs of 

 certain algae, e. g., Fucus and will be considered at another time. 



The significance of the axial differences in susceptibility as 

 indicators of general metabolic rate or condition has been suffi- 

 ciently discussed elsewhere (Child, '130, '156, Chap. III., IX., 

 '15^, Chap. III.). The similarity of results with different agents 

 shows very clearly that the general susceptibility relations depend 

 not upon the specific chemical constitution of a particular agent, 

 but rather upon the fact that many different agents injure and 

 kill protoplasm and that the physiological or metabolic condition, 

 vitality, or whatever term we prefer to use, is a factor in deter- 

 mining their effectiveness as killing agents. 



In considering alterations of the gradient it is necessary to 

 distinguish those which occur in low concentrations of KCN and 

 other highly toxic agents or in slightly toxic agents such as neutral 

 red, from those which occur in high concentrations of highly 

 toxic agents such as HgCla and CuSC>4. 



