88 



C. M. CHILD. 



of highest susceptibility being in general the region of the vege- 

 tative point, at least when the hair is in good condition. 



In Fucus and Castagnea, for example, where the vegetative 

 point is basal, the gradient is normally acropetal, while in all other 

 forms examined the vegetative point is apical and the gradient 

 normally basipetal. 



FIG. 5. 



Under depressing conditions which are not extreme enough to 

 kill rapidly the gradient may be more or less completely obliter- 

 ated or even reversed, and such changes are of course a con- 

 sequence of the differential susceptibility originally present. 

 Obliteration or reversal of a gradient by external factors means 

 simply that a region of highest susceptibility or metabolic rate, 

 is affected by the external factor to so much greater extent than 

 a region of lower susceptibility or rate that the original differences 

 are obliterated or even reversed. In some cases an external 



