84 C. M. CHILD. 



particularly in the more basal cells of the older hairs. Similarly 

 in plants found detached reversed intracellular gradients were 

 often observed. Such plants torn loose by the waves and 

 washed into shallow water may be exposed to depressing con- 

 ditions of temperature, light, or partial drying at low tide. There 

 is of course no way of determining what or how extreme these 

 conditions have been in a particular case but the fact that reversed 

 intracellular gradients appear frequently in the hairs of such 

 plants but are not present in material which so far as can be 

 determined is in the best physiological condition, together with 

 the data on experimental reversal, indicate clearly enough that 

 such reversals are the result of depressing or toxic factors. 



Whatever the processes involved in such reversals, they, like 

 other reversals described in this and an earlier paper (Child 'i6b), 

 are dependent on the preexistence of a basipetal susceptibility 

 gradient in the cell, and they represent the differential action of 

 external factors within certain limits of concentration or in- 

 tensity upon this gradient. The difference in susceptibility 

 between different cells, even two adjoining cells, is usually greater 

 than the differences between the two ends of a single cell, and the 

 difference between apical and basal cells of a hair is of course 

 very much greater than any intracellular differences. The 

 persistence of the hair gradient with reversal of the intracellular 

 gradient indicates merely that the relations between the lesser 

 differences in susceptibility are more readily reversed than those 

 between the greater differences. 



The establishment of the fact that the acropetal intracellular 

 gradient is a reversal of the original gradient at once raises the 

 question whether the gradient of the hair as a whole can be reversed. 

 Various attempts at such reversal have been made with some 

 success, particularly in the younger hairs, but in the full-grown 

 hairs the axial differences in suceptibility are so great that com- 

 plete reversal has not as yet been induced and is perhaps im- 

 possible. 



In Chondria the hairs occur in large numbers near the apical 

 ends of branches of the thallus, and in cases where extensive 

 hair-development occurs the hairs appear macroscopically as 

 white tufts covering the branch tips. A well-developed tuft of 



