ALTERATION OF THE AXIAL GRADIENT. 



223 



most basal cell of the group. Here the original susceptibility 

 gradient persists in the three cells, as indicated by the arrow, 

 while the rhizoid, like any other rhizoid, represents a new gradient 

 in the opposite direction. The rhizoid, however, represents a 



3 



5 



lower level of susceptibility than other parts of the individual 

 and is the last portion to die. 



In Figs. 2 and 3 the original gradient has undergone reversal, 

 in Figs. 4 and 5 it still persists. Cases of reversal, like Figs. 2 

 and 3, were in general more frequent in the material than cases 

 of persistence. This is to be expected, since cell separation is 

 associated with obliteration or reversal of the gradient. The 

 original gradient may persist in cells or cell-groups in which the 

 susceptibility is for any reason exceptionally low and the differ- 

 ential injury consequently less marked, and probably also in 

 cases of partial acclimation (Child, '15^, Chap. III., 'i6d) after 



