SUSCEPTIBILITY GRADIENTS. 77 



an hour or more and killed in KCN the frequency of basipetal 

 gradients is lower and that of acropetal gradients higher, i. e., 

 the neutral red tends to reverse the susceptibility gradient at 

 least to some other agents as well as to itself. Discussion of 

 these and other cases of reversal is postponed to the final section. 



Ceramium rubrum is in general highly resistant to depressing 

 conditions and the hairs are much less sensitive than those 

 of many other forms. It has been observed, however, that 

 in plants kept in the laboratory for several days the hairs 

 are usually less susceptible, and acropetal gradients are 

 more frequent than in fresh specimens. 



The visible death-changes in these hairs are very similar 

 to those in other cells (Child, 'i6a, 'i6b). A slight basipetal 

 staining gradient may or may not be visible. Before the 

 aggregation and decoloration of the protoplasm the usual 

 change in color to deeper red occurs, and this color change 

 commonly shows a basipetal gradient where the death 

 gradient is basipetal. Often death is preceded by plas- 

 molysis which proceeds in the same direction as death and 

 not infrequently a more or less regular gradient in length of 

 the plasmolyzed portions appears, the most apical portion 

 being shortest and the length increasing basipetally (Fig. i). 

 In other cases the protoplasm may separate into only two 

 or three portions, and death may also occur without pre- 

 ceding plasmolysis. Instead of separating into distinct 

 portions the protoplasm often undergoes a plasmolytic con- 

 traction, as a single mass, retracting slightly as a rule from 



the apical end and basally from one third to two thirds of r 



FIG. i. 



the hair length. Both the gradient in length of plasmolyzed 

 portions and the difference in apical and basal retraction in- 

 dicate a graded difference in protoplasmic condition along the 

 axis. 



The final change in the protoplasm is the apparent collapse of 

 the protoplast or its separate portions and the aggregation into 

 small masses stained almost black. Where the basipetal gradient 

 is well marked this change begins apically and can be followed 

 basipetally along the hair. Where the gradient is obliterated 

 it may begin at almost any level, or at several levels, or may be 



