76 C. M. CHILD. 



other parts of the plant may remain alive and apparently in good 

 condition. 



In the earlier observations on the hairs (Child, '16) the 

 extreme sensitiveness and rapidity with which alterations in 

 the gradients may occur was not fully recognized, consequently 

 some of the statements made at that time require correction. 



Since the hairs are without pigment and their protoplasm is in 

 most cases almost entirely devoid of visible structure, except 

 under high powers, in the living condition a susceptibility gradient 

 can be made visible only by staining with neutral red or some other 

 vital dye and then killing with the proper concentration of some 

 reagent. As killing agents neutral red, KCN and HgCla were 

 chiefly used. 



UNBRANCHED UNICELLULAR HAIRS. 



Ceramium rubrum constituted the chief material for the study 

 of hairs of this kind. In the earlier paper it was stated that "in 

 the long full-grown hairs the susceptibility gradient is distinctly 

 acropetal while in those which are apparently still growing it is 

 basi petal" (Child, '16, p. 104). The later observations indicate 

 that the normal or primary gradient is basipetal in all hairs, but 

 that it may readily undergo reversal. In the observations of 

 1915 the hairs were first stained with neutral red and then killed 

 in KCN m/ioo. In 1916 it was found that when neutral red was 

 used both as staining and killing agent the more susceptible 

 hairs which died during the first I to 2 hours showed almost 

 without exception a basipetal gradient, i. e., death began apically 

 and proceeded basipetally, while among many less susceptible 

 hairs which died later basipetal gradients were much less fre- 

 quent, many hairs showed no definite death gradient, and acro- 

 petal gradients were often observed. Apparently if the neutral 

 red kills the hair rapidly enough the death gradient is basipetal, 

 but where death occurs only after a longer time the original 

 gradient may be obliterated or even reversed. In short the 

 neutral red may obliterate or reverse the original gradient in 

 susceptibility to itself. 



Hairs stained for a few moments in neutral red and then killed 

 in KCN usually show a basipetal gradient, but when stained for 



