SUSCEPTIBILITY GRADIENTS. 8 1 



originally somewhat less suceptible, and these are "almost" or 

 "partly" killed and their metabolic rate so decreased and proto- 

 plasmic condition so altered that they become very insusceptible 

 in killing agents. Still farther distally the effect of the altered 

 conditions is less marked until finally little or no change occurs in 

 the most distal cell. If we now subject a hair altered in 

 this way to the action of a killing agent such as neutral 

 red, KCN, etc., we find that the altered condition appears 

 in the altered susceptibility to the killing agent. The fact 

 that the changes produced by some external agent or com- 

 plex appear as altered or reversed susceptibility to another 

 is of interest and brings up the question whether the action 

 of the two agents is additive or not. But whatever the 

 nature of the processes, the fact of reversal is sufficiently 

 evident, and, as in other cases, supplements the direct evi- 

 dence for the existence of a dynamic gradient. 



Castagnea tuberculosa bears very fragile unbranched 

 multicellular hairs, which like those of Fucus develop from 

 a basal vegetative point concealed below the surface of 

 the thallus. Fig. 3 shows the general appearance of the 

 hair, the concealed basal portion being diagrammatically 

 indicated in dotted lines. In my earlier paper (Child, 'i6a, 

 p. 101) I stated that the gradient in these hairs is in general 

 basipetal, but with frequent irregularities, and the earlier 

 observations of 1916 also showed in general a basipetal \-\ 



' I 



gradient. In all these cases, however, the plants used had f!3 

 been in the laboratory a day or more and further investiga- FlG 3 . 

 tion on fresh material showed that the gradient is pri- 

 marily acropetal, but very readily undergoes more or less com- 

 plete reversal as in Fucus. In fact, even in freshly collected 

 material apparently in good condition, but with long well- 

 developed hairs, reversal has been observed in many cases. 



At the time of the earlier observations I did not realize how 

 readily reversal of the gradient might occur nor was I aware that 

 the vegetative point in these hairs is basal and not apical, and 

 when a well-marked basipetal gradient was found as a rule in 

 plants which had been in the laboratory a day or two, I failed to 

 realize the necessity of checking up the results on perfectly fresh 



