SUSCEPTIBILITY GRADIENTS. 79 



to the wall of the cavity and is therefore not visible in intact 

 specimens, but the fact that the length of the cells increases 

 from the most basal region visible toward the free end suggests 

 that cell division is basal instead of apical. Fig. 2 shows a hair 

 of Fucus, the basal concealed portion being diagrammatically 

 indicated in dotted lines. 



In freshly collected plants the hairs are extremely sensitive 

 and even in dilute neutral red death begins within a minute 

 or two, often before the preparation can be placed under 

 the microscope, and in a few minutes the whole hair is 

 dead. In such hairs the gradient is acropetal, i. e., the 

 basal region of the hair is the region of highest suscepti- 

 bility, The acropetal gradient is very distinct and uniform 

 in the progress of death from cell to cell and in the longer, 

 older cells nearer the free end the acropetal intracellular 

 gradient can usually be seen if death is not too rapid. In 

 the shorter, younger cells an intracellular gradient is usually 

 not visible in the concentrations ordinarily used for killing. 

 The high susceptibility of the hairs makes it impossible to 

 stain with neutral red and then use other agents for killing. 



At the time of my first observations on the susceptibility 

 of these hairs I was entirely ignorant of the fact that they 

 grew from a basal instead of an apical vegetative point or 

 "growing tip," but the presence of very distinct acropetal 

 instead of basipetal gradients in all hairs of freshly col- 

 lected plants constituted practically conclusive evidence for 

 the existence of a basal vegetative region and examination 

 of the cellular structure of the hair and reference to the 

 literature confirmed the conclusion. In this case then the 

 direction of the gradient made possible the immediate 

 recognition of the rather unusual growth form. 



In freshly collected plants in good condition and very 



" n 



carefully handled, particularly in small young thalli, there S 

 may be practically no irregularities or exceptions to the FlG 2 

 regular acropetal course of death in the hairs of the whole 

 thallus. After a few hours in the laboratory, however, or after 

 exposure to unfavorable conditions, e. g., partial drying at low 

 tide, the change in the condition of these hairs is very striking. 



