SUSCEPTIBILITY GRADIENTS. 87 



found that in some plants the gradients of the youngest hairs 

 were almost completely reversed after two or three days in the 

 laboratory and after a day or two more these hairs were dead. 



Of course intermediate conditions between the normal and 

 reversed gradient are frequently observed. In such conditions 

 the intracellular gradient may be absent or very irregular and 

 the hair gradient may be slight or even absent over a length of 

 several cells. The differences in susceptibility between apical 

 and basal end are, however, so great, particularly in the older 

 hairs, that at the stage when the gradient is obliterated in one 

 region of the hair other portions are either still normal, already 

 reversed or dead. 



Where the gradient is obliterated or reversed the hairs show 

 a marked tendency to separate into their individual cells. Ap- 

 parently the persistence of the orderly multicellular axis is as- 

 sociated with the persistence of the gradient. Other cases in 

 which this separation appears will be described later. 



The hairs of Polysiphonia are similar in structure and growth 

 to those of Chondria, but in P. variegata and P. fibrillosa, the 

 only species examined thus far, they are considerably more sus- 

 ceptible than the Chondria hairs and therefore not so favorable 

 for experimental work. The gradient is primarily basipetal 

 both in the hair in general and in the cell, and while more or less 

 reversal has sometimes been observed, experimental reversal has 

 not been attempted. 



The hairs of Griffithsia differ from those of Chondria chiefly in 

 the whorled arrangement of branches (Fig. 5) and as we should 

 expect, since growth is primarily apical, both general and intra- 

 cellular gradients are basipetal when the hair is in good condition. 

 After two or three days in the laboratory, however, irregularities 

 and reversals appear, and the hairs soon separate into cells or 

 fall off entire. 



DISCUSSION. 



It is evident that at least the hairs of these three kinds among 

 the algae show axial susceptibility gradients like those in the 

 vegetative axes of the thalli. Moreover, these gradients show a 

 direct relation to the location of the vegetative point, the region 



