AXIAL SUSCEPTIBILITY GRADIENTS IN ALG/E. 427 



In the course of 10-18 hours in KCN, alcohol or ether the 

 median apical region begins to show a deep brown or reddish 

 brown color, and this color eventually progresses basipetally 

 along the midrib, and a brownish tint spreads laterally over the 

 thallus, the apical regions preceding, but the midrib always 

 remains more deeply brown. In HC1 this secondary change is 

 not so clearly marked, the midrib merely becoming more yellow- 

 ish than the lateral regions of the thallus. 



In all cases the color changes begin in the median apical region 

 and progress laterally and toward the base. It is impossible 

 to determine just when death occurs at any particular level of 

 the body in these various agents but it is probably either when 

 the first color changes begin or when the loss of color occurs. 

 The first change in color must mean that the reagent has pene- 

 trated the cells at least to some slight extent, and the loss of 

 color must mean that the protoplasm or the pigment or both are 

 so altered that the pigment is no longer held and diffuses out 

 into the water. There can be little doubt that the cells in the 

 regions concerned are dead when this occurs. 



The point of chief interest at present, however, is the progress 

 of the changes from the median apical region laterally and basally. 

 This progress is more distinct in the more apical region than 

 elsewhere, and it is in this region that the change from the 

 embryonic to the differentiated condition is most marked. In 

 some cases among the older thalli the cylindrical stem is appar- 

 ently slightly more susceptible than the basal part of the flattened 

 thallus, perhaps in consequence of the activity of secondary 

 growth in thickness. The reddish brown color is probably a 

 secondary change after death, rather than a death-change prop- 

 erly speaking, but its progress from the median apical region 

 basipetally and laterally is none the less interesting, as indicating 

 still another aspect of the axial gradients. 



Callithamnion. 



From the physiological point of view it seems best to describe 

 the gradients in Rhodophyceae with primarily monosiphonous 

 thallus and large cells before taking up species with more complex 

 axiate structure. 



