AXIAL SUSCEPTIBILITY GRADIENTS IN ALG.E. 433 



In regions more than 5-10 mm. from the apical end of the 

 axes the color change is often somewhat irregular and appears 

 first in small areas scattered for some distance along the branch, 

 but even in these regions the progress of death is in general 

 basipetal. At these levels secondary growth in thickness is 

 occurring and it may be that the areas of higher susceptibility 

 represent the apical ends of groups of the monosiphonous axes 

 composing the plant body, which are growing more rapidly than 

 others about them. 



In large fronds 15-25 centimeters long the middle regions of 

 the main branches or stems for several centimeters are very 

 commonly less susceptible than either more apical or more basal 

 regions. That the low susceptibility of this region represents a 

 real difference in physiological condition in fronds where it is 

 present is clearly shown by the fact that it is thickly covered with 

 the colorless unicellular hairs characteristic of the species while 

 other parts of the plant show few or none of these hairs. Usually 

 also the color is somewhat lighter than that of other parts of the 

 plant. Undoubtedly this region of low susceptibility is of 

 secondary origin since the younger fronds and main branches 

 do not show it, and the fact that in the plants examined it was 

 limited to these parts of the main branches and stems which 

 were most thickly surrounded by other branches suggests that 

 it may be merely a result of insufficient light or oxygen, or 

 possibly of injurious metabolic products, in other words that it 

 is an incidental result of the crowding of the numerous axes in 

 this region. 



The great development of hairs in these regions of low suscepti- 

 bility suggest that hair development is associated with a low 

 metabolic rate in the cells from which the hairs arise. If this 

 suggestion is correct, the hairs appear first in this middle region 

 because for some reason the metabolic rate is lower there than 

 elsewhere. As the plant becomes physiologically older and its 

 metabolic rate in other regions decreases, hairs may of course 

 appear elsewhere. I have found that plants thickly covered 

 with hairs usually show r a lower susceptibility than those with 

 few or no hairs. 



In neutral red partial reversals of the gradient in the extreme 



