256 Pub. Puget Sound Biol. Sta. Vol. 2, No. 48 



hhe more basal regions attain before other parts the physiological condition 

 which determines the formation of stichidia, and as the plant axis grows 

 longer or physiologically older or both, and each level attains this same 

 physiological condition, stichidia arise successively at higher and higher 

 levels. In other words, there is a gradient represented by a time sequence 

 in the appearance of stichidia on the axis, beginning basally and progress- 

 ing toward the apical end. That this order of development of stichidia 

 along the axis is in some way associated with the differences in physiological 

 condition indicated by the susceptibility gradient is at least highly probable, 

 though of course it does not follow that the quantitative differences in 

 physiological condition indicated by the susceptibility gradient along the 

 vegetative axes are themselves directly responsible for the origin of stichidia. 

 These quantitative differences may be accompanied by, or may bring about 

 qualitative differences in condition, and these latter may be the immediate 

 determining factors. But whatever the actual situation may prove to be, 

 there is every reason to believe that the gradient in origin of stichidia is 

 related in some way to the conditions of which the susceptibility gradient 

 is an indicator. 



Observations made with the vital dyes neutral red and methylene blue 

 show certain points of interest with respect to the question of the passage of 

 these substances into and out of the cell. All of the algae examined can be 

 stained with neutral red, and in most cases the dye enters all cells of the 

 plant readily and accumulates in the cells until its concentration becomes 

 much higher than outside and is finally toxic. In most species, however, 

 an axial gradient in staining with neutral red appears which is in the same 

 direction as the gradient in susceptibility, i. e., the rate at which the neutral 

 red enters the cells decreases from the apical end of the axis basipetally. 

 Usually this gradient is distinct only for a few moments at the beginning 

 of staining and all cells become uniformly red. The most strongly marked 

 staining gradient observed appeared in a Cladophora thallus in which the 

 staining of successive cells from the apical region occurred very slowly, so 

 that the apical cell became opaque black by the time the third or fourth 

 cell was markedly red, and death and decoloration began in the apical cell 

 soon after this. Here the gradient in death and decoloration followed the 

 staining gradient closely down the axis. In other Cladophoras examined 

 at Woods Hole the staining gradient in neutral red was slight and evan- 

 escent as in other forms and the death gradient much more clearly marked. 

 The very strongly marked staining gradient in this case may have been 

 associated with very slow growth, in consequence of which the difference in 

 condition between successive cells was unusually great. 



When neutral red has once entered an algal cell it usually does not 



