AXIAL SUSCEPTIBILITY GRADIENTS IN ALG^E. 425 



tion is necessary to clear up this apparently anomalous behavior 

 which differs from that of other algae examined as well as of 

 other specimens of Cladophora. It is possible that these peculiar 

 results are due to the neutral red rather than to the Cladophora, 

 for with both the fresh-water and the marine form the neutral 

 red used was a different preparation from that which had been 

 used in other cases. 



Fucus vesiculosus. 



In this species young plants ranging in length from 12-15 mm. 

 to 40 mm. constituted the material. Early in the course of 

 experiments with this form it was found that the change in color 

 and loss of the natural pigment of the plant was a more satis- 

 factory indicator of differences in susceptibility than the de- 

 coloration after staining with neutral red and the results described 

 below were obtained by this method. 



In the earliest stages of development the plant is more or less 

 club-shaped and circular in cross section, but in consequence of 

 change in behavior of the apical cell the thallus soon assumes a 

 flattened form except in the basal region, and a thickened midrib 

 develops (Fig. i). The plant is not, properly speaking, bi- 

 laterally symmetrical, since there is no differentiation of dorsal 

 and ventral surface, but it is biradial, i. e., there are two distinct 

 axes of radial symmetry, one parallel, the other at right angles 

 to the flattened surface. Since the plant grows primarily from 

 an apical cell situated in the median apical region and since 

 secondary growth in thickness occurs along the midrib, we might 

 expect to find the regions of highest susceptibility apical and 

 median and susceptibility gradients extending laterally and 

 basally from these points, perhaps modified in the more basal 

 regions, at least in later stages by the increased activity of 

 secondary growth. Such a gradient appearly very clearly with 

 various agents. 



Sooner or later dichotomous branching begins (Fig. 2), each 

 branch growing from a new apical cell which arises from the 

 original apical cell of the axis undergoing dichotomy. Each 

 branch then may be expected to exhibit the same sort of gradient 

 as the original unbranched thallus and this is actually the case. 



The natural color of the thallus is a dirty greenish brown or 



