AXIAL SUSCEPTIBILITY GRADIENTS IN ALG^E. 43! 



and often one, two or three cells next below it are less susceptible 

 than any other part of the axis, and in this group of cells the 

 gradient is usually acropetal, the apical cell being least susceptible 

 of all. Similar results are obtained with CuSO.4. This partial 

 reversal is a characteristic feature of susceptibility to concentra- 

 tions above a certain limit of agents which are powerful co- 

 agulants of protoplasm such as HgCl 2 and CuSO 4 . Acclimation 

 is not concerned here, for it is the higher concentrations not the 

 lower which produce the reversal. Apparently these agents 

 decrease the permeability of the cells to themselves and the de- 

 crease is greatest in the most apical cells, where the protoplasm 

 is most susceptible to alteration. This differential action f 

 such agents is itself another demonstration of the existence of 

 the gradient, and it is of interest to note that an external agent 

 can reverse the axial gradient in permeability to itself. Various 

 data indicate that other agents in sufficiently high concentration 

 will give similar results, but the details are not yet worked out. 



Age differences in the susceptibility of the apical as well as 

 other cells are evident in Griffiths-la. In general a small apical 

 cell, i. e., the cell which has more recently undergone division, is 

 more susceptible than a larger apical cell, which has passed 

 through a longer period of growth without division. Since 

 different apical cells may be subjected to different external or 

 internal conditions which influence their activity these com- 

 parisons often show exceptions to the general rule. The most 

 uniform results as regards these age differences are obtained with 

 a single main axis bearing a number of branches. In such a 

 system the susceptibility of the apical cells usually varies in- 

 versely as the size. 



A few observations on the form known as var. tennis or as 

 Griffithsia tennis, with greatly elongated slender cells, gave results 

 similar to these already described. 



Cystodonium, Agardhiella, Chondrus, Lomentaria. 



In these forms each apparently simple stem or branch repre- 

 sents the orderly growth-activity of one or more monosiphonous 

 axes and their branches, i. e., each macroscopically simple axis 

 is in reality a complex system of monosiphonous axes. In 



