42O C. M. CHILD. 



mined by the visible changes in aggregate condition of the proto- 

 plasm, and in many of the red and brown algae by the diffusion 

 of the pigment out of the chromatophores and out of the cell. 

 The protoplasmic changes are in many cases much more readily 

 seen if the plant has been previously stained with neutral red. 

 The general character of the death changes has been described 

 (Child, 'i6a) and some special observations are recorded below. 

 In the work of 1915 the chief agents used to measure suscepti- 

 bility were KCN, ethyl alcohol and the so-called vital dye, neutral 

 red. In 1916 various other reagents in addition to these three 

 were used, including ethyl ether, HC1, CuSO 4 and HgCk 



THE SUSCEPTIBILITY GRADIENTS IN THE THALLI. 



The chief result of this further study is the same as that of the 

 earlier, viz., that in the definitely axiate forms or parts examined 

 a gradient in susceptibility exists along the axis, the apical 

 region being primarily most, the basal least susceptible to toxic 

 agents in high concentration. The regularity of this gradient 

 is most marked in plants in good physiological condition and in 

 the younger axes or the younger portions of axes. In some 

 forms the original gradient may persist throughout the length of 

 the axis, at least during the vegetative period, while in others 

 it may undergo modification in the later stages or in the older 

 regions of the body. In general it is also true that the more 

 definitely axiate and orderly the growth form of the plant, the 

 more definite and regular the susceptibility relations between 

 different parts. 



Since different species behave somewhat differently and require 

 different modifications of method the data for each form examined 

 are briefly given. The genera include, among the Chlorophyceae, 

 only Bryopsis and Cladophora, among the Phaeophyceae, Fucus, 

 and among the Rhodophyceae, Chondrus, Cystoclonium, Agard- 

 hiella, Lomentaria, Griffithsia, Callithamnion. 



Bryopsis plumosa. 



The demonstration of an axial gradient in this form seemed 

 to me of particular interest since the whole plant body consisting 

 of creeping rhizome-like axes from which arise vertical axes with 



