1919 Child; on Susceptibility of Algae 253 



thallus are closely crowded together, as is often the case, irregularities 

 are more frequent than in cases in which an axis has been free on all 

 sides. Probably differences in oxygen content of the water among the 

 crowded stems and differences in illumination, possibly also differences in 

 hydrogen ion concentration in the water among the crowded stems, are 

 factors in determining these irregularities. It should also be noted that 

 in the more sensitive forms even an hour or two in standing water, particu- 

 larly if an}' marked rise in temperature occurs, is sufficient to produce very 

 marked irregularities in the gradients, or, since the axis shows a gradient 

 in susceptibility to these conditions as well as to chemical agents, to oblit- 

 erate the gradient almost or quite completely. In Desmarestia, for exam- 

 ple, fresh specimens of both species examined show very distinct and 

 regular gradients, but after a few hours in the laboratory very marked 

 irregularities are found and in many axes the gradient is absent or re- 

 versed, the apical regions being less susceptible than the basal. Moreover, 

 these changes occur before the plant shows any change in color or any 

 other visible indications of injury or death as a result of the altered 

 environment. In fact, distinctness and regularity of gradient is a very 

 satisfactory criterion of good physiological condition. 



In forms with branching thallus among those examined not only does 

 a basipetal gradient appear in each axis, but the thallus as a whole, or each 

 system of main stem and branches shows a gradient corresponding to the 

 growth-form of the species. In forms in which the distinction between main 

 stem and lateral branches is strongly marked and particularly in cases in 

 v.hich lateral branches are short, the apical ends of the most apical axes are 

 the most susceptible regions of the plant and the susceptibility of the differ- 

 ent axes decreases basipetally in regular order. In forms in which the dis- 

 tinction between a chief and a subordinate axis is less marked and in forms 

 in which all axes grow equally or almost equally there is little or no dis- 

 tinction between them as regards susceptibility. In the plant as a whole, 

 then, as well as in the single axis, the differences in susceptibility indicate 

 a graded difference in physiological condition and such difference is very 

 clearly related to the growth-form of the species. 



The monosiphonous hair-branches of Dasyopsis and various other 

 forms which themselves represent axes with apical growth, show basipetal 

 susceptibility gradients like other vegetative parts of the plant. Moreover, 

 the hair branches nearer the apical end of the axis from which they arise are 

 more susceptible than those farther basal. In Dasyopsis particularly this 

 gradient in the hair-branches is very distinct and uniform. 



The fact that in the vegetative portions of the thallus differences in 

 susceptibility exist not only in each axis but between the different axes and 

 that these differences in susceptibility are definite in character and corre- 



