1919 Child; on Susceptibility of Algae 261 



the result would necessarily be a more or less nearly spherical mass. In 

 this form, however, not only is there a definite growth-form of the axis as 

 a whole, but the diatoms imbedded in a gelatinous substance, are arranged 

 more or less regularly in longitudinal rows. 



Although presumably no protoplasmic continuity between the indi- 

 vidual diatoms exists, growth and division are evidently limited largely to 

 the apical region of each branch, 1. e., the diatoms of the apical region, or 

 certain of them, continue to grow and divide, while growth and division 

 must cease, at least to a large extent at a greater or less distance below the 

 tips, as in the axes of true thalli with apical growth. 



This being the case, it might be expected in the light of the results 

 obtained with other algae that a gradient in suceptibility would appear in 

 this Navicula pseudothallus, and this is actually the case. The extreme 

 tip is the most susceptible region of each branch, and from the tip down- 

 ward the susceptibility decreases over at least several millimeters. In the 

 more basal, older portions of the pseudothalli irregularities appear, as in 

 the true thalli of many other algae. With KMnOj a basipetal color 

 gradient appears, as in thalli with apical growth. 



Both as regards susceptibility and growth-form, then, the axes of the 

 Navicula pseudothallus behave like physiological axes. In view of the 

 facts, there seems to be no escape from the conclusion that some difference 

 in physiological conditions must exist between the apical and other regions. 

 As regards the nature of this difference, there are apparently two possi- 

 bilities : either physiological correlation of some sort, similar in general 

 effect to the influence of the growing tip on other parts in other plant axes, 

 must exist between the apical region and other levels, or else growth and 

 division are gradually inhibited by the formation of the gelatinous envelope, 

 so that those diatoms which lie at the tip, where little of the gelatinous 

 substance has been produced, are capable of the most rapid growth and 

 division, and the rate of these processes decreases as the gelatinous sub- 

 stance increases. On the basis of the first alternative, the increase in gela- 

 tinous substance is probably a consequence of the decrease in rate of growth 

 and division, while on the basis of the second, it probably determines the 

 decrease. 



It is impossible at present to determine wliich of these alternatives 

 represents the facts, but certain points may be briefly noted. The second 

 alternative, that decrease in rate of growth is determined by the increase 

 in the gelatinous secretion, does not account for the regular bifurcation of 

 the apical growing regions, since these regions consist, not of single but of 

 many diatoms. At present it seems impossible to account for this process 

 and its regularity of occurrence without some sort of physiological correla- 

 tion between the component diatoms. Second, since both susceptibility and 



