260 Pub. Puget Sound Biol. Sta. Vol. 2, No. 48 



the basal portion of the flat frond. Even in these regions of high suscepti- 

 bility in stipe and frond a susceptibility gradient appears. In HCl m/200 

 the change in color and loss of turgor show a gradation and require l/o — 1 

 hour to progress over the length of the growing region and usually several 

 hours more to reach the base of the stipe and the tip of the frond. The 

 presence of this gradient in the growing region undoubtedly means that 

 growth is most rapid at the apical end of this region in the stipe and at its 

 basal end in the frond. 



With other agents used, KNC m/100, KOH m/100, ethyl alcohol 10%, 

 the changes in color are less conspicuous, but with all agents the loss of 

 turgor occurs. In all cases the susceptibility gradients in stipe and frond 

 are the same. In this species then, as in the others examined, the regions 

 of most rapid growth show the highest susceptibility and the susceptibility 

 gradients represent gradients in physiological condition. 



The only form examined in which a distinct gradient in susceptibility 

 was not observed was Callymenia phyllophora. In the elliptical or nearly 

 circular fronds of this species the susceptibility was almost uniform 

 throughout, some irregular areas here and there showing a slightly higher 

 susceptibility than other parts. Fronds ranging in length from 20 — 120 

 Dim. gave the same results. The form of the frond in this species indicates 

 clearly that growth cannot be solely or chiefly apical, but must occur more 

 or less in all directions in the plane of the frond. The irregular areas of 

 slightly higher susceptibility may perhaps indicate more or less definitely 

 localized regions of more rapid growth, or they may be due to incidental or 

 chance conditions. No very early stages in development were obtained, but 

 it seems not improbable that such early stages might show a more definite 

 gradient in growth and susceptibility, and that the gradient disappears 

 relatively early in development as in the case in the leaves of many flowering 

 plants 



The pseudothallus of Navicula 



The observations on the susceptibility of the pseudothallus of the 

 diatom Navicula have a certain interest, as indicating the existence of con- 

 ditions in this form similar to those in the thalli of algae with apical 

 growth. This pseudothallus grows in the form of flattened stems of almost 

 uniform width and thickness, with repeated, rather regularly arranged 

 branches resulting from repeated bifurcation of the tips. The existence of 

 a definite growth-form of this sort in this pseudothallus must evidently 

 result from definite growth relations between the different diatoms com- 

 posing it, moreover, such a growth-form is impossible except where the 

 chief growing region is apical. If all diatoms composing the pseudothallus 

 grew and divided with equal rapidity and without any regular arrangement, 



