1916 Hurd; on C odium 113 



studied must have been a period of rapid growth, when large numbers 

 of new pith-filaments were starting up from the holdfast-cushion. One 

 would expect that these filaments would have to push out between the 

 very oldest ones of the preceding crop. The fact that none of the very 

 large utricles were found at or near the base must mean that the utricles 

 of the lower part of the frond never grow large; this must indicate that 

 they are physiologically different, probably without the power to re- 

 produce, since gametangia were never found on the lower part of the 

 frond. 



The fact that the mucronate tips of the basal utricles appear marked- 

 ly different from those of the young utricles of the same size at the tip 

 confirms the supposition that they are really different. The character- 

 istic mucron of the base is longer, often more acute, and more distinctly 

 striated than the common form at the tip {Fiy. 9). The latter rarely ex- 

 ceed 30 ju, in length {Fig. 14), while mucrons 70 fx long and more are 

 common at the base. The longest of these basal ones measured w'as 250 ^i 

 long. The basal ones are very often oddly thickened and misshapen in 

 plants on which the mucrons of the rest of the frond are all normal {Fig. 

 11). Then, too, utricles of the base often have enormously thickened 

 Avails at the end, not found in any other part of the plant {Figs. 12, 13). 

 Also the basal utricles differ in shape from those of the same size above. 

 The former are more tapering at the end than the latter, and thus appear 

 more slender than the young ones of the same diameter at the tip. It is 

 improbable that the difference in the pressure to which the utricles are 

 subjected could cause such wide variations. 



An interesting feature of the genus Codium is the so-called plugs 

 which serve as cross-walls to divide the pith-filaments into compartments. 

 Although similar structures are found in other genera of this family, 

 notably Bryopsis and Dichotomosiphon, they differ from those of Codium 

 in several essential respects (7). Those of Codium are not of the same 

 composition as those of the other genera, and completely close the tube 

 of the filament when mature. 



Plugs occur very frequently throughout the entire plant. When a 

 young pith-filament starts up from the base of a utricle, a plug develops 

 just above its origin from the latter, so that a plug always occurs in a 

 filament near a utricle, almost invariably just above it {Figs. 17, 18, 23). 

 They may also appear at irregular intervals anywhere along a pith-fila- 

 ment, and one is usually present near a branch {Fig. 19). Rarely there 

 is a plug in the base of a utricle. There is always one at the base of a 

 gametangium and often at the base of a utricle hair {Figs. 28, 11). 



Plugs evidently function as cross-walls They hold the cell con- 

 tents in place and thus prevent the loss of a whole filament when an in- 



