Costaria, Undaria , and Laminaria. 711 
Laminaria , even after they have attained to several centimetres in length, 
still keep their primitive structure. In a blade of 5-6 cm. in length we 
usually have the apical portion in a monostromatic state, and the greater part 
of the area in the four-layered condition. The diagrammatic figure showing 
the boundaries of the different numbers of strata is represented in Fig. 39, 
PI. LV. Compare this with Fig. 13, PL LIII, which is of Cos/aria. Only 
in the transitional region do the precortical layers leave some spaces between 
them filled with a gelatinous matrix ; they are in close contact in the 
remaining area. The meridional area can only be distinguished from the 
marginal when a blade has attained 10-12 cm. in length. In the younger 
stages the marginal parts of course have a more simple structure when 
compared with the middle. Still, we can by no means regard the latter as 
the indication of the meridional area. 
In most Japanese forms of Laminaria , the meridional area is much 
thicker than the marginal regions. It has, in some species, a different 
structure when compared with the latter. For example, the meridional 
area of some species is provided with double layers of lacunae while there 
is a single layer in the marginal regions. Still more remarkabty, the meri¬ 
dional area is depressed on one surface and elevated on the other. Along 
each side of the area, in Laminaria ochotensis , there is a narrow canalicula- 
tion which dies out near the transitional area. In some species the 
meridional area is comparatively narrow, and in others much broader. 
The cross-sections of a blade, therefore, give a bilaterally symmetrical 
undulating figure, characteristic of the species (Fig. 46, PI. LV). Actual 
observation of the growing plants reveals that the depressed meridional 
area is always turned towards the source of light unless disturbed by an 
external mechanical force. The appearance of sori mostly begins on the 
shaded side, and is often limited to it. Thus in the blade of Laminaria , 
at least in the Japanese forms, there is a dorsiventral differentiation. 
The dorsiventrality of the blade in the species under consideration is 
indicated in the transitional region when a plant has attained 2 cm. in total 
length. The one side, probably the future under-side, is elevated, while 
the other remains flat (Fig. 38). The characteristic canaliculation, however, 
becomes recognizable in much more advanced stages, viz. after a blade has 
attained 50-70 cm. in length. In some individuals two rows of dimples 
with regular intervals appear on both sides of the meridional area, no doubt 
owing to the difference in the speed of growth of the two areas. The 
canaliculations are not really distinct until the dimples have disappeared. 
The differentiation in the internal structure of the stipe is first to be 
noticed when a plant has attained 4-5 cm. in total length. The medulla is 
not sharply delimited from the inner cortex, but may be more or less dis¬ 
tinguished by the cellular arrangement. The loose anastomosing structure 
of the hyphal cells is limited to the base of the transitional region (Fig. 38). 
