Costaria, Undaria, and Laminaria. 
697 
Longitudinal sections of fronds at a similar stage of development have 
revealed that the constituent cells of the precortical layer are isodiametrical, 
but much flattened. The length and the breadth are 3-4 times as large as 
those of the pre-existing cells, but the height is nearly alike. The relative 
sizes of the cells of the three layers may also be recognized by observing 
such fronds in surface view after being stained and clarified in toto. 
When the lamina has attained to 5-7 mm. in length, its general outline 
becomes lanceolate or elliptical-lanceolate with tapering base and acute apex. 
The length of stipe varies according to the place where the plant is growing. 
. Those found among the caespitose filaments of Sphacelaria or such-like 
Algae naturally have the stipe longer (Figs. 6-10, PI. LIII). 
Kuckuck 1 has delineated a cross-section of a frond of Laminaria 
apparently corresponding to the above-mentioned stage. He has not given 
an explanation in full of what he has delineated. But his illustration 
suggests a conception diverging too widely from what I have observed on 
my material. I reproduce here the figure given by him (Fig. 22, PI. LIV). 
Others besides myself might suppose from this figure that the middle layer 
is the primary one, and that the peripheral layers have been formed on 
both surfaces of the initial layer. 
The process of gradual increase of the cell-layers is essentially the 
same in the blades of Costaria , Undaria, and Laminaria treated in the 
present paper. They agree in the formation of the peripheral or epidermal 
layers, as they may be termed, previously to that of the precortical layer. 
And so far as I have observed in Dictyotae, Zonariae, Punctariae, &c.* 
which have one or more layers of parenchymatous cells between the two 
single epidermal layers, the formation of the latter from the former is a view 
hardly to be accepted. 
The multiplication or the addition of layers has its centre practically at 
the transitional region. The process extends more rapidly in the longitudinal 
than in the transverse direction. The boundaries between the successive 
layers may be readily traced by observing such a lamina under a low 
power of the microscope after the specimen has been stained and clarified by 
a suitable method. Fig. 13, PI. LIII, shows diagrammatically a frond which 
measures about 1-5 cm. in total length. In Costaria the formation and 
interpolation of the tertiary layer follows directly after the formation of the 
distromatic layers. The genuine distromatic structure is therefore in most 
cases able to be traced along the outer margin of the tristromatic area* 
Generally speaking, the boundaries of the different numbers of layers form 
concentric lines parallel to the periphery of the lamina, converging towards 
the transitional region. 
Comparing the blades at these various stages of development, the 
1 Kuckuck: Bemerkungenzurmarinen Algenvegetation von Helgoland. Wiss. Meeresuntersuch., 
N. F., Helgoland, 1906, p. 250, Fig. 19. 
