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The Attaching Discs
The development of the attaching discs follows the same course in the two species, and it is scarcely possible to distinguish between them.
In very small plants of D. antarctica the central region is obviously continuous with the medulla of the stipe, and consists principally of hyphae In the central region of the lower margin of the disc, these hyphae grow out, and swelling at their tips, form a close attachment between the disc and the substrate.
The expanded portion of the attaching disc is clearly cortical in nature and shows complete continuity with the cortex of the stipe. The cells are arranged in regular rows perpendicular to the surface and have firm, highly refractive walls. Zones of cells parallel to the upper surface with denser contents recall the growth rings in the stipes of laminarians (Text-figure 8 c). At the junction of the stipe and disc the inner ends of these rows of cells merge into an irregular network, as in the stipe, and hyphae freely interweave in this region. Lower
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Text-fig. 8—Diagrams to show the distribution of the tissues in a—a stipe mature plant of D. antarctica, in b—a stipe of a mature plant of D. willana Scale = × 2. h = region of unswollen longitudinal hyphae; med. = medulla; s. e. = secondary cortex. c. An attaching disc of 20 cm. plant of D. antarctica. The lines in the expanded portion indicate the direction of the rows of cortical cells, and the bands parallel to the upper surface zones of cells with denser contents. et. = region of crushed cells, immediately above which is the cortical meristematie region shown in detail with Text-figure 9. Scale = × 12.
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down, however, the cell lumina become distended and the walls thick, and quite a different appearance is presented (cf. Naylor, 1949, Text-figure 6 a).
As the disc increases in size, the cells towards the lower and outer margins become very swollen, and the regular arrangement becomes obscured. Frequent anticlinal divisions of the meristoderm appear to account for much of the increase in diameter of the disc. Frequently from the cells of the lower surface of the disc there arisc hyphal-like outgrowths which penerate into the crevices of the rocks.
At the upper surface of the disc, a few layers of cells contain numerous plastids, but deeper down the cells contain very few plastids. Immediately beneath the photosynthetic cells, there is often a region of collapsed cells, where the regular arrangement becomes obscured. These cells are meristematic in nature, and by periclinal divisions add to the depth of the disc. This cortical meristematic zone (Text-figure 9) resembles that of the stipe of the Fucus, Ascophyllum, etc., where it replaces the meristoderm which becomes worn off in the older plants. Similar erosion of the surface tissues occurs in the old discs of Durvillea.
In the older discs all the cell lumina become considerably enlarged, so that the whole structure assumes a parenchymatous appearance in which it is difficult to distinguish the relationships of the tissues, but the developmental study clearly shows the cortical origin of the disc, so different from the usual condition in the Fucales and more closely resembling that in the Laminariales.
Text-fig. 9—Vertical section through the upper region of the attaching disc of a 5 cm. plant of D. antarctica, showing the meristematic region (m) in the inner portion of the secondary cortex (c). Cell outlines drawn with the aid of a camera lucida. Scale = × 390.
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Habit of the two species showing the horizontal stipes of D. antarctica (A) and the vertical ones of D. willana (B). The fronds of D. antarctica. are completely uncovered by the low tide, those of D. willana only partly so.
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Longitudinal sections of the lamina of the two species. A, C, E. D. antarctica B, D, F. D. willana A and B, immediately behind the apex of a 2 cm. plant. Scale = × 520. C and D, immediately behind the apex of a 20–30 cm. plant. Scale = × 225. E and F, the oldest portion of the lamina of the same plants as shown in C and D. Scale = × 100.
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R.L.S. of the medulla of stipes at varying stages of development. A Stipe of 2 cm. plant of D. antarctica, showing early stages of the swelling of the hyphae. At P can be seen a lateral outgrowth which will form either a horizontal hypha on a secondary pit. The cells are showing division by thin horizontal walls. Scale = × 360 B. Stipe of a mature plant of D. antarctica showing the widely separated, much swollen longitudinal hyphae with the interweaving horizontal hyphae. Scale = × 140 C and D. D. willana. C the stipe of a 20 cm. plant and D of a mature plant, showing the more compact, parenchymatous organisation, In D, a few horizontal hyphae can be seen cut transversely. Scale = × 140.
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A and B. Longitudinal sections through the laminae of old plants showing conceptacles of three years' reproductive periods at successively deeper levels in the thallus. A. D. antarctica B. D. willana. Scale = × 40. C. Vertical section through a mature conceptacle of D. willana. Scale = × 120