The New Zealand Species of Durvillea^*

By Margaret Naylor, University of Otago

it from all other species of Durvillea. D. willana resembles the more subdivided forms of D. antarctica, but is easily identified by the stiped proliferations of the same appearance as the main frond which arise from all sides of the stipe. The plants are darker in colour than D. antarctica and are readily distinguishable at low tide by their stout, vertically growing stipes, in contrast with the shorter stipes of D. antarctica (Pl. 58), which are usually horizontal.

Both the species occur on exposed coasts, often together, when D. willana is found at a slightly lower level. This is the case, for example, at St. Clair, Dunedin, where the material for this investigation was principally collected. Here D. antarctica is easily accessible, as its zone, extending for the whole length of the beach, is partly uncovered by all except for a few neap tides, and completely so by spring tides. D. willana, on the other hand, is more difficult to collect. It occurs only at the two ends of the beach where flat, sand-covered rocks form a suitable substrate, and is exposed only by the spring tides, and then normally only over that part of its zone which overlaps with D. antarctica.

Methods

Comparisons were made between plants of the two species 2 cm. long; 5 cm., 10 cm.; 20–30 cm.; about 1.5 metres long, but before any signs of conceptacle development were visible to the naked eye; and between fully mature plants which had been reproductive for a period of several years. At all stages comparisons were made at representative levels from apex to disc.

The fixative found to be most satisfactory was a modification of Karpechenko's fluid (Laing, 1941). Formalin-seawater caused excessive swelling both of the hyphal walls and of the primary walls, particularly in D. antarctica. After fixation, the material was stored in 50 per cent. alcohol In some cases alcoholic glycerine was used, but although this has the advantage of keeping the material supple, it also tends to cause swelling of the walls. D. willana was found to be more resistant to swelling than D. antarctica. For the anatomical structure a 0·05 per cent. solution of gentian violet in 50 per cent. alcohol was used, and for the reproductive bodies, Heidenhain's iron alum haematoxylin.

An account has already been given (Naylor, 1949) of the structure of the adult plant of D. antarctica, but no young plants were then available. This present investigation, in general, confirms the earlier account, but indicates that the fixatives which had been used had caused excessive swelling of the cell walls in the lamina, and had obscured the cell cavities and given them a distorted appearance.

Morphology of the Young Plants

Very young plants of both species were collected in December, 1950. The smallest were between 0.5 cm. and 1.0 cm. in length and were probably between two and eight months old, as the period of fertility in 1950 lasted from April to September.

In both species these youngest plants are similar to young laminarians, with an entire, ovoid blade; a short, cylindrical stipe and a conical attaching disc (Text-fig. 1 A–F). As the plants increase in size, the form varies. In some cases dissection of the blade begins at an early stage, so that the split extends almost down to the stipe (Text-fig. 1 G and 2 C). In other cases the blade remains entire until the plant is much larger, and fine branches arise from the distal end of the

by which the species may be distinguished. Prior to the appearance of these proliferations, young plants can only be referred to their species with any assurance if they are collected from zones of pure growth where there is no intermingling of the two species.

It was found that in the case of material from St. Clair, the plants of D. willana showed a relatively greater development of stipe—both in length and diameter—to blade area than did plants of D. antarctica (Pl. 58). This may possibly be related to the vertical growth of the former, or to its rather more exposed habitat. It is of interest that plants of D. antarctica from Piha, where they are growing in a more exposed position than at St. Clair, show a greater development of stipe in proportion to blade (Text-fig 2 c-e) D. willana is not present in this locality for comparison.

Anatomy

The Lamina

The lamina of the 2 cm. plant of D. antarctica (Text-fig. 3 a) resembles the structure already described in the growing tips of the older plant (Naylor, 1949) but on a simpler scale. The cortex is only about 2 cells deep, the cells being slightly elongated in the direction of the longitudinal axis and irregularly shaped. The cell walls are thin and the cells closely packed. The medullary cells are more regularly elongated and arranged in longitudinal series. Throughout the lamina

Text-fig. 3—Longitudinal sections in the plane perpendicular to the plane of flattening immediately behind the apices of 2 cm. plants. A D. antarctica R D. willana. cor. = cortex; hy. = hyphae; med. = filaments of medullary cells; mer. = meristoderm. Cell outlines drawn with the aid of a projection apparatus. Scale = × 340.

the cells contain numerous and conspicuous plastids. There is slight production of mucilage in the inner cortex and in the medulla, apparently due to the gelatinisation of the cell walls. A few hyphae interweave with the primary medullary cells, even right up to the apex. These hyphae have firm, highly refractive walls and wide lumina. The hyphal walls stain intensely reddish-purple with gentian violet, and are thereby very conspicuous, as the primary cell walls do not stain at all with the gentian violet.

of D. antarctica. The swelling is usually more advanced in the basal regions of the stipe, indicating that elongation is probably carried out principally in the upper regions of the stipe. The cell lumina of the hyphae swell to many times their original diameter, so that the cells become first sausage-shaped and finally spherical (Text-fig. 7). At an early stage, the cells become divided by thin, horizontal septa (Plate 60 a). This swelling recalls the formation of the diaphragms in Himanthalia lorea (Naylor, 1951).

In D. willana the longitudinal hyphae are always tightly packed and the swelling takes place in a regular manner so that a compact medulla of parenchymatous appearance is produced (Plate 60 c and d). In D. antarctica, on the other hand, there is a certain amount of gelatinisation and the longitudinal hyphae become widely separated. Secondary pit connections (Plate 60 a) are established between the longitudinal hyphae which also give rise to horizontal hyphae (Plate 60 a), which interweave between them (Plate 60 b). In D. willana secondary pit connections are also formed, and in the latter stages of development small numbers of horizontal hyphae are formed (Plate 60 d), but they are not such a conspicuous feature as in D. antarctica. Thus, although in D. antarctica a compact medulla may ultimately result from the swelling of the pit connections and horizontal hyphae, at most stages of development its rather looser and less regular structure can be distinguished from the more regular and compact arrangement in D. willana.

In both species there is a cylinder of unswollen hyphae, probably the most recently formed, in the region of inner cortex and outer medulla (Text-fig. 8). The degree of swelling of the cells increases towards the centre of the stipe.

In both species the primary medullary cells become distended as well as the hyphae, often at an earlier stage, when they appear as filaments of sievetube-like cells. But these are relatively so few in number that they are of little importance in the great increase in diameter that takes place.

Thus, although there is apparently great contrast between the adult stipes of the two species, the basic underlying construction is the same in both. The wide secondary cortex of D. willana recalls that of many laminarians, and possibly this parenchymatous structure is responsible for the rigidity of the stipe of the of this species, which enables it to stand erect when no longer supported by the water at low tide.

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

Distribution of Cellulose and Alginic Acid

Cellulose is widely distributed in the tissues of both species. At all stages it forms a lining layer to the protoplast of the primary tissues, but the reaction is particularly pronounced through the walls of the hyphae. As in Himanthalia lorea (Naylor, 1951), this coincides with the region of the wall which stains red with gentian violet. Also, the most marked reaction is in the tissues concerned with a mechanical function.

In both species extraction of alginic acid with dilute hydrochloric acid and sodium carbonate causes disintegration of the primary tissues, although the layer of the wall next to the protoplast is unaffected Thus, alginic acid appears to be responsible for the cohesion of these tissues, and is either intercellular or forms part of the outer layers of the wall. In all cases, the interweaving hyphae remained intact after the extraction.

Conceptacles

Both species of Durvillea are dioecious. Conceptacle development does not begin until the plants are at least in their second year, and there are many points of similarity between the two species.

The conceptacles of Durvillea and Sarcophycus have been described by earlier writers (Durvillea—Skottsberg, 1909; Grabendörfer, Herriott; Sarcophycus—Whitting), so it is only necessary to make a few general observations which concern only the female conceptacle.

In both the species under investigation the conceptacles occur scattered all over the lamma. They are slow to develop, becoming evident as early as June,

Text-fig. 10—Longitudinal section through a mature conceptacle of D. willana showing oogoma (og) at all stages of development separated by moniliform hairs (h). Sterile, basally growing hairs (per) project beyond the ostiole. Cell outlines drawn with the aid of a projection apparatus. Scale = × 170.

but not becoming fully mature until the end of the following April, when they produce a succession of gametes for a period of up to five months. When all the gametes have been shed, the spent conceptacles become occluded by the ingrowth of hyphae from the surrounding cortex. This occlusion has been recorded previously, but its significance has not been fully understood (Decaisne, 1841; Grabendörfer, 1885; Naylor, 1949). The plants are fertile for several years before they die, and as they age the periclinal divisions of the meristoderm cause the old conceptacles to be buried in the cortical tissues. Meanwhile, new meristoderm cells give rise to further conceptacle initials, and another crop of conceptacles develops at surface level. Thus, in an old plant, it is possible to tell for how many seasons it has been reproductive by the series of conceptacles at successively deeper levels in the cortex (Plate 61 a and b).

In D. antarctica the majority of the oogonia are borne on branched hairs, as described by Skottsberg (1907). Herriott, Grabendörfer (in D. harveyi) and by Whitting (in S. potalorum), but sometimes they are borne directly on the wall of the conceptacle. In D. willana, on the other hand, I have only seen the oogonia arranged parietally and separated by moniliform hairs (Text-figure 10 and Plate 61 c).

The oogonia are quite indistinguishable in the two species, both in the arrangement of the oospheres and in size. Each oogonium contains four oospheres and is divided by two transverse septa, the middle loculus being further divided by a longitudinal septum.

Extrusion

The mode of extrusion of the oospheres, fertilisation and early development of the young sporeling are identical in the two species.

The structure of the oogonium wall is simple. It consists of two layers, an outer exochiton and an inner endochiton immediately surrounding the oospheres.

Immediately prior to extrusion, there is no pronounced swelling of any intervening mucilaginous layers, as in Fucus. The exochiton ruptures distally—possibly due to hydrostatic forces developed within the eggs themselves—and the packet of four eggs is released with a sharp, jerking movement (Text-figure 11 c), and, guided by the paraphyses, passes out through the ostiole.

This stage of extrusion appears to take place whilst the plants are exposed between the tides, and the exudate accumulates in mounds over the ostioles. The sexes can easily be distinguished by the colour of the exudate, that of the female being brown in colour and that of the male white.

Once free of the restricting exochiton, and on access of seawater, the egg packet as a whole increases in size, and the oospheres lose their angular shapc and round off so that the four-chambered nature of the endochiton can be clearly seen.

The individual oospheres now escape from their compartments. The oosphere bulges towards, and presses against, the outer wall of its compartment, causing it to bulge and finally to rupture (Text-figure 11 g and h). The oosphere then flows rapidly through the small hole thus formed, becoming considerably constricted during the process. Very rapid movement of the cytoplasm is seen during the process. Finally, the oosphere spins free and rapidly resumes a spherical shape This indicates that the changes in shape during the escape are due to mechanical causes and not to the active movement of the oosphere

itself (cf. Behrens, 1886; amoeboid movement in Fucus). All four oospheres escape in a similar manner, either simultaneously or in succession, leaving behind the firm, four-chambered empty endochiton (Text-figure 11 I-L).

Germination of the Oosphere

Once the oosphere is free of the endochiton, fertilisation can take place. Large numbers of antherozoids are attracted to each egg and swim round it for a while until one of them enters it. A wall is soon formed round the fertilised oospherc.

Both species were grown in culture for several months, and in both the early segmentations followed the same course.

The first sign of germination—as in Fucus—is the development at one end of the oospore of a protuberance which marks the position of the primary rhizoid (Text-figure 12 b). The first wall divides the oospore into two unequal parts, a large upper portion and a smaller lower one from which the primary rhizoid is cut off by the second wall (Text-figure 12 c–e, p–q).

The upper cell is then divided by vertical walls into quadrants, after which further horizontal and vertical divisions follow in quick succession. The body increases in size and becomes elongated. Simultaneously vertical divisions, followed by horizontal ones, take place in the lower cell. Meanwhile the rhizoid elongates and becomes multicellular, and in many cases, branched. The rhizoidal system is surrounded by a layer of mucilage which cements it firmly to the substrate.

When the sporeling is about a fortnight old, the wall which surrounded the fertilized egg becomes apparent as a cap (Text-figure 12 h and t), and is shed. Shortly after this the sporeling becomes erect, so probably this wall plays a part in attachment in the early stages.

The radial symmetry is soon lost, and a definite flattening is developed (Text-figure 12 m and n). The only further developments which were seen in a period of four months were an increase in size due to further cell divisions, and an increase in the degree of flattening, so that a flat, plate-like lamina, two cells deep, was produced, and in some cases (Text-figure 12 L) other rhizoids besides the primary one developed.

These early segmentations are thus seen to follow the usual course in the Fucales, and in the production of a single primary rhizoid, Durvillea resembles the intertidal Fucaceae and not the Cystoseiraceae and Sargassaceae, where the initial divides to give a tuft of rhizoids. At no stage was there any sign of an apical groove, nor an apical tuft of hairs such as develops in sporelings of Fucus and Carpophyllum after two to three weeks' growth in culture, indicating that the lack of any special apical cell is a primary feature of this genus.

Discussion

This anatomical investigation shows that, although superficially the spongy, open thallus of D. antarctica differs from the firmer and more compact thallus of D. willana, the underlying construction is the same in the two species. The principal difference is in the nature of the cell wall, which tends to gelatinise in D. antarctica but not in D. willana This may possibly be related to the level on the shore at which the two species occur. Within the genus Fucus, Zaneveld (1937) has shown that the thickness of the cell walls is greater in the species at

S. simplex. Both he and Skottsberg (1921) question the existence of D. montagnei as a separate species. Skottsberg considers only one species to exist, which he states should, on grounds of priority of nomenclature, be called D. caepestipes (Mont.). Skottsberg (1941) questions whether Oliver's narrower form of D. antarctica might be D. harveyi, but this cannot be the case on account of the inflated lamina and the nature of the attaching disc.

Sarcophycus simplex Kütz. (1860) I have had no opportunity of examining, and so do not know what should be regarded as the position of this species. As the diagnostic of this genus includes the same nature of the division of the oogonium, possibly it, too, is rightly considered as a species of Durvillea

It seems, therefore, from a study of the material and literature, that the two genera should be united into the single genus Durvillea (Bory) containing the following species:

D. antarctica (Chamisso) Hariot.

D. caepestipes (Mont) Skotts. (= D. harveyi Hook fil.).

D. potatorum (Labill.) Areschoug.

D. willana Lindauer.

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