Art. 38.—Some Morphological Notes on the New Zealand Giant Kelp, Durvillea antarctica(Chamisso).
[Read before the Philosophical Institute of Canterbury, 7th December, 1921; received by Editor, 15th December, 1921; issued separately, 22nd May, 1923.] Plates 54, 55.
Table of Contents.
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|Morphology and Anatomy||551|
|Morphology and Anatomy—continued.||(3.) Lamina—continued.|
|Peculiar Morphological Features||560|
|Bounty Island Specimen||562|
|List of Works referred to||564|
During the past years several references have been made to this fucoid in reports of the various Antarctic expeditions. Skottsberg, of the Swedish Antarctic Expedition, and Gain, of the second French Antarctic Expedition, have both recorded it from the smaller islands of the Southern Hemisphere, but have not given any details as to its anatomy. For such a study it is almost necessary to work with fresh material collected where the plant is growing. Plants that have been washed up and allowed to dry for any length of time give unsatisfactory results; the tissues shrink and the cells become distorted. Notwithstanding the abundant material available, and the interest of the plant as probably representing, in spite of its enormous size, one of the lowliest forms of its order, no account of its anatomy has been given by any New Zealand botanist. The greater part of this paper, including the anatomical sections, was prepared several years ago.
Durvillea antarctica was first recorded under the name Fucus antarcticus by Chamisso (Voy. Choris, p. 7, tab. 7). In 1826 Bory redescribed it in Flore des Malouines (p. 588), and formed a new genus, Durvillea, for its reception. He, however, bestowed on the plant a new specific name, utilis, disregarding the older name antarcticus. The plant was then known as Durvillea utilis Bory, and the error was not corrected till 1892, when Hariot pointed it out and restored Chamisso's specific name. Later, in 1908, Skottsberg also noted the mistake that had been made (1908, p. 140), though evidently he did not then know of Hariot's correction. The plant should now therefore be known as Durvillea antarctica (Chamisso) Hariot. For a list of synonyms and references see Gain's account of the plant (1912, p. 51).
The genus Durvillea belongs to the order Fucaceae of the group Phaeophyceae, or Brown Algae. This order is distinguished by the complete absence of any method of asexual reproduction. As, however, Lloyd Williams has recently described sexual reproduction in the order Laminariaceae, where it had previously been supposed to be wanting, it is possible that future investigation may show some form of asexual reproduction in this order. Hooker's description of the genus is as follows: “Root scutate. Frond stalked, dark olive-brown or black, flat, expanded, very thick and coriaceous or honeycombed transversely internally, palmate or pinnate, without distinct organs. Fruit dioecious. Conceptacles scattered over the whole frond in the cortical stratum, containing either obovoid subsessile spores or branched filaments bearing obovoid antheridia” (1867, p. 654).
The genus belongs to the Southern Hemisphere, and usually two distinct species are recognized, D. antarctica and D. Harveyi. Of these, only D. antarctica is known from New Zealand. Hooker (1867, p. 654) describes the species thus: “Frond dark brown or black, often 30 ft. long, forming an immense flabellate palmately lobed laciniated lamina, contracted at the cuneiform base into a short stipes as thick as the wrist; segments or thongs often 1 in. thick, honeycombed internally.”
The chief point of difference between the two species appears to be that in D. antarctica the lamina is much divided and inflated, whereas in D. Harveyi it is more or less undivided and solid throughout. Hooker (1847) also describes the “root” of D. Harveyi as fibrous, while in D. antarctica it is disc-like. Skottsberg (1921, p. 53) points out that Areschoug included all the South American forms in one species. He himself would retain the names D. antarctica and D. Harveyi for the two extreme forms, but he says that he has found at least one form which is intermediate between the two, and suggests the possibility of others being found to bridge the gaps, in which case a series might be formed connecting the two extremes. He also points out the different conditions under which these two so-called species are found. D. antarctica, with its much-divided lamina, is found where the surf is very violent, while D. Harveyi is found in much quieter waters, or in tide pools, and is not exposed to such severe treatment. A specimen to be described below from Bounty Island, and subjected to even severer conditions than those met with on New Zealand coasts, has its lamina much more divided than typical D. antarctica, a fact which may add further weight to the suggestion that the form of the plant depends on the habitat.
As regards the distribution, Skottsberg reports it from central Chile to Cape Horn, Falkland, Kerguelen Islands, New Zealand, Chatham, Auckland, and Campbell Islands. “It does not grow in South Georgia, but drifted pieces are sometimes found” (1921, p. 54). Gain, however (1912, p. 111), gave South Georgia as one of the localities in which Durvillea antarctica occurred. Hooker (1847) had described it as having been found floating in the sea off the Cape of Good Hope, and as far south as the 65th degree of latitude on the meridian of New Zealand.
Durvillea antarctica grows attached to rocks “on exposed rocky coasts or steep cliffs, often in narrow crevices where the surf is very violent. During dead low water it hangs from the rocks, with the whip-like segments washed by spray, or in calm weather almost dry for a short while. At high water the whole plant is violently tossed about in the breakers, and in stormy weather with tremendous force that would tear the strongest frond to pieces were it not split up into numerous laciniae, elliptical or circular in section” (Skottsberg, 1921, p. 53).
As Hooker noticed, Durvillea is generally associated with the other large brown kelp, Macrocystis pyrifera. Durvillea grows close to and attached to the rocks, forming huge banks of brown seaweed (Plate 54, figs. 1 and 2), which are dashed about in the water and against the rocks by every breaker, while Macrocystis forms a second zone farther out, and its thin leaf-like segments can be seen floating on the surface, rising and falling with each passing swell. The structures of these two seaweeds afford striking points of contrast. The frond of Macrocystis is enormously developed, and exposes as great a surface as possible for assimilatory processes, while the stipe is very slender. There is no strain on the stipe, its purpose being merely to connect the various portions of the frond and to act as a channel of conduction, for which purpose it has developed its peculiar sieve-tubes. In Durvillea, on the other hand, the stipe is of very considerable thickness, one having been measured of 12.5 cm. diameter. It grows out horizontally from the rock-face, and during low water has partially to support the huge segmented lamina which hangs from it; it must consequently be tough and leathery, and yet very pliable, to meet the great strain upon it. Portions of the frond torn away from the plant may frequently be found, and sometimes the stipe itself is broken across. The force of the waves may even tear away the holdfast with portions of the supporting rock; and on looking down at a bed of Durvillea one may see ample evidence of the rough treatment to which it has been subjected.
Morphology and Anatomy.
Three distinct regions can be recognized in the plant-body. These may be called (1) the holdfast (the “radix” of Agardh and the “haftscheibe” of Skottsberg), (2) the stipe or “stem,” and (3) the lamina (Plate 55).
(1.) The Holdfast.
The holdfast attaches the plant to the rock. It is usually quite regular and circular in outline, forming one continuous disc. In one specimen (fig. 1) collected on the New Brighton beach the tissue was torn apart, leaving fissures in the structure similar to that described by Grabendorfer for D. Harveyi. The upper surface is light-yellow in colour. much lighter than the lamina and stipe, the lower surface, which grows towards the substratum, being dark brown.
The holdfast grows stretched over the rock, adhering to it much after the manner of a leather sucker to a stone. The tissues grow round the jagged edges of rock and fit into the crevices, thus obtaining a firm hold. Holdfasts, of course, vary in size, according to the size of the plant attached, increasing in area as the plant increases in size from year to year. The diameter is always several times that of the stipe attached, the smallest plant measured, in which the three regions of the plant were differentiated,
having a diameter of 2–5 cm. for the holdfast and 3 mm. for the stipe. In some large specimens the holdfast measures from 30 cm. to 60 cm. across, and in these it is not uncommon to see several stipes arising from one holdfast: Agardh (1848, p. 188) mentions six as the highest number observed; the writer has seen four, three adult plants, several feet in length, rising from the central portion of the holdfast, and near the outer edge a small plant, measuring only 15 cm. in length, whose lamina was still a flat thallus which had not begun to segment (fig. 2)
Fig. 1.—Holdfast, with portion of stipe showing pits, &c.
Fig. 2.—;Small plant of Durrillea antarclica (× ¾).
Where the plants grow together in such masses that the holdfasts of the separate plants have not sufficient room to develop independently, one will encroach on another, the two finally growing together, forming one common holdfast in which the tissues have united so completely that there is in most cases no mark to show where the union has taken place.
In one specimen examined the stipe seemed to branch just at its junction with the holdfast; but this apparent branching was probably brought about by the germination of two oospheres close together, so that the two holdfasts united at a very early stage, while the two stipes by growth
of the cambium layer increased in thickness, and finally met. Continued growth and the constant friction of the tissues may then have caused the adjoining tissues to unite, and so to appear finally as branches of one main stipe.
The tissue of which the holdfast is composed is very elastic; when pulled away from the rock it contracts greatly, the lower layers contracting most and drawing the upper layers over them. When washed up on the beach and left exposed the holdfast loses its water and contracts to a hard horny mass.
Figures 3 and 4 show details of the anatomical structure of the holdfast. These sections were cut from a young specimen. The tissue appears to consist of (a) an outer layer about seven cells in thickness, the cells being small and cubical or slightly rounded, abundantly filled with protoplasm and a light-brown colouring-matter. The outermost layer is not specially marked off from the underlying cells, as it is in both stipe and lamina
Below this comes (b), a meristematic tissue composed of several layers of smaller cells in a state of active cell-division. On staining, each of these cells is seen to contain a large nucleus.
Below this again comes tissue (c), of small rounded cells, and this passes into the central tissue (d), consisting of large irregularly shaped cells with sparing cell-contents. This tissue makes up the greater part of the body of the holdfast. Its cells contain some small round bodies (fig. 3, k) which stain a faint yellow with iodine; this is probably the Phaeophycean starch of Schmitz (Zimmermann). The presence of such bodies in the holdfast would support the theory presented by Harvey (1849, Introduction, p. xx) that the holdfast acts as the storehouse for reserve food, and may be compared with tuberous or tap roots of higher plants.
On the under-surface of the holdfast are about three layers of cells which contain a dark-brown colouring-matter, as described by Grabendorfer (1885, p. 7) for D. Harveyi. Their function is probably protective.
Sections were also cut from older holdfasts. These had been exposed for some time to wind and sun, and had therefore lost much of their water. The outermost layer consisted of cubical-shaped cells, but the great part was composed of very irregularly shaped cells embedded in a horny matrix, to which the peculiar woody appearance of a cut holdfast is probably due.
When such dried specimens are placed in fresh water they swell up enormously, but the constituent cells then present a very distorted appearance, with very irregular lumina. Such a section represents a structure very unlike that shown by fresh material. The study of dry material does not give satisfactory results in this case, and it is probably owing to the fact that Grabendorfer's examinations were made mostly on dry material that his descriptions of D. Harveyi are so hard to reconcile with examinations of D. antarctica.
In the holdfast growth is effected by a definite growing tissue (fig. 3, b), and in this it differs from stipe and lamina, in which growth is evidently effected by division of the outermost layer of cells.
(2.) The Stipe.
The stipe, as it arises from the holdfast, is cylindrical, but in most cases it gradually becomes flattened and passes into the lower portion of the frond In other cases the original stipe continues its growth, retaining its cylindrical form, while it gives off secondary stipes along its axis; This is the pinnate arrangement mentioned by Hooker (1864, p. 654). The secondary stipes then bear the segments of the frond. A small plant of
this type had a primary stipe measuring 90 cm., and one of the secondary stipes, with its lamina, measured 2–55 m.
The stipe increases in thickness from year to year, the diameter in some of the larger specimens varying from 5 cm. to 7–5 cm.; one exceptionally large one measured 12-5 cm. Such thicknesses are exceptional for seaweeds, but in the case of Durvillea, as mentioned before, the stipe has partially to support its huge segmented lamina when it is left uncovered at low tides, hence its need of a stout tissue. The thickness of the stipe of Durvillea is exceeded by the big Laminarian Postelsia palmaeformis, whose huge stipe is strong enough to maintain an upright position in the air and support its tuft-like lamina (Campbell, 1902, p. 127).
Three distinct tissues may always be recognized in the stipe, whether young or old: (1) an outer cortical assimilatory layer, (2) a filamentous conducting tissue, and (3) a central tissue.
The surface layer of cells (figs. 5, 6, a) appears to be differentiated from the rest of the cortical tissue. The constituent cells are rather small, somewhat cubical in shape, and at the same time more densely filled with cell-contents. Below this comes the rest of the cortical tissue (b), several cells in thickness, containing abundant protoplasm and rich in brown colouring-matter. These cells are arranged in radial rows round the stipe. The innermost cells of this layer lose their characteristic oval form and show a tendency to branch and elongate. They pass gradually into the next tissue (c), which is a mass of interwoven filaments. In longitudinal section (fig. 6) these appear to lie close together, and run more or less in a longitudinal direction. In transverse section (fig. 5) they appear to be arranged in rows continuous with the rows of the outer layer. Towards the centre of the stipe this arrangement becomes less regular, and in figs. 5 and 6 (c) the longitudinal filaments are seen cut across. These filaments are interrupted by other septate filaments running in a transverse direction, which serve to bind them together and make the tissue firmer.
The central tissue (d) is composed of a mass of cells of varying shapes. There are few filaments to be seen both in transverse and longitudinal sections, but the greater part of this tissue is made up of large cells, arranged in chains, about ten times the size of the smaller ones of the outer tissues. These have sparing cell-contents, but contain small round bodies (k), similar to those of the holdfast, which probably represent the reserve food material in the form of Phaeophycean starch. This central mass of tissue appears much darker than the other two layers in a freshly cut stipe, but a section across does not reveal any cause for this. The contents are colourless, or nearly so. The two outer layers may also be distinguished in a fresh stipe by their colour. The outer cortical tissue is coloured a light brown, while the inner conducting tissue is nearly colourless, and merges gradually into the darker central tissue.
A stipe with a diameter of 4 cm. was taken and cut into thin layers about 0–5 cm. thick. Some were placed in water, others in 30 per cent. alcohol, and others in 50 per cent. alcohol, and left all night. In the morning the piece in water was much the same. Of the piece in the 30 per cent, alcohol, the outer tissue remained the same colour, the second tissue had become flesh-coloured, the central tissue being lighter; some of the dark colouring-matter had dissolved out. Of the piece in the 50 per cent., the outer tissues were the same as in the 30 per cent., but the central tissue had turned a bright green. When left longer in the alcohol this green colouring-matter dissolved out. A section of the central portion showed that the green colour was due to small round green bodies in the larger
Fig. 3.—Vertical section of holdfast.
Fig. 4.—Tangential section of holdfast.
Fig. 5.—Transverse section of stipe.
Fig. 6.—Longitudinal section of stipe.
Fig. 7.—Transverse section of lower portion of lamina.
Fig. 8.—Transverse section of upper portion of lamina.
Fig. 9.—Tissue breaking down to form air-chambers.
Fig. 10.—Portion of lamina with upper tissue removed to show air-chambers.
Fig. 11.—Structure of wall of air-chamber.
cells of this tissue. These bodies would appear to be connected with the colourless bodies in the fresh specimen, and it is probable that they were identical with them.
Growth takes place as in the lamina by the active division of the outermost layer of cells, which are more densely filled with the brown colouring-matter than the other cells of the cortical tissue. These cells divide only from their inner side, and so give rise by continued division to the radial rows of the cortex. The cells of the cortex again gradually pass over into the next layer by continued growth, and so each tissue increases in size from the one directly outside it.
On experimenting with a stipe from an adult plant, after Detmer's method (1898, p. 20), to extract the brown colouring-matter from the outer layers, it was found that it differed greatly in this respect from portions of the frond. The colour of the stipe is always much lighter than that of the frond, and the brown colouring-matter is not extracted by boiling water or by alcohol, while it is readily extracted from the frond in either way, leaving the green chlorophyll in the cells.
The stipe used had a diameter of 6–25 cm., and a traverse section about 1–5 cm. long was cut in quadrants. These pieces were placed in hot water, and in half an hour the inner portion corresponding to the storage-tissue contracted greatly; the two outer layers did not contract at all, and were therefore drawn over this central portion. The same result is obtained in a less degree by cutting up a stipe and leaving it exposed to the air for a day or two. Hence it is seen that the inner tissues of the stipe are in a state of tension, and when the tension is released they contract owing to loss of water.
General Structure.—The original stipe usually gives rise to a palmately segmented lamina, which often attains a great length. Numerous specimens were measured that had been washed up after a strong gale. The smallest stipe of a group of four attached to one holdfast bore a lamina broken across part of its length, but the remaining portion measured 10-8 m. The lower portion of the lamina is a continuation of the stipe, and is usually flat and expanded, measuring from 30 cm. to 45 cm. in its widest part. This then divides up, more or less closely to the stipe, into numerous long whip-like segments, which appear to have arisen quite irregularly. The segments are of various lengths; some branch repeatedly, others remain unbranched and occasionally terminate bluntly, but usually each segment tapers to a fine point.
The broad lower portion of the lamina is solid throughout, but as it breaks up into segments the central tissue is torn apart, and forms air-chambers, in which gases collect, causing an inflated appearance in the fronds. Air-chambers are not always present, but there is usually some indication of them. On the segments occur the conceptacles, which contain the reproductive organs. The conceptacles cover the frond thickly, and may be recognized by the pores, or ostioles, which are plainly visible as little dots on the surface. In the winter months, which are the best for studying the reproductive organs, these ostioles are covered with the extruded contents of the conceptacles. These contents may be recognized by their colour; in the case of the oogonia they are dark brown, while the colourless antheridia show as white dots.
There are only two clearly defined tissues present in the lamina, the outer cortical or assimilatory, the inner medullary or conducting.
In the lower portion of the lamina where no air-chambers are present there are only two kinds of tissue; the inner storage-tissue of the stipe disappears. Fig. 7, a section of this portion of the lamina, shows the arrangement of this tissue. The outer layers (b) are composed of cells arranged in more or less regular radial rows. In sections which had been preserved in alcohol after picric acid these cells were flask-shaped, and the rows were distinctly separate. The cells were filled with darkly stained cell-contents. The inner cells of the layer lose their regular form, and show a tendency to elongate in various directions, and gradually to pass into the second medullary layer (c), which is composed of a dense mass of filaments running parallel to the longitudinal axis of the plant.
Sections of those segments of the frond which possess air-chambers show the same structure (fig. 8), but here the medullary tissue (c) is broken down in the centre of the very young portions of the segments, and the origin of the air-chambers can be seen (fig. 9).
Growth takes place here as noticed by Grabendorfer (1885) in the outermost layer of the lamina. These cells are richly filled with protoplasmic contents, and are in a state of active cell-division, both by tangential and radial walls.
Air-chambers.—The air-chambers found in the frond of Durvillea antarctica are a distinctive mark of this species; they increase the width of the segments of the lamina, their gases distending it considerably. The chambers vary in size and shape, but the largest measured was not more than 1–25 cm. across. They may be hexagonal, square, triangular, or quite irregular in cross-section, and are separated by walls at right angles to the surface (fig. 10). These walls are composed of interwoven filaments (fig. 11), each of which is surrounded by a gelatinous wall, clearly distinguished on staining with methyl blue. In fresh specimens the walls are tough and elastic, but when dried they become papery.
The gas contained in air-chambers of the Fucaceae was stated by early experimenters to be nitrogen. “Wille added to this oxygen, and denied the presence of carbon dioxide ” (Murray, 1895, p. 48). Some of the gas was collected at Taylor's Mistake and brought up to Christchurch, and analysed with Hempel's apparatus. There was no CO2 present. In 55 c.c. of the gas after passing it through KOH there was no difference in volume, thus proving the absence of CO2. After treating with pyrogallic acid, 42-6 c.c. remained, thus giving a volume of 12-4 c.c. of oxygen present. This gives a percentage composition of 22-54 oxygen and 77-45 nitrogen, showing the gas to be richer in oxygen than ordinary atmospheric air. There are no special organs for absorbing the gases, which must therefore pass through the cell-walls in solution in the water, and they can do this only very slowly.
Professor Warming states that the fucoids are able to absorb every trace of air present in the surrounding water, and, when that is all absorbed, to draw on the stores of oxygen contained in the air-chambers, and so carry on the process of respiration. This would account for the state of collapse which has been noticed in some of these air-chambers, as in the seaweed which has been washed up in a shady place where little moisture can reach it. The gas can pass into the plant only in solution in water, and when no water is present the plant is compelled to draw on its reserve stock. When the plant is exposed to the sun the heat expands the gas, and so keeps the chambers distended, while it also dries up the tissue of the plant, so that the process of assimilation cannot go on.
Conceptacles,—Durvillaea is dioecious, male and female conceptacles being found on different plants. The conceptacles are flask-shaped cavities (fig. 12), opening to the exterior by a narrow mouth, or ostiole. The conceptacle itself extends down into the medullary tissue, which is specially differentiated around it, forming a layer of two or three cells in thickness, composed of elengated cells tapering at both ends and dovetailing into one another. The structure of the conceptacle does not differ much in the male and female plants.
The apical cell, by division of which the conceptacles arise, does not persist as in Splachnidium, and there are no hairs protruding from the ostiole. The cells lining the cavity grow out into short papillae just near the mouth, the shortest being found nearest the exterior. Both male and female conceptacles are abundantly provided with hairs (d), both branched and unbranched, on which the sexual organs are borne. These hairs are the outgrowths of the cells lining the cavity, and they are found in various stages of development, some as short club-shaped papillae, others two or three cells in length; some branched, others again still unbranched. Each hair is surrounded by a thick gelatinous coat, which shows up plainly on staining with methyl blue. Its cell-contents contract somewhat, but take the stain much more deeply than the outer coat.
These hairs show a tendency to swell up at the end into a club-shaped body more densely filled with protoplasm. This is apparently the origin of the antheridia and oogonia.
In the female conceptacles (fig. 12) there are both branched and unbranched hairs (d), and the oogonia (o) are borne on both, or may be subsessile. Hooker (1864) found them to be subsessile, and in one specimen examined there were no branched hairs at all, the oogonia being directly attached by a basal cell to the wall of the conceptacle. In all the others there were numerous branched hairs, and the oogonia were in these found on lateral branches, although in some cases they were subsessile on the wall of the conceptacle. Mr. R. M. Laing (1885) stated that the older ones were found on branched hairs; but this is not always the case. There does not seem, to be any distinct rule about their arrangement. Each oogonium is cut off, either from its basal cell or from the rest of the hair on which it arises, by a cell-wall. The outer coat can be always plainly recognized by a suitable stain. The oogonium is at first globular (fig. 12) It is rich in cell-contents, at first colourless but later on taking a faint yellow tinge, deepening to a dark-brown colour when mature. The young oogonium is also rich in food material in the form of small round bodies scattered in the granular protoplasm. The protoplasm divides into three transversely, the middle portion again dividing into two longitudinally, so that there are finally four oospheres within the oogonium (fig. 14). This mode of division was first noticed by Hooker, as stated by Hombron and Jacquinot (1845), and he called the four resulting oospheres “ tetraspores.” It was first rightly interpreted by Kutzing in his examination of Sarcophyous potatorum (Phyco. Mem., p. 38). The four oospheres, surrounded by an inner coat, escape from the oogonium into the water outside the plant by rupture of the wall at the apex, and leave the empty oogonium still attached to the wall of the conceptacle.
Oogonia in all stages of development may be found in the same conceptacle. The colourless globular bodies were found in one in which the ripe dark-brown oospheres were being expelled, From this it would seem that the conceptacles are able to produce oogonia indefinitely by the continued activity of the cells lining the cavity. Mr. R. M. Laing (1885) states
Fig. 12.—Section through female conceptacle.
Fig. 13.—Young oogonia.
Fig. 14.—Older oogonia.
Fig. 15.—Section through male conceptacle.
Fig. 16.—Branched hair bearing antheridia.
Fig. 17.—Transverse section of stipe, showing pit.
Fig. 18.—Transverse section of stipe, showing pit (Bounty Island specimen).
that the reproductive organs may be found at any time of the year, though best in the winter months. In specimens collected at the end of March both ripe and immature oogonia were found, and also in others collected in September.
The male conceptacles (fig. 15) possess many more hairs, and they are much more branched, and more strongly developed. One hair (fig. 15) arising just opposite the ostiole extended almost to the ostiole itself, and was densely covered with branches, which were also branched, thus presenting the appearance of a small tree with monopodial branching.
The antheridia are formed usually on branched hairs from the swollen ends of the branches. They are much smaller than the oogonia, and the protoplasm is rounded off into numerous small colourless spermatozoids. Occasionally small unbranched hairs develop directly into antheridia. The spermatozoids escape into the surrounding water, still enveloped in an inner mucilaginous coat, which is dissolved by the water, setting free the actively swimming spermatozoids.
The contents of both conceptacles are forced out at low tide, probably owing to the sudden release from pressure. In this they are aided by the mucilaginous character of the hairs, which swell up and expel the contents in mucilage. The fronds may be seen covered by little white and brown dots, over the ostioles of the conceptacles. These are the male and female organs cohering together in the mucilage. When the tide rises, the mucilage and walls surrounding the organs are dissolved, setting free the oospheres and spermatozoids. Each oosphere when set free becomes rounded off, and is surrounded by numerous spermatozoids carried to it by the water from a neighbouring male plant, and so fertilization of the oospheres takes place. It is noticeable that there is a great difference in the colour of the two plants, the one bearing oogonia being always darker than that which bears the antheridia, and when the reproductive organs are expelled from the mouth of the conceptacle the two may be recognized by the difference in colour of the small dots lying over the ostioles.
Peculiar Morphological Features.
On the stipe and lower part of the lamina are to be found curious markings, which seem to be cracks in the tissue. On the stipe these are usually seen on only one side, and are about 1–5 mm. in depth. The larger the stipe the greater the number of markings (fig. 1); but on quite small plants there are always at least two, just at the junction of the stipe with the holdfast. They are not cracks in the tissue, as was at first supposed. A section taken across one shows that the radial arrangement of the outer tissue of the stipe is continuous throughout (figs. 17 and 18). This formation is probably an adaptation of the plant to its environment. The plant, as before stated, grows in a horizontal position, and these pits are found usually on the upper surface, so that when the plant is left uncovered by the water, and the weight of its fronds are pulling the stipe downward, these pits are wide open, and afford the stipe a means of increasing its surface area, and in this way prevent it from being so easily snapped in two by the weight upon it. When the plant is covered with water it is again subjected to great strain from the constant dashing of the water upon it, and these pits give greater protective pliability to the stipe.
There are smaller linear markings on the lower expanded portion of the frond; these are more numerous, forming a very regular pattern.
The marks vary from about 3 mm. to 6–2 mm. in length, and on that portion of the frond nearest the stipe are arranged in broken concentric
Fig. 19.—Portion of stipe and lamina, showing “budding.”
Fig. 20.—Section through bud and stipe, to show structure.
Fig. 21.—Outline of specimen from Bounty Island.
rings (fig. 1). Farther from the stipe they are arranged somewhat diagonally across it, and finally disappear altogether where the frond
begins to be divided into finer segments. The markings are not deep enough to appear in a transverse section of the frond, but are plainly seen on surface view. They would serve the same purpose as the deeper pits of the stipe, to give greater pliability to this portion of the frond. In both cases they probably owe their origin to the continual mechanical stress on the tissue of the plant as it is growing.
A plant collected in September, the beginning of the spring, presented a curious appearance owing to the presence of numerous “buds ” occurring on all parts of stipe and lamina (fig. 19). This budding was not confined to the outer edge of the frond, but what appeared to be small plants in all stages of development were found scattered irregularly over its surface. The youngest plants appeared as small light-coloured round patches on the surface, interrupting the dark-brown colour of the frond; others were about 3 mm. in length, appearing as little conical protuberances; in others there was some differentiation into stipe and frond, the frond appearing as a flattened blade about the length of the stipe; in the largest the frond was about 7 mm. in length, and in one case it was branched. Wherever the stipe of these small plantlets was differentiated at all it showed the peculiar ring-like cracks in the tissue characteristic of the adult stipe of this species of the Durvillea. In the younger buds the meristematic tissue appeared to have arisen from the layers below the coloured tissue of the frond, and the buds had broken through these layers, which formed a ring of tissue round it. In the older buds this ring of tissue disappears, and the chlorophyllous tissue of the bud is continuous with that of the parent frond Under a high power of the microscope a section through one of these buds shows that the cells of the apex of the bud are irregular in shape, and rather small, like those of rapidly growing tissue (fig. 20), and they present an appearance quite different from the regular radial arrangements of the upper layers of the parent frond, resembling more in shape and appearance the cells of the very young plants described below; no special apical cell could be observed.
From this it would seem that the frond and stipe have an unlimited power of growth in any direction; any cell, apparently, of the outer tissue may become meristematic, take upon itself the functions of an apical cell, and form the starting-point for fresh growth. In these lower forms of plant-life, even though the plant may attain an enormous size, as in the case of D. antarctica, there is not much differentiation amongst the cells, and it is not unusual to find a cell becoming meristematic in this way. It is, however, the first time the peculiarity has been noticed in Durvillea. It may be that at this time of the year (September) fresh growth is incited amongst these lower forms, and finds an outlet in this way. It is possible that the cells of the holdfast may become meristematic and give rise to the young plantlets which have been observed attached to an old holdfast.
Bounty Island Specimen.
A specimen collected at Bounty Island by Dr. Cockayne presented some very striking points of contrast with the specimens from this coast.
(1.) The colour was a very much lighter yellow, and Dr. Cockayne says that the lighter colour is quite distinctive of the Durvillea, found on
Bounty Island. The island consists of a few rocks, almost destitute of vegetation, and this seaweed forms a fringe round it, exposed to most tremendous seas. The Durvillea seen on the other subantarctic islands, of New Zealand was the ordinary dark-brown D. antarctica.
(2.) The frond was peculiarly branched; instead of ending in blunt segments, which may measure as much as 4 cm. across in this plant, the segments were divided up into a number of very fine filaments, which gave a peculiar tasselated appearance to the plant (fig. 21).
On only one portion of the frond were there any indications of air-chambers; the central tissue was just beginning to break down. On this portion were conceptacles filled with branched hairs bearing antheridia. The structure of these conceptacles agreed quite well with that of those of the ordinary form.
The cracks on the stipe were much deeper and usually wider than in the more common form, while the markings on the frond were continued up the main segments and were lost only on the finer divisions.
According to Dr. Cockayne, Bounty Island is subjected to very violent storms, and one would naturally expect a form which managed to maintain an existence under these conditions to show some modifications in its structure. These are found in this case in the finer filamentous segments of the frond, and in the pits and markings, which enable the plant to withstand a great strain. This plant, then, marks an extreme D. antarctica tendency, and may be considered as a variety of that species, which has developed in a manner exactly opposite to D. Harveyi. Further specimens of this form should be obtained and compared with Areschoug's specimens from South America.
A peculiar case of the growth of the young plants was noticed in the month of September, where four young plants varying from 25 mm. to 50 mm. in length were found growing on a large holdfast which still bore the adult plant. The smallest of these plants was differentiated into stipe and lamina, the lamina being dichotomously branched at its apex. The stipe also showed clearly at the base one or two of the characteristic pits. The portion of the holdfast to which they were attached was different from the rest of the tissue. It appeared to form a layer about 3 mm. in thickness over a mass of harder and more compact tissue, evidently the original holdfast of the large plant. This layer was soft, and could easily be cut away from the harder tissue below. A ring of a slightly darker colour could be discerned, separating a portion of the holdfast round each plant, which fact would lend support to the theory that the holdfasts of the different plants, though originally separate, may in course of time coalesce. From all appearances it would seem as though in this case the ripe egg had fallen on to the old holdfast and germinated there, and so saved itself the trouble of forming an entirely new holdfast by making use of the old one, and possibly adding a fresh layer on its surface.
On cutting a section through the holdfast vertical to the surface at the base of this young plant, a cylinder of darker colour appeared to be sunk into and differentiated from the surrounding tissue, thus suggesting that the young plant might possibly have anchored itself on the old holdfast by sending down an organ resembling the haustoria of the parasitic phanerogams, but under the microscope the tissue appeared to be homogeneous throughout.
The study of the anatomy of Durvillea antarctica shows that there is not much differentiation of tissue in the three regions, the tissues of one passing gradually into those of the other. There is no special conducting tissue, as in Macrocystis; the meristematic tissue of the holdfast is perhaps worthy of mention. Oltmann considers this plant one of the lowest of the fucoids, and this is borne out by the results attained in the present study.
The peculiar pits on the stipe and the markings on the lamina are interesting in the relation they probably bear to the conditions under which the plant lives. In this connection also the study of the Bounty Island form should throw further light on the problem of the variation of the plant in response to external stimuli.
The study of large seaweeds such as this is interesting in connection with Church's theory of the origin of land-plants (1919). Here are found the three regions of the plant-body corresponding to the root, stem, and leaves of higher plants, which his theory demands; but, on the other hand, there is no trace of a “basal absorptive mechanism” nor sign of an upward conductive axial tissue which would be necessary if Durvillea were to lead on to anything higher in the scale of plant-forms.
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