Note on the Brown Alga, Ecklonia brevipes J. Ag.
[Read before Hawke's Bay Branch, August 15, 1945; received by the Editor, August 17, 1945; issued separately, December, 1945.]
J. Agardh (1877) Alg. N.Z. Mar., p. 5, in describing Ecklonia brevipes, did so from a fragment which did not exhibit its true characteristics. Dr. E. Marion Delf (1937) Proc. Linn. Soc. Land., p. 112, using material supplied by the writer, briefly referred to the plant's proliferating habit in connection with regeneration of certain brown algae. Specimens were distributed in Landauer, Algae Nova-Zclandicae Exsiccatae, Fasc. 1, No. 7. It is here proposed to discuss the plant in fuller detail.
Ecklonia brevipes J. Ag. Plate 34; Figs. 1–5. Plate 35; Figs. 6–8. Frond olive-green, membranous, up to 1 metre high, arising from a wide-spreading holdfast of long, dichotomously branched hapteres; stipe very short, soon flattening and widening cuneately into a blade, either narrow or broad; branching pinnate at first, later irregular and flabellate; marginal teeth numerous, developing here and there into adventitious hapteres; sori in patches on blade and lobes.
The plant, being extremely variable in habit, has probably been confused in the past with E. richardiana and E. flabelliformis; for it may be either narrow-bladed with regular, simple, strap-like pinnae, or conspicuously broad-bladed with compound, overlapping segments. Its special characteristics, however, make the species easily recognizable. They are as follows: (1) an unusually short stipe; (2) erratic development of adventitious hapteres from the marginal teeth in such a way as to be adapted to the vegetative reproduction of the individual; (3) extraordinary development of the pinnae themselves; (4) exceptional growth of the branched holdfast; (5) a thinner and softer texture than usual.
The writer has seen no juvenile plants which could be definitely identified as maturing sporelings; on the other hand small individuals, measuring a centimetre or two in height, are frequently found entangled in the holdfast of the mother plant; these possess true Laminarioid fronds with distinctly serrated margins, but have the appearance of having been severed from the parent plant.
In E. brevipes the development of the basal pinnae is often abnormal and excessive and, should these be destroyed, the upper pinnae produce a bewildering and erratic series of proliferating marginal laminae at all angles. It is obvious that every serration in the margin of the frond has the latent possibility of developing into a complex pinna; in fact the dominant urge of the plant appears to be to lengthen the pinnae rather than the blade and to use every
possible serration in throwing out proliferations which often resemble E. richardiana-like plants attached to the margins at irregular angles.
Likewise, it is possible for every dormant serration, whether at the tip of a pinna or not, to produce itself into a hapteron which, owing to its greater density, will drag down the pinna, finally anchoring it to the substratum. At this point, either by the disintegration of the blade (which process gradually extends downwards), or to the stress of the surge during storms, the tender frond which, in this species, is the most delicate of any New Zealand Ecklonia, is severed at the spot where the pinna joins the blade, thus leaving a new plant attached to the substratum. During storms, too, large numbers of lacerated lobes, freed from the parent plant, drift in the lower strata of the water, dragging along the sea-bed. These, as has been proved by observation, always drift with the hapteres downwards, and it is certainly probable that many of them anchor themselves to the rough, Lithothamnion-covered or sandy bottom of their particular habitat.
One peculiarity of the plant is that the erosion of the tip of the blade is often imperfect and, where the blade is excessively broad and oval-shaped, it splits from apex to base, so that frequently large portions of the upper blade, aged but still intact, are present on the newly severed young plant.
As mentioned above, adventitious hapteres may be developed from any dormant serration, and it often occurs that the margins of the frond are adorned with dangling hapteres in great numbers. Their development is in two stages: (1) the formation of a grapnel—or hook—or anchor-like organ; (2) the growth, above this, of dichotomous root-branches. A hapteron at first appears as a small papilla, being a transformed marginal tooth, which increases in length to become a cylindrical prolongation about 1–1.5 mm. wide and up to 5 cm. or more in length. It is probable that a new meristematic region has already been developed at the point at which the papilla has arisen and that all growth in that area is intercalary. At this stage one or two offshoots may be sent out at right angles to the prolongation, near its tip; opposite to each other should two be formed; and these mark off the length of the presumptive stipe; for all below will eventually become holdfast, the length of the future stipe depending primarily upon whether the offshoots are relatively close to, or distant from, the point of origin of the papilla. In some cases offshoots appear after the stipe has grown a few millimetres only, in others no offshoot whatever is developed. In any case very little, if any, further growth of the stipe in length will take place, for it seems that that portion of the meristem at the new transition-place which normally adds to the length of the stipe, ceases to function perhaps because of the urgency in the formation of the allimportant holdfast.
The stipe, having been marked off by the off-shoots, the primary hapteron-axis continues its growth as an extension of the stipe, becoming
strongly bent and hook-like. It is at this stage that the organ, by its greater specific gravity, forces the pinna to the substratum (particularly when there are many hapteres dangling from the neighbouring areas) and attaches itself. Frequently the hook, having served its purpose, ceases to grow; at other times it divides indefinitely and takes part in the rooting-system.
The second stage is now reached—namely, the formation of the holdfast proper. This is developed from the offshoots already mentioned, which divide dichotomously, growing greatly in length so as sometimes to exceed the lateral spread of the pinnae. It seems that the arresting of the growth of the stipe is compensated for by the excessive growth of the root-branches which, owing to the loose nature of the substratum, is rendered necessary.
It is noticeable that the holdfasts of these plants, washed up in exceptionally rough weather, are never attached to large shells or stones, so characteristic of the holdfasts of other New Zealand Ecklonia. Indeed, the plant may readily be distinguished by the holdfast alone. Occasionally the hapteres are found entwined around small nodules of Lithothamnion, but they are usually free from all debris, a point which suggests further that the majority of the plants grow among loose sand. Another proof of this is the fact that no matter how severe the storm, specimens are never found broken off at the trasition-place or severed just above the holdfast, a common occurrence with plants growing on rock from which they have been torn by the force of the waves.
Perhaps the most remarkable circumstance connected with the vegetative reproduction of the species under discussion is the fact that the pinnae which, up to a certain point, have increased in length by growing outwards towards the apex, finally (in the initial stages of the growth of the adventitious hapteres) develop new meristematic tissue at the apex, from which all further new growth proceeds, and the apex of the pinna becomes the base of what is to become a separate plant even before separation takes place, continuing growth in its now inverted position as if it were actually attached.
In this connection, the remarks of the late Professor W. A. Setehell (private correspondence) who substantiated this theory, may prove interesting: “The matter of reproducing by these proliferations, while amazing in itself, is accompanied by a physiologically structural behaviour which seems almost without parallel, and that is in the absolute reversal of polarity. Your “Double-headed-monster” shows this. Every lobe which is proliferating shows it. The apex of a lobe becomes the base of a new plant. The reaction to the direction of the action of the force of gravity seems to reverse itself. New meristematic tissues arise, and what is “distal” becomes “proximal”; what is apex becomes base; where the growth is proceeding away from the original point of attachment produces new hapteres, and the direction of growth is so influenced by these as to proceed in entirely the opposite direction. I know of no plant which has come in to me which seems to upset and overturn our
Ecklonia brevipes J. Ag.
Fig. 1.—Showing development of a hapteron from tip of pinna.
Fig. 2.—A juvenile plant. Natural size.
Fig. 3.—Sketch, somewhat diagrammatic and simplified, to illustrate method of vegetative reproduction.
Fig. 4.—A severed, growing pinna. × ½.
Fig. 5.—A severed, growing pinna, showing sori of new plant and part of sorus and blade of the mother-plant. × ½.
Fig. 6.—Ecklonia brevipes J. Ag Photograph of a plant. × 1–6.
Fig. 7.—Ecklonia, br [ unclear: ] vipes J. Ag. Photography of a plant [ unclear: ] ooted at both ex-tremities. × 1–6.
Fig. 8.—Ecklonia breripes J. Ag. Photograph of three pinnae severed from mother-plant, growing independently, but as yet not separated from each other. × 1–6.
general ideas so completely. If if were only the basal segments which were concerned it might not seem so strange, but the very apical segments seem to behave in the same way.”
The development of the adventitious hapteres normally takes place on the margins of the frond. It seems that, should the margin be torn away, as is often the case, or, equally frequently, the lobe be split. longitudinally from the upper end, no regeneration of the torn edges takes place, and, therefore, proliferations are absent, for it appears that they develop normally from marginal tissue. Should, however, one surface of the frond become ruptured so that the opposite surface is raised in the form of a ridge, that ridge will continue to elevate itself and eventually form a proliferation even to the extent of becoming a new pinna, bearing hapteres.
Fertile specimens of E. brevipes are somewhat rare. The sori are formed on both blade and lobes generally in large, longitudinal, medial patches either at the upper end of the blade or, when on the pinnae, somewhat close to the point of insertion. Sori are also sometimes present close to the tips of hapteres-bearing pinnae immediately above the newly-forming holdfast. Less frequently are mature sori found lower on the blade in full grown plants.
E. brevipes has been definitely recorded only from Long Beach. Russell, Bay of Islands, New Zealand.