Art. XXX.—Notice of the Establishment of Vallisneria spiralisin Lake Takapuna, together with some Remarks on its Life-history.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 29, 1896, Page 386

Art. XXX.—Notice of the Establishment of Vallisneria spiralisin Lake Takapuna, together with some Remarks on its Life-history. By T. F. Cheeseman [Read before the Auckland Institute, 3rd August, 1896.] Nearly three years ago my friend Major Mair showed me an aquatic plant which he had gathered in Lake Takapuna, and which he rightly concluded to be a stranger to our flora. When first seen it formed a dense mass of no great size in comparatively deep water in front of his boathouse, and several similar patches were noticed not far away, all being on the southern side of the lake. None of the residents near the lake knew the plant, and none of them could give him any information as to the mode of its introduction. I had little difficulty in identifying it as the well-kown Vallisneria spiralis, a common species in still water in most tropical and many temperate countries, and found plentifully as near to us as Australia and Tasmania. During the following summer additional specimens were brought to me by residents at the lake, all of whom agreed in stating that it was spreading rapidly on the southern shore of the lake, covering large areas in water varying from 6ft. to 15ft. in depth. During a short visit to the lake I saw the plant, and succeeded in obtaining flowering specimens. All of these were males; nor have any females been found up to the present time. In the winter of 1895, when the water of the lake stood at a high level, I heard but little of it; but towards the end of the spring, and during the past summer and autumn, the case was altogether different, and I was assured that it was increasing with extraordinary rapidity, and that it had already crossed the lake and had established fresh colonies in many places along the shores. In the original locality it had formed submerged masses of dense vegetation occupying areas of considerable size, in one or two cases blocking up the foreshore to such an extent as to compel the owners of boatsheds to clear passages in order to gain access to the lake. As the level of the water fell during the summer months the tips of the submerged leaves were laid bare, and where they were very numerous formed a compacted floating mass over which ducks and other aquatic birds walked as freely as on land. Every gale washed up numerous plants from shallow water, and these were frequently drifted to other portions of the lake, where they became rooted in the mud, thus forming fresh centres of distribution.

Although I had no doubt that Vallisneria had been purposely introduced, it was some time before evidence of the fact was obtained. I was at length informed by Mr. E. Bartley that in 1885 he received some living plants from Dr. Ralph, of Melbourne. These he cultivated for some time in a small aquarium in his garden, but eventually planted one of them, a male, in a secluded nook on the south side of the lake. A year or two afterwards he revisited the locality, and failing to find the specimen, concluded that it had died. It is from this single plant, however, that the lake has been stocked. As only one sex was planted, propagation by seed has been out of the question, and the whole of the plants now growing in the lake have been derived from offsets accidentally detached and drifted to fresh stations. Vallisneria has increased so rapidly during the last two years, and has shown such aggressiveness and facility of distribution, that it will probably increase still further. Its spread in the past has naturally alarmed people not fully acquainted with the conditions of its growth, and surmises have been made to the effect that it will gradually spread into deeper water until a considerable portion of the lake is occupied with it. The water-supply of the Borough of Devonport is drawn from Lake Takapuna, and the Mayor and Councillors of the borough have several times had under their consideration the probable effects of the spread of Vallisneria upon the purity of the water. I am of opinion, however, that there is little cause for alarm. From what I can learn of the growth of Vallisneria in other countries, it appears that it usually lives in water from 6ft. to 12ft. or 15ft. in depth, rarely or never flourishing in deeper water. It is precisely in waters of these depths that it has become established in Lake Takapuna, and beyond those limits it probably will not advance. Fortunately, Takapuna is a deep lake, a great portion of it varying from 12 to 25 fathoms in depth. In many places the shallow water fringing the shore is a mere strip, and with the exception of a bay at the northern end it as nowhere very wide. Consequently there is no risk of Vallisneria doing more harm than will result from the formation of a narrow fringe around the shores of the lake, interfering to some extent with bathing and boating. The question at once suggests itself whether Vallisneria, having effected a lodgment in this country, will spread into other localities. The shallow lakes and sluggish streams of the middle Waikato basin are places where it might cause serious trouble by choking channels of drainage or even navigable streams. We have before us the instance of the American Elodea (Anacharis) canadensis, a plant closely allied to Vallisneria, which made its appearance in Britain about the

middle of this century, and in a few years spread over a great portion of the country. It caused an immense amount of harm by impeding the navigation of canals, interfering with the working of water-mills, and by choking the outlets of drainage-works and reservoirs, but of late years has shown signs of decrease. But Elodea is exceptionally well adapted for spreading into distant localities. Its slender branched stems are easily broken into fragments, any one of which is capable of reproducing the plant. In a country like Britain, with its network of inland navigation, it is easy to see how the passage of barges along canals might be the means of carrying the plant from place to place, to say nothing of the transporting of the fragments by currents or by aquatic birds. But Vallisneria, though capable of rapid increase in any locality where it becomes firmly established, is not so easily carried, to a distance. Its stems are much stouter and heavier, and portions large enough to form fresh plants could not be carried by birds. Consequently it is hardly to be expected that it will spread by natural means from Lake Takapuna, which has no connection with other lakes or rivers. The object of this paper was simply to place upon record the facts connected with the naturalisation of Vallisneria in the colony, and, having done this, perhaps I ought to conclude. But it has occurred to me that there are some questions connected with the life-history of the plant which, although well known to botanists, may not be equally familiar to others, and which are certainly both curious and interesting, and well worth the attention of any one who takes an interest in natural science. I allude to the remarkable manner in which the flowers are fertilised. With your permission, therefore, I will give you a brief account of the structure of the flowers and the mode of their fertilisation. In the first place, for the sake of clearness, it is perhaps necessary to mention a few facts which are probably known to all who read this paper. In all flowering-plants, before perfect seed can be produced, it is requisite that the pollen, or male element, should be transferred to the pistil, or female portion of the flower. In the vast majority of flowering-plants this is effected in two ways, and in two ways only. These are, first, by the agency of wind, which simply blows the pollen from flower to flower. Our pine-trees, oaks, willows, most grasses and sedges are familiar examples of wind-fertilised plants. Secondly, by the aid of insects, birds, or other small animals, who, unconsciously to themselves, carry the pollen from flower to flower. Almost all plants having brightly-coloured or highly-perfumed flowers belong to this class. But in addition to these two main divisions some few plants exist—not numbering more than three or four score—

whose flowers are fertilised in quite a different manner, and of these Vallisneria is a typical example. In their case the pollen, instead of being blown by the wind or carried by insects, is conveyed to the female flower by means of rafts floating on the surface of the water. Let us now see how this takes place. The stems of Vallisneria are short, and almost concealed by the mud amongst which they grow; but here and there they give off offsets or stolons. These are easily detached from the parent plant, and readily put down roots into the mud, thus starting life on their own account. It is by their means that the rapid increase of the species is insured when seeds are not produced, as is the case at Lake Takapuna. The leaves are in tufts at the top of the stem, and vary in length according to the depth of the water, sometimes being quite short, at other times reaching a length of 12ft., or even 15ft. They are always narrow for their length, resembling long ribbons, and float freely in the water with their tips just below the surface. Like many water-plants, they are so delicate in texture that if the water is removed they are unable to support their own weight, and sink limp and withered on the mud. The flowers are unisexual, and the two sexes are borne on different plants. The male flowers are small and very numerous, and are closely packed within a delicate bladder-like spathe. Each little flower consists of three perianth leaflets, enclosing two or three stamens. The stalk which supports the flowers is short, seldom reaching more than 6in. above the surface of the mud, so that the flowers come to maturity far below the surface of the water. The female flower is single, and is at first enclosed within a long tubular spathe. It has three broad perianth leaflets, within which are three large fringed and lobed stigmas. The stalk of the female flowers lengthens in an extraordinary degree as the flower approaches maturity, and has been known to grow at the rate of more than a foot in twenty-four hours. When it reaches the surface of the water the tubular spathe splits, and the flower emerges, just floating above the surface. The stigmas protrude between the perianth leaflets, and the flower is ready for fertilisation. It will naturally be asked how this can take place, seeing that the male flowers are matured close to the bottom of the water, often several feet below the level of the female flowers. But Nature is always full of resources, and the plan which is followed is as remarkable as any known in the whole range of the vegetable world. When the male flowers are perfectly mature the globular spathe which surrounds them splits into three pieces, which hang downwards out of the way.

The little flowers, still in the bud stage, then detach themselves one by one from the stalk, and, from their natural buoyancy, rise to the surface of the water. Having arrived there, they very shortly open, the three perianth leaflets bending backwards and downwards, thus forming a little raft, above which is borne the projecting stamens with their load of pollen. Although the pollen is so close to the water it is very seldom wetted, for the little raft responds to every movement, and rides over the waves quite as safely as many larger vessels. The male flowers are produced in such great profusion that the surface of the water often becomes covered with the tiny rafts and their cargoes. They are drifted about by currents or by wind, and sooner or later some of them are certain to be washed against the female flowers, with their protruding stigmas. If so, the pollen, which is exceedingly viscid, is certain to be smeared over the stigmas, and fertilisation consequently effected. Immediately after this takes place the long flower-stalk coils up in a spiral manner, thus drawing the flower under water. The coils gradually become more numerous and more closely packed until at length the flower (or, rather, young seed-vessel) is brought down almost to the muddy bottom of the water. Here the seed-vessel remains until it is ripe, when it opens and discharges the seeds into the water. The peculiar mode of fertilisation which I have just described is confined to a few species closely allied to Vallisneria. The great majority of water-plants produce flowers raised well above the water, whose pollen is carried either by the wind or by insects, as is the case with most plants.