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Isolated Ponds: A Botanical Study
by Edward F. Linton, M.A.
IT is an interesting question how ponds that have no water communication with streams or long existent streams of water acquired their variety of aquatic or waterside plants. The ponds I am writing about mostly exist for the sake of furnishing cattle with a water-supply, and are filled by surface water--i.e., by the rain that falls in its immediate neighbourhood. And yet such pools, even though made in recent times are seldom without aquatic or riparian vegetation; in fact, I may say never, unless overstocked with ducks or geese, or greatly disturbed by horses, cattle, or sheep. That the plant-life of ponds is destroyed or kept down in this way may be seen by comparing one that has been for some time railed in, or protected, as is often the case, on three sides, with another such as one often sees on a common or near the entrance to a farmyard, which is easy of access to all sorts of bibulous creatures on every side. The latter may often be found to have no sign of vegetable life in it or on its margin; mud and desolation reign supreme.
When pools of water have an outlet or overflow, which is usually the case where they are fed by a rivulet of a strong perennial spring, it is obvious that in the lapse of time water-loving plants may have arrived by the waterway. On this ground I limit my inquiry to isolated ponds, which have no water communication. I will also at once dismiss all Cryptograms--the liverwort, the moss, or the occasional fern--since we know that their multitudinous spores are carried freely in the air, to germinate wherever they alight on congenial soil.
Let us note the contents of such a pond as we are studying. Here is one that will do for a start in a retired field, far from farmyard or poultry-rearing cottager, and safe from the depredations of the omnivorous duck. See, the surface is uniformly green, except at the side where a gentle slope invites thirsting cows to drink. With a stick we draw out a number of small oval fronds, 1/8--1/4 in. in length, that floated so thickly strewn as to cover this surface; this is the Lesser Duckweed; these fronds with a rootlet form the whole plant; they very rarely flower, but young plants are freely reproduced laterally, sessile on the sides of the old, soon to become independent and have families of their own. It is almost the smallest British flowering plant and is easily carried about from pond to pond, adhering to the legs of waterfowl in the same way as it adhered to the stick we used just now. The smallest flowering plant belongs to this order, Wolffta arrlitza by name; it has no rootlet and is only 1/30 inch long!
In many a pond, if not quite smothered with duckweed, we may find one species or another of Pondweed (Potamogeton), a genus of submerged aquatics, which only raise their flowers above water during the process of fertilisation, and sink them again while they mature their fruit. You may sometimes see oval leaves floating on the water, 2-4 in. long, which are of a brownish hue while young, then pass through olive to green. These are the floating leaves of Potamogetan natans, the commonest species of this order in ponds, and on their lower side are often deposited the eggs of water-snails. Another species, of a ruddier hue (P, rufescens), which also has some floating leaves, I have found in a pond in Lancashire, and also in a small pool at the great elevation of nearly 3000 ft. in Perthshire, which was isolated except when flooded by rains. At this height the plant never flowered, but spread only by stolons or clongations of its creeping rootstock; the plant was dwarfed also in this exposed situation, averaging only 4 or 5 inches instead of 2-3 ft. P. pusillus, P. crispus, P. densus , three species which develop on floating leaves, but have all their leaves arranged alternately on the stem, have also been observed. The last bears some resemblance to the "American Weed" (Elodea canadensis), which has so often caused obstruction in canals and rivers in this country by its rapid spread. It has a peculiar mode of fertilisation, resembling that of Valisneria; the pistillate flowers rise by long slender flaccid stalks or tubes, till the stigma can rest on the surface and await the chance arrival of the fertilising pollen--an arrival for which it waits in vain in this country; for the plant is dioecious, and the pollen-bearing plant has not been met with in England. It really spreads, however, wherever water carries fragments of its brittle stem. Occasionally it has been observed in ponds where its presence is not easy to account for; but as its leaves area pellucidly thin, and cling, when taken from the water, to any object they touch. It may possibly have travelled adhering like the duckweed to the legs or feet of waders.
In a large modern pond in the grounds of a Suffolk rectory a Water--crowfoot and a Chara grow. The latter was C. fragilie, and this is perhaps the most frequent member of the whole order to be found in ponds. I have, however, observed Nitella opaca in an isolated moorland pit in Skye. The Water-crowfoot is perhaps the commonest plant occurring in ponds or pits; and its showy white flowers with their yellow centres are no little adornment to the sheets of water on which they float. But it must be remembered that this aggregate name embraces numerous varieties or closely allied species. Another species of this genus, Ranumculus Lenormandi, grows in very shallow water, or on the muddy margin. I have seen it in a moorland pit in Glamorgan. No doubt the commoner R. hederaceus would occur; R. Flammula (lesser Spearwort) is frequent and some of its forms spread by rooting at the nodes. Seedling plants of this buttercup have a tuft of narrow stalked leaves, which remind one of a very much rarer species belonging to an entirely different order, Limosella aquatica (Mudwort), a humble herb with inconspicuous flowers. The rare plant is sometimes overlooked, I daresay, from its resemblance to young plants of the much commoner species, and is so far protected by the resemblance and, if mimicry exists among plants, as it no doubt does among insects and their larvae, this may well be a case in point. I have met with the two growing together by a roadside pond in Derbyshire.
The large order of Compositae , or compound flowers, which have their corollas aggregated together within a general involucre, as the calyx enveloping the head is called, contributes to aquatics to the flora of ponds. There are, however, two or three members of this order which haunt the margins. For example, the two species of Bur--marigold (Bidens), so called, presumably, because, while their heads are something like a marigold, their seeds stick like burrs. Their seeds are of a narrow oblong shape, much compressed and four-angled, and from the upper end of the angles strong bristles project, two in one species, four in the other. These bristles are armed with recurved teeth, and consequently stick fast in any soft material they penetrate. I had an experience of these barbed seeds last September when the plants had matured their fruit. After walking round part of one of the smaller meres in the Mere district in Shropshire, when I was keeping near the edge in order to observe any plants growing in the water, I noticed in a little while that I was carrying about one or two hundred Bidens seed! Clusters of them were standing out at right angles to the cloth in which they had buried their barbs. This gave me a forcible illustration of the manner in which some seeds can travel. Those of Bur-marigold take hold of the soft fur of rabbits or the hairs of dogs, or perhaps, even of the feathers of waterfowl, and take their chance of being dropped sooner or later in a spot favourable to their germination. This mode of transportation is not very uncommon, but it is quite unusual with water plants.
The meres of Shropshire are for the most part isolated ponds on a large scale, their overflow being carried off by a subterranean passage, often of considerable length. One or two are connected with the Ellesmere Canal. Before these artificial outlets were made, some of the meres appear to have had no outlet at all. Round their borders several of the sedges had their habitation. The common Bulrush (Scirpus lacustris) abundantly fringed the shore in places, growing in water 1-3 ft. deep, and rising 3-5 ft. above the surface. The tiny Eleocharis arisularis, a plant closely allied to the last, creeps in the shallow water or in wet turf here and there. E. palustris of course abounds. The handsomest plant (British) in this order, Cladium Mariscus, is to be found there, its rigid branching panicle with numerous clustered spikelets admirably adapted for decorative purposes. But take care how you handle the leaves? Their edge is like a fine saw, and quite as capable of giving a deep cut to the unwary hand that is drawn along it.
Another handsome plant abounded by some of the meres, the Reed-mace (Typha latifolia); it seemed to threaten the very existence of Sweatmere, a small sheet of water adjoining Cosemere its tangled roots and erect stems, here and there crowned with a rich brown cylinder of fruiting flowers, floated out on the deep water and mud of the mere, far beyond the farthest point to which I dare venture on the quaking mass--I cannot call it ground, where the substance is nearer akin to fluid than solid. This is, however, the way in which a good deal of ground has been formed, by the annual decay of the vegetation and the progressive extension of aquatic growth closing in by degrees by the lessening sheet of clear water that remains. Many a "moss" in Lancashire and Cheshire is the outcome of an ancient mere, that in course of ages has been choked with aquatics, and even submitted to be drained and tilled!
Some seeds are provided with feathery appendages, or a tuft of silky hairs, by aid of which they can float long distances before the breeze. In this way the seeds of both willows and willow-herbs are carried to the moist banks of ponds, and the presence of these plants here is easily accounted for. But aquatic plants generally have no such contrivance for locomotion. Their seeds are usually smooth, and many are of fair size. The Bur-weed, (Sparganium) has smooth pear--shaped fruits with a conical top, which no wind could carry. Yet I have seen Sparganium simplex in a pond on the high level of a moorland common a few miles from Swansea; and S. ranosum, with its still heavier fruits, has been observed in ponds far away from waterways not unfrequently. How did the original seeds reach such stations?
There can be little doubt that waterfowl have usually been the transporting agents. Obtaining their food from rivers, lakes, and ponds, they seldom alight far from some acquired haunt. Their feet, commonly more or less webbed, carry away some of the soft mud as they rise from the water's edge, and with its aquatic seeds, which abound in such mud, will ether adhere to these unconscious porters and be dispersed wherever next the birds take to their natural element. In this way the roundish seeds of Pondweed, or the pear-shaped fruit of the Bar-reed, or the smaller seeds of Callitriche or Zannichellia may travel long distances through the air, and the mud which conceals them will be more likely to be washed off in water than to drop off in any other way.
It is possible that some of the harder seeds may accompany birds in their flight as inside passengers, and so take the chance of being deposited in their watery haunts. We know that the seeds of holly and yew, of hawthorn and roses, are distributed by this means to the promotion of germination in the case of these woody seeds. Yet it does not follow from this that any seeds of aquatic plants can endure the heat of the bird's body and preserve their vitality. None of them are woody: and some proof may well be demanded before accepting a hypothesis against which so strong an objection lies. But there is nothing irrational in the theory that water-birds convey seeds of aquatic in the mud that adheres to their feet, when they take flight as they often do, from a soft muddy margin.
The last aquatic mentioned, Zannichellia, is a genus that grows entirely submerged, of the same order as the Pondweed. I have seen it in a brickyard pool of modern origin, and it has been seen in a similar situation, as well as in a pond on the Sussex downs, by another observer. The same naturalist mentions, having found Hornwort in a pond on the downs, but only once. Water Starwort (Callitriche) is not an unfrequent occupant of still water, some three varieties having come under observation.
There are some plants which one might expect to find in these situations, which, I believe, are usually absent. Watercress, Arrowhead, ad Flowering-rash seems to require running water. Helosciadium nodiflorum, related to celery, has been found half--starved (var. ochreatum) by stagnant water; but its close ally, H. inundatum, a submerged Umbellifer with finely divided leaves, I have met with flowering and fruiting feely in a heathland pond. The common horsetail (Huippuris vulgaris) is reported from ponds in Hampshire, e.g.--but I have no evidence of its occurrence in isolated ponds. All three species of Water Milfoil have been noticed in such places; one of them (M. alternifolium) I detected last year in a dew--pond near the top of a high ridge near Swanage. These grow entirely submerged; but, like the Pondweed, exsert their spike of flowers for fertilisation.
There is an interesting species of St. John's Wort (Hypericum chdes) which looks very different from the rest of the genus, as we know it in Britain; the upper half of the plant is emerged during its flowering and fruiting season, and the masses of roundish leaves, grey with thick pubescence, and graceful few-flowered panicles, readily catch the eye. I have seen this twice at least as a denizen of a pond, on one occasion with a quantity of Alisma ranunculoides, a Water Plantain with flowers that remind one of a white buttercup. The common Water Plantain is not unfrequent as an occupant of ponds and brickyard pits, but usually absent from chalky ponds.
Very little reference has been made to sedges, and none to grasses and rushes, for the reason that so many of them are inhabitants of damp ground rather than water plants; and when found flourishing by a pondside, it may be that they occupied the spot before the pond was dug. There are, however, two or three species of Glyceria which are confined to watery places, and are not unfrequently at the edge of isolated ponds--as, e.g., in S. Derbyshire, where G. pedicullata is usually present. A rare fox-tail grass (Alopecurus fulvus) is in my experience a thorough aquatic; I have seen it in a pond and by a mere, both in Norfolk and entirely isolated, coming into flower only when the water had subsided to a low level with summer drought, but submerged the greater part of the year.
Litttorella lacustris, a diminutive member of the Plantain order, is of similar character, often clothing the shallow bottom of lake or pond with its tufts of linear leaves, but I believe only flowering when the subsidence of the water exposes it to the air. There are, however, a few plants which actually flower and are fertilised under water, and, what is more, produce good seed. I have already mentioned Zannichellia, and Callitrich. The two Elatines grow in water from a few inches to 1 1/2 ft deep; and I have found both of them fruiting in water that was permanently over a foot in depth. This was in one of the three large Cutmil Ponds which are entirely isolated. There is, too, a little Crucifier, Subualria aquatica (Alwort), which flowers and fruits (1-3 ft.) below the surface; and also the rare little Naias flexilis; but these I believe are only found in lakes or pools which are drained or fed by watercourses.
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