VIGNETTES FROM NATURE.

Otago Witness, Issue 3335, 13 February 1918, Page 55

VIGNETTES FROM NATURE.

By G. M. Thomson, F.L.S. XXI.—A WAYSIDE POOL IN TARANAKI. It is only a very commonplace little pool by the wayside, not 100 miles from the famous mountain. A cross-country road had been' thrown across a gully near its head, and had dammed back the surface waters, leaving only a small culvert for the overflow, and Nature had done the rest—just a common pool, but one which could furnish interest and study to the naturalist for a lifetime. Even the conspicuous things were in abundance, and these alone can claim our attention as we spend an hour on . its margins; the- man .with a microscope could never exhaust its beauty and wonder. All the country round this neighbourhood is superficially soft, and is easily cut into by running water, for we are away from the hard lava-flows which have poured out from Mount Egmont in the days of its activity. Only the scoria and the tufa remain, lying on the top of the papa rocks which form the mass of this country. Once a stream-bed, however small, has been started on the surface, it cuts down its.channel deeply and with great rapidity: So, as we stand on one of the slight rises of this rich country and look around us we seem to be on a nearly levej. or slightly-sloping plain. But in travelling across it the surface is found to be sharply cut into deep valleys with rather steep sides. This is true both for big and little erosions. So in making these cross-country roads, when streams or gulfies have to be crossed, the track sidles more or less gently down one side of the ravine, which is usually spanned at the bottom by a bridge placed very much at right angles to the descent (very much to the dislike of the numerous motorists of this butter-fat land), and then climbs obliquely up the other side. But when only the head of a ravine is crossed a slight cutting is made on each side, a culvert is built near the bottom, and the material from the cutting is thrown across the embankment. That' is how our little wayside pool has been formed. The original culvert seems to have- been allowed to choke up, and a- pond has resulted, from Avhich a pipe outlet has been provided in more recent times. One chief point of interest in this little sheet of water lies in the way that it has become populated with vegetable and animal life l . In its limited way it constitutes a microcosm. Most of the forms of life which now tenant it and clothe its surroundings must have been brought from outside since its formation either by Avind, by birds, or by other animals. There is only one plant in it which is out of the common, and that is an Australian water-weed (Ottelia ovalifolia), of which more -anon. The gully' has been filled up bv the debris washed down by the rains of the last score of years, and the water is probably quite shallow, though the mud is deep. On the sloping edges is the usual admixture of native and introduced plants (mostly of the latter), which thrive in ground where only cattle, or occasionally horses, graze. Where sheep can get at the surface the vegetation quickly changes its character to some extent. Beginning on the dry ground with rcasfcure grasses and weeds." it passes into the damp region, where gunnera thrives on the clay faces and rushes grow in chimps, and so down into the mud, covered in parts with.sheets

of the bright green Callitriche. Why are so many water-plants degenerated in structure? I assume it to be degeneration where the plants have lost their attractive whorls, but belong to groups or orders of flowering plants, which are usually furnished with conspicuous organs. One assumes, of course, that the typical flowering plant had such organs, and that their descendants, in acquiring an aquatic habit, had retrograded in structure. _ To take a familiar example, a lily has six brightlycoloured perianth leaves, with usually six stamens and three carpels, and -the thousands of species which belong to the great order of tho Liliaceaa retain this structure. Now, all the true rushes (Juncus and its allies, numbering some 250 species) have nearly the same structure —six perfectly regular perianth leaves, six stamens, and three carpels,—though the latter are often suppressed, and only one appears to be present. There is very little doubt that rushes are descended from the same group as the lilies, but they have mostly adopted an aqueous or sub-aqueous habit, and with it (but whether it is a case of cause or effect, I do not know) a retrograded flower. Rushes have regular and beautiful little flowers, but they quite lack the size, colour, and beauty which, characterise the true lilies. Wood-rushes (Luzula)' and true rushes (Juncus) are abundant in the ground which drains into our. pond, and the latter invade its muddy margins. I think most rushe3 bear self-fertilised flowers; but I have come across masses of the common toad-rush (Juncus bufonius) covered with small, pale greenish-yellow flowers, which opened brightly to the noonday sun. They were quite pretty to look at, and were apparently being visited bv small flies.

The most conspicuous plant which occupies the shallower waters of the pond is the common and übiquitous bulrush, or raupo (Typha angustifolia). At first sight, nor, indeed, after some examination, one does not realise the posibilities of relationship between bulrushes and lilies, but there is little doubt that the-two are very remotely derived from a common ancestor. The long brown head which crowns the tall, solid raupo stem is composed of an immense number of minute flowers crowded together. These flowers have nearly quite lost every, traco of a perianth. The female or 'pistillate flowers, of which the greater portion of the spike is composed, consist each of an ovary mounted on a relatively long stalk, and on this stalk are numerous silky haire, which are all that represent the perianth. The ovary is one-celled, and contains one pendulous ovule. Altogether this is an extremely different flower in all respects from that of a lily. The male or staminate flowers are usually crowded at the upper end of the spike, forming the upper 3in or 4in of it, and often separated by a short space from the female flowers below. Each male flower consists of a few stamens mixed with thin, hair-like scales. It would take us too far to enter on a discussion as to whether the whole group of monocotyledons, to which lilies and bulrushes alike belong, were derived from one trimerous type —that is, Jiaving its parts arranged regularly in threes, or whether some of them have evolved from a much simpler form, and have never in the history of their family development possessed ancestral forms with trimerous flowers. The most of the evidence favours the first theory. On the other hand, the übiquitous distribution of the simpler forms seems to point in the opposite direction. Many of the water weeds are very widely distributed. Our bulrush is a cosmopolitan plant. The forms common to the whole Australasian region differ in a few details from those found in other regions of the world; but all belong to the one species (Typha angustifolia). In the wealth of our natural resources in New Zealand we do not use the plant nowadays, but it possesses great interest and possibilities. The Maoris made extensive use of these. The pollen, made into cakes -with water, was baked and used as food, while in times of scarcity the starchy rhizomes were similarly employed. It is somewhat remarkable that in times of intense food scarcity, such as prevails in so many countries of Europe at the present time, more of the- natural vegetable resources of the land ha,ve not been realised. Bulrushes are very common in many parts of Europe, and it might be quite possible to make excellent food material both from the pollen and the starch. This is one of the numerous lines into which scientific research might well be directed. In building their houses and especially in decorating them the stems and leaves of the raupo were freely used by the Maoris, and in imitating them we might come back to primaeval simplicity without any fear of retrogression into primitive savagery. - I referred earlier to the Australian water-weed (Ottelia ovalrfolia) as a denizen of this pond. The plant belongs to an order not represented in the native flora (the Hydrocharideffi), the best-known number of which, the Canadian water-thyme (Elodea canadensis) has invaded so many of our ponds and slow-running streams, like the Avon in Christchurch, and has nearly choked them. Ottelia first appeared in the Auckland district, but I havo no information as to how it_ got into this Taranaki pond. Its first introduction to this country might well bo explained by the bringing of water-lilies and other aquatics from Australia to Auckland._ Its later spread is most probably due to birds. Wild ducks and other water birds fly long distances from lake to lake. Darwin, in tho " Origin of Species,'' gives some interesting information about the manner in which birds carry seeds either on their legs or feathers, sometimes attached to their feet by mud. In some such way, probably, the seeds of this plant have been carried from tho Auckland district into Taranaki. In that caso_ it will be found in the intermediate region and will quickly spread further throughout New Zealand. I have, however, no evidence on this point.

In some of the shallow puddles round the pool and spreading out on the surface of tho water are sheets of duckweed (Lemna minor), forming a beautiful green mantle over the muddy parts. This is tho smallest flowering plant in New Zealand, perhaps in the world, for the whole organism is seldom more than a fourth of an inch long, and commonly only about half that size. Each plant consists of a scale-liko green frond, which is biconvex or lens-shaped, and nearly oval in outline, with one end rounded and the other pointed. From each frond a thread-like root hangs down into the water. This ends in a root cap, which is a familiar object of study in every botanical laboratory. " We call tho green discs of duckweed fronds and not leaves, because they bear roots and flowers. Functionally, they are at once leaves and stems. During tho summer they bud continually. A pair of minute rudiments appears on tho upper surface of a pond, while it is still very small and concealed within the parent frond. Each of these small rudiments becomes enclosed in a special sheath formed by an overgrowth of the frond upon which it is borne. The new fronds are invariably ■ paired at' first, but one ''generally outstrips the other, and often only one comes to maturity. The pointed end of every frond marks the place it was attached to the parent. , Four or five successive generations may be found still fastened together, all of which are destined to break away sooner or later. "When a frond is studied microscopically it is found to be built up of small green cells. There is a faint midrib, not easily made out, however, ■ and a pair of lateral veins. A considerable part of the interior is occupied by air-spaces, which are large in proportion to the cells, and arranged in one, two, or three layers according to the depth of the frond. It is these airspaces which give to the frond its remarkable buoyancy. The upper surface repels water strongly when the plant is in good health; the lower surface is always wet."

I have never seen duckweed in flower, but Professor Miall, in his . Nature-study work, "Round the Year," gives an interesting account of his seeing them in Yorkshire. He says: "To-day I was walking to Barden, when I stopped to hunt for aquatic insects among the duckweed. I saw a peculiar yellow light reflected from the ■ floating duckweed, and on looking closely perceived that aimost every frond was in flower. The yellow light was reflected from the anthers, which stood out from clefts in the edges of the fronds. A few years ago I had" never seen duckweed in flower, and supposed that it seldom or never flowered in England. A botanical friend took me to see it in flower, and since that time I have discovered how common the flowers are, and how easily they mav be seen by an attentive observer." Well, I have still to look forward to the pleasure of seeing them, so the following account is again quoted from Miall: " The flowers spring from clefts in the margins of the fronds, and are enclosed by minute scales or bracts, the outer forming a sheath, which is burst at the time of flowering. On one or both sides of the flowering frond appears a group enclosed by bracts, and consisting of a pistil and two stamels. The pistil is flaskshaped, and sivrmounted by a hollow style open at the top. Each stamen bears two separate anther-lobes, slightly divided into two cells. The seeds ripen in -autumn. They are minute (less than 1 mm. or a twenty-fifth of an inch long), oval and ribbed along their length. They float in water, and germinate at the surface in the course of the following spring." Are these miniature plants evolved unwards from a more primitive type, or retrograded from forms with trimerous symmetry? Probably the latter. But"in that case they have travelled very far to lose all leaf-structure, floral envelopes, and even their svmmetrv of .number.

I must return to the duckweed and other denizens of the pond in another paper.