New Zealand Plants and their Story
Chapter VII. — Plants of Fresh Water, Swamps, and Bogs
Plants of Fresh Water, Swamps, and Bogs.
Scarcity of aquatic plants in New Zealand—Water-ferns—The red Azolla—The pond-weeds — The water-milfoils—Fresh-water algae—Vegetation of hot springs—Effect of plants on changing the land-surface—Swamp vegetation —The niggerhead—Economic importance of swamps—New Zealand hemp —Bogs and; bog-plants—Sphagnum and its peculiarities—Flesh-eating plants —A vegetable trap.
It has already been shown that in her forests, meadows, shores, and shrubberies New Zealand possesses plants which do not yield in beauty or interest to those of any other land. With her seaweeds, too, she is well able to hold her own. But when it comes to the fresh-water plants she must take a lower place. Rivers and lakes there are in plenty which offer first-class inducements for occupation by aquatic plants, but none of the more beautiful kinds have accepted the offer; in vain we look for water-lilies like those of the sister continent. Still, for all that, our waters are not without plant-life, some of which, from the biological standpoint, is interesting enough.
Take the case of the floating water-fern, Azolla rubra. The red masses of this curious plant, covering still pools so thickly that one might think them dry land, must be known to all. The outer surface of the leaves is covered with minute excrescences, so that they cannot be wetted, and, in consequence, drops of water frequently begem them, glittering in the sunlight like diamonds. An individual plant is quite small, and consists of a thin, much-branched stem, putting down roots into the water from its under-surface, and bearing overlapping leaves. Each leaf consists of two lobes, which, except on close examination, look like separate leaves. Each lobe is adapted for a totally different condition of life, so there is a distinct division of labour in the one leaf. The upper lobes are comparatively thick, provided with leaf-green, and are therefore food-producers, and they are never submerged. Each contains a large cavity full of slime, page 106and inhabited by a fresh-water alga, which, however, does its host no damage, but, like a respectable lodger, probably pays for its accommodation. The lower lobe is partly submerged, and quite thin, so that it can absorb water. Moreover, the close arrangement of the leaves as a whole furnishes cavities where air can lodge, and so provides the necessary buoyancy for the floating plant.
To see other aquatic ferns the town-dweller must go much farther afield, visiting those solitary lakelets far in the mountainous region of the South Island that are traces of the ice-plough of ancient glaciers. On the gravelly beds of such cool waters lives the alpine quillwort (Isoetes alpinus), looking more like a tiny rush than a fern; and here, too, but in the deeper water and on a more muddy bottom, is the home of Pillularia novae-zealandiae, which also might easily be mistaken for a small rush. Some of the lakes in the Waikato and in the Taupo districts also contain another species of quillwort (I. Kirkii).Indeed, it is highly probable these plants are commoner than is generally supposed.
The Pond-weeds and Water-milfoils.
In most parts of New Zealand one may see, floating on the surface of slow-flowing rivers or calm sheets of water, the oval brown leaves of some species or other of pond-weed (Potamogeton), the commonest of which is P. Cheesemannii. Besides the above leaves there are others which live always submerged, and which differ considerably from the floating ones. These submerged leaves are very thin, erect, more or less ribbon-shaped, and are also extremely numerous. Since there can be no danger of want of water, such leaves are entirely without any protection on that score; on the contrary, they are so constructed as to be able to absorb water over their whole surface just like the filmy ferns of Chapter III, and thereby secure at the same time the oxygen which it contains. Their ribbon-like shape is well adapted to withstand damage from the currents of water, while sufficient extent of leaf-surface is provided by increase in number of leaves. It is also an interesting fact that these submerged leaves are similar to the early seedling ones of the pond-weed, and that this particular shape of leaf is common even amongst the land members of that great division of plants to which Potamogeton belongs. Some of the pond-weeds also never produce floating leaves—e.g., Potamogeton ochreatus and P. pectinatus.*
* Found frequently in slightly brackish water.
The water-milfoils (Myriophyllum) differ from the pond-weeds in that they have no floating leaves, but boldly raise their upper portions above the water-surface. They agree, however, in the fact that the aerial leaves differ from the submerged ones. This is the more interesting because no line of demarcation on the erect stem separates the two except the water-surface—that is to say, the same tissue can change its leaf-form according to change of outer circumstances. The water-milfoils are graceful, feathery - looking plants, with the leaves frequently given off, four or more, from the same height round the stem. The submerged leaves are cut into fine segments, a very common occurrence in many water-plants, whereas the aerial leaves are broader and much less cut. Some of the water-milfoils are bog-plants rather than aquatics—e.g., M. Votschii.*
It is highly probable that all seed-bearing aquatic plants are descended from land plants, which took to the water through competition with rivals better suited than they to their original stations. This is no place to discuss this question, but it may be pointed out that some plants can live equally well on land and in water, and even do not mind being submerged. The water-starworts (Callitriche) are examples.
In many streams the native aquatic vegetation has been ousted by the introduced watercress or the American water-weed (Elodea canadensis).It is interesting to note how much the former varies in both leaf and flower when growing on dry ground by the sides of ditches. Both plants are noxious weeds, and it costs the country much money annually to keep open the watercourses where they flourish.
Rivers, ponds, lakes, stagnant pools, moist soil, and many other stations are the homes of the fresh-water algae, or the pond-scums, as one section may be popularly called. They very frequently form green, slimy masses on the surface of the water. Common forms consist of what look like very fine, long, green hairs. Under a fairly strong power of the miscroscope these are seen to consist of long tubes, divided by thin walls into compartments, which contain plant-green, sometimes in the form of bands.
The fresh-water algae are a very large family, and, although page 108occupying a low position in the plant-world, their structure is at times fairly complicated, and, their methods of reproduction are quite elaborate. To this family belong the diatoms, the stoneworts (Chara, Nitella), and many others. In the hot springs of the North Island are some peculiar forms, belonging to the blue-green algae, which are able to exist in water of a very high temperature. These were recently studied by Professor Setchell, of the University of California, and in a letter to the author he states that none of the New Zealand forms can endure a temperature greater than 167° Fahr., which seems a bath quite hot enough in all truth!
These hot-water algae are sometimes cited to show how living organisms could exist in the early days of the earth when cold water would be unknown, and how such organisms may have persisted since those distant ages, and they or their congeners be the ancestors of our present plant-life.
Relationship between Lakes and Meadows.
Between lakes, swamps, bogs, and meadows there is. a close connection. Sedges, raupo, rushes, and rush-like plants growing in the shallow water near the margin, of a small lake may in time, through their decay, turn that part into dry ground, and advance farther and farther until a water-surface is no longer visible, the whole having become a raupo or phormium swamp. From this, the transition to meadow land is, in many cases, only a matter of time.
The blocking of watercourses with aquatic plants can soon convert a meadow into a swamp. Even on shingly river-beds, swamps at various stages of growth may be observed, and toetoe grass, palmlilies, and phormium break the monotony of the scene.
Sinking of the land may bring about great changes in the plant societies, and remains of plant-life in bogs can teach much as to recent changes in the land-surface.
In the swamps in the neighbourhood of Christchurch large numbers of fallen trees are found, the remains evidently of a large coastal forest, which must have been replaced by swamp during a sinking of the land. So, too, on that narrow peninsula to the far north of Auckland is much kauri-gum to be met with in the bogs, a sure sign that the land stood considerably higher at the time it was occupied by the kauri forest, since that plant is most rare in swamps.
A journey on any of the New Zealand main lines shows the traveller that swamps are a very common feature of the landscape, for they can be recognised at a glance by the dense growth of phormium or raupo (Typha angustifolia) and by those most curious plants, the niggerheads (Carex secta). Formerly, too, the scene was enlivened by that fine bird, the pukeko (Porphyrio melanotus), gay with red legs and bill and blue breast. Before the days of settlement these swamps were much more extensive, but some of those reported as being undrainable by-the early surveyors now bear rich crops of grain or "roots."
Besides the New Zealand flax (Phormium tenax) and the raupo (Typha angustifolia) many other plants occur in swamps. The manuka has been mentioned in an earlier article. Other arborescent growths are the palm-lily (Cordyline australis) and Coprosma propinqua. Very characteristic is the niggerhead (Carex secta), a species of sedge which builds for itself tall and stout "trunks" out of its dead roots and root-stocks, from the summit of which, like shock-heads of hair, the long leaves droop. In such a position the plant is raised high above the water of the swamp, having thus made for itself a dry position very much better for its well-being. On the "trunk" of the sedge, the fern Blechnum capense finds a congenial home, as well as some small native plants—e.g., the marsh - pennyworts, species of Hydrocotyle. Swamps are rich in several species of willow-herb, of which the beautiful Epilobium pallidiflorum and the very tall E. erectum need mention. Two buttercups, Ranunculus macropus and R. rivularis, are common, the latter being easily distinguished by its finely cut floating leaves. Here is also the home of the sedge family, to which the so-called "cutty-grasses" belong.
Swamps are of considerable economic importance in New Zealand. The manufacture of fibre from phormium— "New Zealand hemp," as it is now called—has become one of the staple industries of the Dominion. At present P. tenax is not cultivated, except for ornamental purposes, the wild supply being sufficient. But this will not last for ever. As the swamps are drained, the supply of flax diminishes. The old Maoris were experts in its manufacture, and recognised varieties much more suitable for their mats, &c., than the rank and file of the flax-plants. To such they gave special names. Now what the uninstructed Maori did could be performed much more thoroughly by page 110modern science. We live in the age of scientific plant-breeding. Day by day its importance is realised. Scientists are only now for the first time delving deeply into the mysteries of hybridisation. By scientific breeding, the sugar-beet has had its sugar-content increased more than 10 per cent.; and there seems no reason why the fibre-content of phormium, both as to quantity and especially as to quality, should not be augmented, and pedigree breeds of our "flax" come into cultivation.
Bogs and some of their Plants.
According as water varies in regard to the chemicals it holds in solution, so does it offer different stations for plant-life. Certain waters are rich in lime; in others this is wanting to a great extent, and acids are present instead. Therefore the presence of lime-rich or acidrich water sharply separates water-soaked ground into the two classes, swamps and bogs. Of course, transitions exist between these. In addition, the water of a bog is coffee-coloured, and contains a large quantity of organic matter. Those small plants called bacteria, which play such an important rôle in adding the all-important nitrogen-compounds (nitrates) to the soil, are also scarce in bogs.
At a glance, bog-vegetation can be distinguished from that of swamps. The bog - moss (species of Sphagnum) is nearly always present in the former, and forms rounded cushions and hillocks of a whitish colour, on which many of the bog-plants make their home. For-some unexplained reason bogs are physiologically dry, and consequently many of their plants must be protected against drought. It is with these unfortunates, indeed, a case of "water everywhere and not a drop to drink." Many bog-plants, owing to these adaptations, can exist excellently well in dry places. Phyllachne Colensoi, e.g., a beautiful green cushion-plant of most dense habit of growth, thrives equally well in subalpine bogs and at the heads of alpine shingle-slips.
Bogs occur both in the lowlands and mountains. They are common on the narrow peninsula of the far North, and also occupy much ground in Stewart Island, the same brownish wiry-stemmed rush-like plant (Hypolaena lateriflora) being common in both localities. Many identical species occur in both lowland and alpine bogs, and those of Stewart Island at almost sea-level bear a close relationship to those of the Southern Alps.page 111
Sphagnum possesses some characteristics which distinguish it from most other mosses. Its stems at their periphery are provided with thin-walled capillary cells, stiffened by fibrous thickenings, and communicating with one another and with the exterior by round openings. Thus water is rapidly sucked in by the plant and stored up, while by the capillaries formed by the cells it can be conducted downwards to all parts of the plant. Although the surface on which the sphagnum grows may be extremely wet, but little water comes from below, and then only for a very short distance. Thus a sphagnum bog is altogether dependent on the rainfall, and can only exist where this is abundant, an excessive precipitation allowing the plant to occupy even a rock-surface. As the upper portion of a sphagnum cushion grows, its lower part dies, and is converted into peat, great masses of which frequently accumulate. Such peat is used for fuel in many parts of the world, and at Waipahi, in Southland, is cut for that purpose to some extent, though such New Zealand peat is generally formed by many other plants in addition to sphagnum, or this latter may be altogether wanting. The upper surface of a sphagnum bog continues to rise in height, and any plants growing thereon must, like dune vegetation, be able to grow upwards faster than they are buried. The small pine, Dacrydium Bidwillii, common on subalpine bogs, is frequently buried by the too rapid growth of the moss, and may be observed in all stages of burial. On the sphagnum cushions themselves many plants will grow, owing to the absorption of pure water, which cannot live on sour peat itself.
Where a mountain-stream on flattish ground is unable to take away all the water, an excess accumulates, and a bog is formed. In such places shallow pools are frequent, between which are the sphagnum hummocks. Here is the home of another cushion-plant, much resembling Phyllachne, Donatia novae-zealandiæ, and a sedge of similar habit, with leaves arranged like a comb, Oreobolus pectinatus. Other plants occur in plenty—e.g., a small celmisia (C. longifolia var. alpina), another with broader leaves (C. glandulosa), the slender grass Deyeuxia setifolia, certain plants belonging to the rather rare family Restionaceae, Gaimardia ciliata (fig. 53), G. setacea, and G. pallida, all cushion-plants, and looking like mosses. Here, too, will be a dense turf formed by a woody plant, Dacrydium laxifolium, that smallest species in the world of the pine-tree family.page 112
Very characteristic of bogs are the sundews (Drosera), and they deserve, at any rate, a passing word. As shown above, the bog-water lacks available nitrogen. The small, spoonlike leaves of the sundews are provided with glandular hairs, at the end of which a shining drop of fluid may usually be seen. This contains a substance which has the power of acting on animal matter in much the same manner as the gastric juice. Should a small insect alight on a drosera page 113leaf, it becomes entangled in the sticky fluid, and at the same time the hairs bend over and pin the victim fast. Thus does this tiny but bloodthirsty plant procure some of its nitrogenous food. All this family do not dwell in bogs; D. auriculata is a plant of the heath, and climbs over grass-stems, &c. D. pygmaea has a remarkable distribution, having so far been found only at the two extremes of New Zealand—viz., on the Bluff Hill and in the far north of the North Island. It is a tiny plant, no bigger than one's little-finger nail, and so may be easily overlooked.
Species of the genus Gunnera are frequent in lowland bogs. Those near Invercargill contain abundance of G. prorepens. The New Zealand species are quite small—mere pygmies, indeed, in comparison with their huge-leaved Chilian relation (G. chilensis). But, notwith-standing this, both equally afford house-room to a species of Nostoc, a fresh-water alga, somewhat after the manner of Azolla before described. Perhaps the prettiest denizen of the bogs is the pale-blue liliaceous Herpolirion novae-zealandiae, which, when not in flower, may be mistaken for a grass. A companion plant is Oreostylidium subulatum.
The bog umbrella-fern (Gleichenia dicarpa) frequently occupies large areas of boggy ground, its pale-green leaves and brown stems rendering it very conspicuous. A creeping club-moss (Lycopodium ramulosum) is frequent in Stewart Island and on the west of the South Island, and farther north the somewhat similar L. laterale is encountered.
Before leaving the bogs, another flesh-eater must be mentioned, the bladderwort (Utricularia monanthos), a plant with small, showy, purple flowers. The bladderworts are quite without true roots, metamorphosed leaves functioning as such. In some instances the leaves develop in another abnormal way: they construct themselves into small bladders, which are furnished with a lid, which can open only from without inwards. This leads to an arrangement like that of certain mouse-traps, so that a minute aquatic animal may easily enter the bladder, whence it cannot escape, and so is digested in due course by the plant.