This formation appears to follow on directly after the Sphagnum, so soon as the ground has become a shade drier. It is now altogether a more common formation than the Sphagnum proper, but it also is rapidly being destroyed, chiefly through the agency of fires. Although Lepyrodia
traversii itself is by no means uncommon in the northern part of the island, the formation there, although at first sight looking distinctly a primitive one, has probably been burnt, but reproduced almost unchanged. Even on the tableland there was only one small piece of this formation that I could feel sure had never been exposed to the influence of fires and stock. This is situated on the south bank of Lake Rangitapu and occupies a space of an acre or more.
The soil consists of peat completely saturated with water, into which a stout stick 2 m. in length can be thrust up to the hilt with the greatest ease. Water can be squeezed out of this soil as from a very wet sponge by quite a slight pressure of the hand. At a depth of 20 cm. the soil is rather of the consistency of porridge. So powerful is the water-holding capacity of such a soil that, if a deep drain is cut through ground such as the above, it will remain saturated with water for years right up to the margin of the drain; even the abundant natural drainage by means of many creeks flowing at a considerable depth below the surface of the peaty tableland, and having a final fall of 210 m. into the sea, have no apparent effect on reducing the water of the bogs.
The vegetation of the formation is extremely dense in most places, and consists of Lepyrodia traversii mixed with Olearia semidentata and Dracophyllum paludosum. In such a dense patch one has to walk right on the top of the L. traversii, which sinks with every step; but it is then not on the ground that one is walking, but on the Lepyrodia itself, while the soil is at a considerable distance beneath. In such a place L. traversii reaches to one's neck. Neither O. semidentata nor D. paludosum are usually quite so tall as the Lepyrodia, though in some places the Dracophyllum is the tallest of all. The lower parts of the Dracophyllum and the Olearia are usually quite leafless, owing to the density of the Lepyrodia. In such a dense portion there is no visible undergrowth on the wet, black, peaty ground, nothing being there but the remains of the decaying vegetation and the many matted roots. If a small hole be scooped out in this soil, it will very quickly become filled with water.
The surface of the formation is not all at one level. The Lepyrodia, being bent downwards either by the wind or owing to its rigidity not being sufficient to keep it upright, overlies and becomes entangled with the other shrubs, its shoots lying mostly towards the south, owing to frequently violent north winds. In places, not quite so dense as that described above, Gleichenia circinata puts in an appearance, its long wiry leafstalks raising up the laminæ to within less than 50 cm. of the surface of the vegetation. Here, or more frequently in more open places, the surface of the ground is covered with various
liverworts and mosses, especially with Sphagnum, a relic of the parent Sphagnum formation. In places where the three dominant plants are growing more thinly still, and the underlying Sphagnum is exposed to the light, seedlings of Olearia and Dracophyllum occur; also Drosera binata and Gentiana umbellata make their appearance, but these two latter are by no means numerous.
In some places the original Sphagnum is still quite thick; there the shrubby growth becomes at once much reduced, and its height quite one-half less. In such a place the sunlight can have some effect, and in consequence more plants appear. For instance, Corysanthes macrantha and Carex sp.; while Gentiana, Drosera, and Utricularia occur in greater numbers.
Here and there through the formation are a few stunted plants of Phormium tenax which have strayed from the drier ground, but which certainly do not really belong to this formation. The most important constituents of this formation are Lepyrodia traversii, Olearia semidentata, and Dracophyllum paludosum. of these Lepyrodia traversii, a strongly marked xerophyte—as, indeed, are the other dominant plants—occurs only in very wet bogs, and a slight diminution in the wetness of the ground will cause it to disappear. It is furnished with a strong rhizome 3 cm. in diameter, which creeps through the Sphagnum or the peaty ground at a depth of 5 cm. From this rhizome upright rush-like shoots are given off at intervals, each bare for the lower third of its length, but above branching laterally. These upper branches are terete, dull-brown in colour, very smooth, stiff but quite flexible, and about 1 mm. to 1.5 mm. in diameter. These stems function as leaves, and are provided with a dense palisade parenchyma. A transverse section shows an irregular-shaped lacuna in the centre of the stem, surrounded by a large-celled parenchyma. This, again, is enclosed in a ring of stereome, and round the periphery is a one-layered epidermis with a very thick cuticle. The strong rhizome is of great importance to the plant in assisting it to spread, and in so preventing the advent of other plants. Whether L. traversii is able to grow on dry ground like its relative Leptocarpus simplex only experimental culture will prove, though its structure should certainly fit it for very dry stations. Mr. Cheeseman, who first pointed out that L. traversii occurred in New Zealand as well as the Chatham Islands, where it was thought to be endemic, thus writes (4, p. 325): “In the Ohaupo locality Sporadanthus”—the genus under which the plant was then placed—“is seldom found near the margin of the swamp; but towards the centre, where there is a great depth of peat, which affords ample room for its creeping
rhizomes and long stringy roots, it occurs in immense abundance, often covering hundreds of acres to the exclusion of all other vegetation. Mr. J. Stewart, C.E., informs me that the workmen engaged in constructing the railway dreaded to encounter it, as its thick matted roots not only made it difficult to open out the drains, but were always a sure sign of a very bad part in the swamp.”
Olearia semidentata varies in form according to the position in which it is growing. When it forms a constituent of the wettest portion of the Lepyrodia bog it is of a straggly growth, its lower part leafless and concealed by the thick mass of Lepyrodia, while only the ends of the branches, which project into the light, bear leaves. When growing where it has more room to spread it forms rounded bushes sometimes 85 cm. in height and 1.24 m. in breadth. Such plants, as I have pointed out elsewhere (128), bear a rather close resemblance to certain cultivated species of south European Cystus. The leaves are variable in size, thick and coriaceous, slightly cottony and shining on their upper surfaces; but the lower surface, which alone contains stomata, is thickly covered with white tomentum. The ultimate branchlets are so dense as to touch one another. Usually the flowers of New Zealand plants are either white or yellow, but O. semidentata is a notable exception, the ray-florets being of the most brilliant purple (see coloured plate, 232, pl. ii.), indeed, a group of these shrubs covered with heads of blossom each 4cm. in diameter is a most beautiful spectacle. The roots, which are of considerable length, project laterally, and not vertically, downwards. This manner of growth is evidently strongly hereditary, since it appears in the seedling at quite an early stage. It would be thought that a plant such as O. semidentata, growing under edaphic and climatic conditions of extreme constancy, would vary little. On the contrary, so far as the leaves are concerned, the amount of variation is very great. I specially measured the leaves of a considerable number of plants growing near one another in the vicinity of Lake Rangatapu. The largest measured 6.5 cm. by 1 cm., and the smallest 3.6 cm. by 7 mm. The following measurements give examples of the variability of length with regard to breadth: 6.5 cm. by 1 cm.; 5.1 cm. by 1 cm.; 4 cm. by 1 cm. The tomentum on the upper surface of the leaf varied, according to my notes, from “none” to “very abundant.” The number of teeth on the margin are also very variable, both on different leaves and on the opposite sides of the same leaf, some leaves having no teeth at all and some as many as nine, while others showed all the intermediate numbers. My notes on this subject conclude thus: “Where I write, three plants
are growing side by side, which, so far as general appearance goes, might be taken for three distinct species.”
Dracophyllum paludosum, like O. semidentata, when growing amongst L. traversii, is only leafy on those stems which emerge from that latter plant. It often occupies much drier ground than even O. semidentata. When fully grown and under the most favourable conditions it may be about 1.8 m. in height, and is of rather a fastigiate habit.* It has long creeping woody underground stems, giving off numerous strong cord-like roots. Seedling plants also show this rhizomatous habit. This spreading of roots or underground stems near the surface of the bog is a special characteristic of bog plants all over the world, for in such a position it is possible to get the supply of oxygen for these underground organs which is so deficient in the deeper layers of the boggy peat. Professor W. F. Ganong describes how he traced a stem of Rubus chamæmorus in a New Brunswick peat bog for a distance of 17ft. without finding an end (19, p. 142). The leaves of D. paludosum are much like those of certain other New Zealand Dracophyllums, being needle-shaped and stiff. They are 4 cm. long by 1mm. broad, and have a short sheathing base. Usually they are semivertical, but sometimes almost quite vertical. The upper surface is slightly concave and the under-surface convex; both surfaces have a strong cuticle, and possess stomata. The vascular bundles are surrounded by stereome, which extends from one surface of the leaf to the other, and alternates with the chlorenchyma.
Originally the Lepyrodia formation must have occupied very large areas not only on the southern tableland, but on other parts of the island, especially on the low-lying ground in the north, where many relics of this formation still exist; such, for example, may be seen in abundance in the boggy ground south of Wharekauri Hill. In another part of the flat land of the north-west peninsula several acres which had evidently once been this formation were covered with young plants of Olearia semidentata. On these northern bogs Cladium gunnii is often quite common.
The xerophily of bog plants is a most astonishing phenomenon, and has received various explanations, none of which, however, seem to me altogether satisfactory. The most generally accepted at present is that of Schimper (48, p. 18), which is thus stated by Dr. Cowles (142, p. 145): “Schimper believes that these structures”—i.e., xerophytic structures—
[Footnote] * Compare Kirk's remarks (42, p. 225) on D. scoparium, which, of course, may be the same species as the Chatham Island plant: “It is a compact plant of fastigiate conical habit of growth, exactly like that of Cupressus sempervirens, and quite unlike that of any other plant.”
“are due to the difficult absorption in peaty soil, the humus acids and the lack of oxygen being detrimental to normal root activities. For similar reasons the normal soil activities of bacteria and fungi are lessened, and, as a result of this relative lack of decay, great quantities of peat accumulate.”