At one time in the history of Chatham Island the Sphagnum bogs must have been very extensive; even when the white man first arrived Sphagnum must have been much more abundant than it is at present. Here and there on the tableland primitive Sphagnum bogs may be encountered; others of considerable extent occur in other parts of the island, of which one on the high ground between Whanga marino and Te Whanga is of great interest, but most likely even this is not by any means in its primitive condition. The Sphagnum bogs of the tableland usually form small islands in the midst of the second stage of bog, the Lepyrodia formation, described in the next section. The Sphagnum, perhaps an undescribed species peculiar to Chatham Island, is extremely wet, and, in the centre of the bog, pools of water lie on the surface. Walking on the surface one sinks up to the ankles, and in the centre of the bog much deeper still. Growing on the Sphagnum, in the very wettest places, is a small quantity of Isolepis sp. and Carex sp. Where the bog is a little drier Hierochloe redolens, Poa chathamica, and Pratia arenaria are
scattered about here and there. Just where the Sphagnum formation abuts on the Lepyrodia formation stunted plants of Olearia semidentata and Dracophyllum paludosum, two very characteristic plants of this latter formation, make their appearance, showing the line of tension between the two formations. Growing on the Sphagnum the two plants mentioned above remain very small, and often flower when only 10 cm. or less in height. Similar dwarf plants are common in peat bogs elsewhere, Professor Conway McMillan, for instance, mentioning spruce-trees in the bogs of Minnesota seventy-five years old, and but little more than 1 ½ in. in diameter (44, p. 460). It must have been such miniature plants of Dracophyllum paludosum that Buchanan referred to Drac. rosmarinifolium in his list of Chatham Island plants (3, p. 338).
In the bog on the Whangamarino Run the Sphagnum forms large rounded mounds, on which grow many plants of Gleichenia circinata and a few of Pteris esculenta. Growing in the hollows between the Sphagnum mounds are Myriophyllum pedunculatum, Drosera binata, Utricularia monanthos, Pratia arenaria, and Poa chathamica. In many places the Sphagnum becomes less abundant; here the soil consists of imperfectly decomposed remains of moss and other vegetable matter 1.2 cm. in depth, below which is black rather sticky peat of a fairly firm consistency. In the upper layer Myriophyllum, growing to a height of 4.5 cm., and forming a dense mat on the ground, stretches its rhizomes, while its roots penetrate for a distance of 5 cm. into the black peat. Growing through the Myriophyllum are many tufts of the Isolepis and Eleocharis gracillima. In places the Isolepis becomes so abundant as to almost conceal the Myriophyllum. At times also that extraordinary fern Schizœa fistulosa puts in an appearance. Where the Sphagnum appears the substratum is much wetter, the decayed remains of the Sphagnum being wringing wet. In such places Isolepis almost altogether replaces Myriophyllum. Where the ground is a little drier Gleichenia circinata forms an unbroken sheet; and growing in company with it are all the bog plants enumerated above, together also with many stunted plants of Dracophyllum paludosum, like those before described. Many of these latter tiny plants were in full bloom.