If a portion of the sea be cut off from the main body of water by an enclosing barrier of sand a lagoon is the result, of which Te Whanga is the most important example. Its waters are usually shallow for a considerable depth from the shore, and so are favourable for plant-life; but, being brackish, only a limited number of phaneroganis can exist in this station, while frequent winds agitate the surface of the water to such a degree that only those plants specially adapted to
resist the action of waves can exist. In consequence the lagoons of Chatham Island possess only a very small phanerogamic flora; nor so far as observed were Algœ, numerous. Ruppia maritima often occupies large portions of such shallow places in lagoons, and its leaves and stems, floating at times on the surface of the water, form a mat of such density as to have attracted the attention of the Maoris, who call it the “eels’ blanket.” The floor of the lagoons consists of sand or of sandy peaty mud, formed from the decay of many generations of plants. Such muddy peat is the commencement of a transition from the bed of a lagoon to salt meadow, for as it gradually accumulates it rises out of the water and becomes at once occupied by an abundant plant population. Such a spot forms the line of tension between lagoon and salt meadow. In several places Te Whanga Lagoon is, as before pointed out, sufficiently shallow to be crossed on horseback, the depth of water at the crossing varying according to the direction of the wind; and with a north-west wind blowing in summer it may be quite dry, and clouds of dust and sand mark its position. On this portion of the lagoon-bed large round patches of Samolus repens are abundant; so here is one of the most characteristic of salt-meadow plants taking possession of the ground almost before the station is fit to receive it. A very slight rise indeed of the land and the bed of the lagoon would be transformed by nature into salt meadow, which probably might be succeeded by forest, especially if the land were elevated a little more. It is also easily conceivable how such a lagoon could be transformed into a bog, and it seems very probable indeed that the low-lying boggy ground in the north-west of the island has had this origin, as suggested by Haast (23).