The coastline of New Zealand extends from 4,000 to 5,000 miles, with a great variety of physical features, sandy beaches, gravel and boulders, rocky stormtossed headlands, and often stretches of sand dunes.
There are about 600 species of seaweeds, some always submerged, others in intertidal zones, which are in three main classes, each with a distinctive colour according to the place in which they live. Agar was previously imported from Japan, for its use in food preparations and for scientific work. Now agar is extracted from Pterocladia lucida and P. sapillacea from our own shores. Corallina officinalis is one of the red seaweeds that extract calcium salts from sea water and their tissues become impregnated with lime. Red seaweeds are submerged, the brown seaweeds occupy the next zone and very common is Hormosira banksii, like a string of beads, found in rock pools. Among the kelps are the giant kelp Durvillaea antarctica, the bull kelp D. utilis, and the bladder kelp Macrocystis pyrifera, which is washed up on shore by a storm. The green seaweeds are found in shallow water, exposed for long periods between tides and represented by sea lettuce (Ulva), the thick fleshy masses on stones formed by Codium adhaereus and Caulerpa brownii which resembles a twig of the tree rimu. The Maoris called this and other seaweeds by the name rimu.
Sandy shores are too unstable to support many plants. In sheltered places the shore convolvulus (Calystegia soldanella) and the shore buttercup (Ranunculus acaulis) are found. On rocky shores there is more variety; many herbaceous and semi-woody plants are found, with more restricted distribution. In the south are a native dock (Rumex neglectus, the bronze Gunnera and Crassula moschata. A few of the more widespread plants are the true flax (Linum monogynum), some hardferns (Blechnum species), a small sedge (Scirpus cernuus), the succulent glasswort (Salicornia), and the yellow-button (Cotula coronopofolia). “Wandering” sand dunes extend inland in some areas and only plants with long underground stems can withstand the constant movement of the sand. Where such are established they arrest the progress of sand which often threatens to invade farm lands. Shore convolvulus (Calystegia), pingao (Scirpus frondosus), and Spinifex hirsutus are useful for this purpose. Sand coprosma (Coprosma acerosa), two species of Pimelia, and the Cassinias form compact shrubby growth in some exposed areas.
Juncus maritimus var. australiensis and Leptocarpus simplex, either singly or in combination, may dominate salt swamps from the North Cape to Banks Peninsula. Further south Leptocarpus only is found.
The mangrove, Avicennia resinifera, is characteristic of muddy tidal estuaries of the North Island to 38° S on the east coast, but not so far south on the west coast. This is a very unusual plant with many peg-like roots and a seed that begins germination before it leaves the plant.
Certain trees form a narrow belt of coastal forest. The pohutukawa, Metrosideros excelsa, common in the north, reaches to 38° S; the karaka, Corynocarpus laevigatus, is in the South Island also and is abundant on the Kaikoura coast to about 42° S; the ngaio, Myoporum laetum, also extends to the South Island as far as Dunedin.
In the sheltered eastern harbours of Stewart Island and the Auckland Islands, the southern rata, Metrosideros umbellata, forms a coastal forest belt reminiscent of pohutukawa in the far north. Large-leaved composites are common on the coast in the south: Senecio reinoldi of Stewart Island, S. stewartiae and Olearia lyallii on the Snares, and O. lyallii in the north of Auckland Islands.
A remarkable group of endemic species with tropical affinities is found on the northern off-shore islands. Among these are Meryta sinclairii (Araliaceae), a large-leaved tree, abundant on Three Kings Islands and Hen and Chickens Islands and representing a genus centred on New Caledonia; Xeronema callistemon (Liliaceae), common on Poor Knights and Hen Islands, is related to X. moorei of New Caledonia. Elingamita johnsoni (Myrsinaceae), an endemic of about a dozen trees on West Island, Three Kings, resembles Tapeinosperma of New Caledonia. Tecomanthe speciosa (Bignoniaceae) is a woody climber related to T. hillii of Queensland, of which only one plant has been found on Great Island, Three Kings.
Plectomirtha baylisiana (Anacardiaceae), a single tree on Great Island, Three Kings, is related to Semecarpus of Indo-Malaya.