New Zealand Plants and their Story
Adaptations of Coastal Plants
Adaptations of Coastal Plants.
Latitude being left out of the question, in all parts of the world coastal vegetation, both in its form and distribution, depends upon certain factors. Of these, salt in the soil and exposure to sea-spray and violent winds are of prime importance. Wherever they occur, genuine seaside plants have various features in common. The most important of these are contrivances to regulate the water-supply, the commonest of which is succulence of leaf and stem, one or both. This succulence is caused by the presence of special tissues which serve for water-storage. Many New Zealand coastal plants exhibit this feature. The ice-plant (Mesembryanthemum australe, pig's-face, horokaka), which so frequently drapes the coastal cliffs page 64with its pale-green leaves, and bears rather large rose-coloured flowers, is a pleasing and familiar, example (fig. 25). This species must not be confused with the Hottentot fig (M. edule), a native of South Africa, now naturalised on many sandhills, but which possesses leaves still "fatter" than those of its indigenous sister, and bears larger and yellow-coloured flowers. Other common coastal succulents are: Salicornia australis (fig. 26), a stem-succulent common in salt meadows, and Suaeda maritima, a leaf-succulent, usually growing in rather wetter ground.
Succulence has been shown experimentally to be brought about by excess of salt in the soil, and certain plants to which salt is not a deadly poison can be made artificially succulent. Some of the introduced plants of this country, as, e.g., the spotted catchfly (Silene anglica, var. quinquevulnera), acquire much fatter leaves when growing near the sea than inland.page 65 page 66
That plants growing in wet stations, such as salt meadows and marshes, should be furnished with appliances to combat drought appears very remarkable. The truth seems to be that for some reason not yet sufficiently explained, although many theories are rife, the plant dare not, use too much brackish water, and so is actually in the same position as a plant of a desert region. When dealing with the bog vegetation it will be seen that it, too, is in a similar condition, and so is that in the neighbourhood of solfataias and the like.
Schimper has summed up these conditions in an excellent manner, pointing out that two kinds of dryness exist. These he has named "physical" and "physiological." Physical dryness arises from want of water in the soil, but a physiologically dry soil may contain any amount of water, but yet of such a quality that its plant inhabitants cannot use it. To quote a common example, the sea is physiologically dry, so far as man is concerned. Physiological dryness alone concerns plant-distribution.