Art. XXIX.—On the Occurrence of Ottelia in New Zealand.
[Read, before the Auckland Institute, 10th October 1898.]
Rather more than a year ago I had occasion to visit the three little volcanic hills at Ihumatao, which unite to form the promontory jutting out into the Manukau Harbour to the south of Weekes Island. Taking a short cut from one of these hills to regain the road to Mangare and Onehunga, my attention was attracted by the large yellowish-white flowers of a water-plant which quite covered the surface of a small pond, and which was evidently a stranger to me. On examination it proved to be Ottelia ovalifolia, a species which is not uncommon on the eastern side of Australia, ranging from Victoria to the North of Queensland, but which had not been previously noticed in New Zealand. Curiously enough, it belongs to the same order—Hydrocharidaceæ—as Vallisneria spiralis, which a few years ago appeared in Lake Takapuna, and has since become extremely plentiful there (see Trans. N.Z. Inst., 1896, p. 386). Ottelia, however, is very dissimilar in appearance to Vallisneria. The leaves, which are numerous and densely tufted, are borne on stout cylindrical petioles, varying in length in accordance with the depth of the water. These petioles are greatly swollen at the base, forming a mass of air-cells. The blade of the leaf is about oval in shape, and from 2 in. to 3 in. in length, and lies flat on the surface of the water. The flower-stalks rise just above the surface of the water, each one bearing a single rather large yellowish-white flower. The flowers are produced in profusion during the whole of the summer months, giving the plant a very attractive appearance.
No doubt Ottelia has been introduced from Australia, but whether purposely or by accident I am unable to say. The locality is a remarkable one for a naturalised plant to make its first appearance, and the few settlers residing in the vicinity have no information to give as to its establishment, beyond the fact that it has existed in the pond for many years past. So far as I am aware, the plant has never been in cultivation in Auckland, either in private gardens or in nurseries. It is certainly not usually cultivated in Australia, and its name does not appear in any of the nurserymen's lists. As the pond is an isolated one, the plant is not likely to spread further; but if by chance it should be conveyed to any of the
lakes or slow-running streams in the northern portion of the colony it may be expected to show a rapid increase.
While on the subject of water-weeds, I may mention that there is some risk of the establishment in the northern portion of the colony of the water-hyacinth (Eichornia crassipes), a plant of very similar habit to Ottelia, although in reality-belonging to a very different order. It is now commonly cultivated in ponds or small tanks, and apparently does well in our climate. Its naturalisation ought to be guarded against, for in Florida, where it was introduced some years since, it has increased to such an enormous extent as to completely block the navigation of several of the more sluggish rivers.