On the Occurrence of Lemna Oligorrhiza Kurz in New Zealand.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 60, 1930, Page 563

On the Occurrence of Lemna Oligorrhiza Kurz in New Zealand. By T. L. Lancaster, M.Sc., Lecturer in Botany, University College, Auckland. [Read before the Auckland Institute, 30th October, 1929; received by Editor, 10th November, 1929; issued separately, March 31st, 1930]. Plate 63. The only members of the family Lemnaceae hitherto recorded as occurring in New Zealand are Lemna minor and Lemna gibba. The former—the common duckweed—is found floating on fresh-water ponds and lakes in all parts of the country, while the latter, if it is present at all, must be exceedingly rare, not having been reported since its original mention by Colenso as occurring in the Poverty Bay district. No information as to the exact locality or the date of its discovery appears to be available but the latter could not have been less than 35 years ago. Some interest, therefore, attaches to the discovery of another species of Lemna which was found by the writer at Awapuni, some 2½ miles from Palmerston North, in January of this year. A sheet of rather shallow water occupying an old abandoned course of the Manawatu River had a considerable amount of this plant floating on its surface, the total area more or less closely covered being perhaps a couple of acres. Growing in the same pond were clumps of Typha angustifolia, various aquatic sedges and other plants. The swollen undersurface of the shoots suggested that the plant in question might be Lemna gibba and in order that its identity might be accurately determined, specimens were sent to Kew. The writer is much indebted to Sir Arthur Hill, Director of the Royal Botanic Gardens, through whose kindness the material was submitted to Professor Moss and Mrs. Young of Johannesburg, who are specialists in aquatic plants and who were able to identify the plant as Lemna cligorrhiza Kurz. This species occurs in India, from which country it was originally described, and also in Malaya and the eastern states of Australia. As it is probable that Lemna oligorrhiza will be found to occur elsewhere in New Zealand the following short description of the salient characters by which it may be recognised, or, at all events, distinguished from the common Lemna minor, is given. Shoots 3–5 mm. in length, 2–3 mm. in breadth, broadly ovate or oblong, usually asymmetrical. Upper surface slightly convex, edged with purple, often with a faint longitudinal ridge near centre. Lower surface more strongly convex or tumid, uniformly purple, or covered with purple reticulations. Roots two or three together from the lower surface of the older segments. Young shoots from pockets towards the base of the older ones, remaining attached for a time so that a plant may consist of three or even more segments, which separate later.

No flowers were seen and it seems not unlikely that this species, like many other members of the genus, rarely flowers, at any rate in temperate regions, but depends almost entirely upon vegetative budding for increase. The amount of anthocyan pigment, to the presence of which the purple colour of the lower surface of the shoots is due, varies a good deal. Similar fluctuation in depth of colour is also noticeable in other small floating water plants such as Azolla rubra and Riccia natans and is related to environmental conditions among which intensity of light is the most important factor. The question arises as to whether Lemna oligorrhiza is indigenous in New Zealand or whether it has been either accidentally or intentionally introduced by man from abroad. The property on which it was found was acquired by the father of the present owner 40 years ago, at which time it was covered with heavy forest, and the latter gentleman is fairly certain that no plants from beyond New Zealand have been introduced into the ponds since the land came into the possession of his family. While this does not exclude the possibility of its accidental introduction it would appear that, until further light is shed upon the matter, Lemna oligorrhiza may be regarded as probably a native plant. When the readiness with which small floating water-plants are capable of being distributed on the breasts and feet of ducks and other aquatic birds is considered, it seems extremely unlikely that this species occurs only in the locality where it was found by the writer. It can scarcely be a common plant, but is almost certain to be reported from other parts of the country. It is by no means impossible that the species described as Lemna gibba by Colenso, at a time when the genus was very incompletely known, may really have been Lemna oligorrhiza. Both agree in having the lower surface of the shoots conspicuously swollen or gibbous, which character is, however, much more marked in the former species. An examination of the ponds and lakes of the Poverty Bay district, with a view to determining what species of Lemna occur there, is consequently a matter, not unworthy of attention.