WATER PLANTS

WATER PLANTS

by Olive Rita Croker, M.A., Botanist, Wellington.

WATER PLANTS

Plants which grow in streams, lakes, and ponds, in drains and ditches, and in the sea are often referred to as water plants. If the term is used to include plants which spend most of their life partly submerged in water, as well as the algae which live, more or less, completely submerged, there is difficulty in defining the limits of what should be included, but there are a few ferns and some flowering plants which are worthy of note for their watery habitat. Some representatives of these will be included.

Algae

The algae are that group of simple plants forming with the fungi the class Thallophyta. Algae contain chlorophyll and obtain oxygen and carbon dioxide from the water in which they live, thus enabling them to carry out the normal functions of green plants though, in seaweeds, this colour is often masked by other pigments, brown or red. There are two main groups of algae, the freshwater and the marine forms. The marine algae or seaweeds are dealt with in a separate article. Though a great deal has been published about the seaweeds found round our coasts, there has been very little published about the large group of freshwater algae, though at intervals there have appeared lists of the names of species found in different localities, showing that a few hundred species are present in our waters.

The freshwater algae are a group of very interesting plants. It is only when these forms are found in large numbers as, for example, as pond scums, that they are visible to the naked eye. Small areas of standing water may appear reddish in colour because of multitudes of Haematococcus, one of the few forms with any pigment other than chlorophyll. But with the aid of a microscope a drop of water from pond or stream is found to contain many varied and fascinating forms of tiny plants. These are of considerable importance for they manufacture food and so form the first stage in the provision of food for animals. Freshwater algae live under varying conditions and are of many forms. They are found in slow-moving rivers and streams, in lakes and ponds, in ditches and stagnant pools, and on damp earth. They are of varied form. Many are completely unicellular, some appear as filaments which form pond scums, while others appear as hollow spheres or in more complicated forms. Their methods of reproduction may be by simple division, or by some elaborate methods of sexual reproduction. Of the free unicellular forms are Diatoms (of many intricate designs), Desmids (often crescent shaped), Haematococcus and Chlamy-domondas (moving quickly with two cilia). Among the pond scums are many forms which appear as long fine green hairs, slimy to the touch, common among these are Spirogyra, Zygnema, Vaucheria, and Oedogonium (with complicated methods of reproduction). The Stoneworts (Chara and Nitella) form long brittle colonies on the beds of rivers and lakes. The hollow spheres of Volvox, whirling through the water, are formed of tiny similar plants, but with complicated methods of reproduction.

Many other freshwater algae might be mentioned as Sphaerella, Cladophora, Ulothrix, Oscillaria, and the tiny Protococcus and Pleurococcus on damp earth and tree trunks, but, though they seem to be common in most places, there is little exact information concerning their distribution in New Zealand. In the thermal regions there are some members of the Cyanophyceae (the Blue-Green Algae) which are noteworthy for the fact that they grow in water of high temperature and on damp clay which is very hot.

Many mosses and liverworts live more or less in water. These groups of plants are dealt with in separate articles. Of special note is Drepanocladus fluitans, a moss found submerged in sluggish streams and pools.

Floating Plants

A very interesting plant which may cover a pond so closely that it appears as a grassy patch is the tiny fern Azolla rubra which floats on the surface of the water, the arrangement of overlapping leaves enabling it to remain buoyant. Each leaf consists of two differently shaped lobes, the upper one being above the water so that a cavity is formed between the two lobes in which a tiny freshwater alga finds a home. This is Anaebena, a member of the Nostoc family. Isoetes alpinus is a lycopod with stiff green leaves, found in Lake Taupo, and in lakes on the mountains of the South Island.

Of the flowering plants we may consider a few. There are no trees or shrubs of fresh water comparable to the mangroves of salty estuaries, but many monocotyledonous plants live on the edges of lakes, rivers, or in wet swamps, and some appear to live floating on the water. Long underground stems are common, and sometimes there is a thick rhizome in which food is stored for the season when leaves are shed. On the surface of slow-moving streams and ponds in most parts of New Zealand may be seen the floating, oval leaves of species of Potamogeton. P. cheesemanii is the commonest, while P. ochreatus and P. pectinatus are abundant also. The latter lives completely submerged; even its flowers and fruit are under water. In other types there are long coiled stems by which the flowers come to the surface where pollination takes place. This happens in Ruppia spiralis, where after pollination the stalk recoils and the fruit ripens at the bottom of the water. There the long stalk on each fruit becomes tangled with others so that large rounded masses of fruits are formed. This plant is very common in Lake Ellesmere.

Among floating plants are the tiny duckweeds, which form bright green masses on the surface of still water, with small hanging roots. Lemna minor is a bright green plant of wide distribution; Spirodela oligorrhiza, slightly larger is found only in parts of the North Island; Wolffia arrhiza, found mainly near Wellington and in Canterbury, is also bright green, and disappears to the bottom during the winter. The water milfoils have tiny, much divided leaves, especially those completely submerged. There are four species of Microphyllum commonly found in New Zealand.

There are three species of bladderworts (Utricularia) described as found in New Zealand, but U. mairii, which grew in Lake Rotomahana, has not been seen since the Tarawera eruption of 1886. This was a floating plant similar to U. protrusa with finely divided leaves. The bladderworts are curious plants with finely divided leaves and bladders in which insects become trapped and, as they disintegrate, form part of the food of the plants. A third species has roots: this is U. novae zealandiae found in the North Island as well as in Canterbury on wet peat subject to frequent flooding.

Swamp Plants

Two other small plants growing in muddy places and often submerged are Mimulus repens and Limosella tenuifolia (mudwort), relatives of the monkeymusk (Mimulus guttatis). Some species of buttercup of similar habitat are Ranunculus rivularis (poisonous to stock), R. fluitans, and R. macropus. Also growing in wet mud, completely submerged at times, are Callitriche verna, the light green starwort; Elatine gratioloides, which creeps under water at edges of lakes; Ludwigia palustris, water purslane, with a mat-like form of reddish colour; and Tillaea sinclairii, found mainly in the South Island. These are all tiny plants. More conspicuous are sedges and rushes. Of many species of Carex a few grow near the edges of water. The genus Scirpus includes species from 3–10 ft high of which the best known are S. lacustris, our tallest sedge, common throughout the country; S. perviridis, usually 5–6 ft, of the North Island, and the shorter S. caldwellii, found in both Islands. Species of Eleocharis and of Cladium are other familiar sedges. The raupo, Typha muellerii, known to the Maoris as koreire, is probably the best known of our plants of watery swamps. Before there was any systematic drainage of the country, extensive areas were covered by raupo. The tall flower stalks have dense masses of tiny flowers, male flowers above and female flowers on the lower part. This plant was useful to the Maoris in various ways: the rhizomes for food, the flower heads and hairy fruits for plugging holes in canoes, and pollen mixed with water baked as cakes. Early settlers used inflorescences for stuffing pillows and beds and the leaves for thatching roofs.

Introduced Plants

In addition, there are some introduced plants which have become very abundant, in some cases blocking streams and rivers. Among these is watercress, Nasturtium officinale, one of the plants introduced by early colonists as a food, which became a nuisance in the Avon River and streams about Christchurch. Pond weeds, which have become very abundant and in certain instances a serious problem of control, are Ottelia ovalifolia, from Australia, and Aponogeton distachyon, a water hyacinth from South Africa. A greater threat comes from another water hyacinth, Eichhornia crassipes, the growing of which is strictly prohibited in New Zealand. It has attractive pink-mauve flowers with yellow and purple markings on one petal. It grows very rapidly, has extensive rhizomes, and may root in mud or float on water.

by Olive Rita Croker, M.A., Botanist, Wellington.

WATER PLANTS 23-Apr-09 Olive Rita Croker, M.A., Botanist, Wellington.