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.