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.