Ferns are abundant in all damp situations in New Zealand forests, forming the undergrowth beneath a dense canopy of evergreen trees. They are found growing also on trunks and branches of trees, on banks of streams, and in the open, even on dry hillsides. There are over 150 species found in this country, representatives of 12 families, and of these species 54 are not found elsewhere; they are endemic. It is not the number of species in New Zealand but their great variety which makes them so interesting, ranging from filmy ferns, only 1 in. high, to large tree-ferns, which often form part of the canopy of the forest.
Ferns may well be regarded as fascinating amphibians of the plant world, for there are two distinct stages in the life cycle of each plant, one of which is dependent on water. The fern plant with which we are familiar usually grows on land; it represents the asexual generation – the sporophyte – and bears spores on mature fronds. These spores are borne in cases, the sporangia, which are usually collected in groups or sori, on the under surface of the leaf. Each sorus usually has a protective covering, the indusium. The sori differ considerably in form and position and are useful as a means of identification. They may be in a cluster, in marginal lines, or the whole of a fertile frond may be entirely covered with spore cases. There is often an intricate mechanism for the release of the spores, each of which is capable of producing a new plant, but of a different form. This is extremely small, a thread-like or heart-shaped structure growing close to the ground, but is green and self supporting. This is the prothallus, and because it bears the gametes, or sex organs, is called the gametophyte. On it are produced male organs, antheridia, from which tiny free-swimming sperms are set free and depend on films of water for finding their way to the egg cells, still within the archegonium, embedded in the prothallus. When a sperm has fused with an egg cell the fertilised egg produces a new plant, unlike itself, the large, conspicuous sporophyte. Thus we see an alternation of generations, two types of plants; the sporophyte bearing asexual spores which give rise directly to the gametophyte on which sex organs are produced. The latter is seldom noticed; it is the sporophyte generation which we regard as the fern plant. Some ferns have also efficient means of vegetative reproduction as the underground stems of bracken, Pteridium esculentum, and the “sporelings” or “bulbils” on the surface of fronds of the hen-and-chicken ferns, Asplenium bulbiferium.