New Zealand Plants and their Story
Take the case of the floating water-fern, Azolla rubra. The red masses of this curious plant, covering still pools so thickly that one might think them dry land, must be known to all. The outer surface of the leaves is covered with minute excrescences, so that they cannot be wetted, and, in consequence, drops of water frequently begem them, glittering in the sunlight like diamonds. An individual plant is quite small, and consists of a thin, much-branched stem, putting down roots into the water from its under-surface, and bearing overlapping leaves. Each leaf consists of two lobes, which, except on close examination, look like separate leaves. Each lobe is adapted for a totally different condition of life, so there is a distinct division of labour in the one leaf. The upper lobes are comparatively thick, provided with leaf-green, and are therefore food-producers, and they are never submerged. Each contains a large cavity full of slime, page 106and inhabited by a fresh-water alga, which, however, does its host no damage, but, like a respectable lodger, probably pays for its accommodation. The lower lobe is partly submerged, and quite thin, so that it can absorb water. Moreover, the close arrangement of the leaves as a whole furnishes cavities where air can lodge, and so provides the necessary buoyancy for the floating plant.
To see other aquatic ferns the town-dweller must go much farther afield, visiting those solitary lakelets far in the mountainous region of the South Island that are traces of the ice-plough of ancient glaciers. On the gravelly beds of such cool waters lives the alpine quillwort (Isoetes alpinus), looking more like a tiny rush than a fern; and here, too, but in the deeper water and on a more muddy bottom, is the home of Pillularia novae-zealandiae, which also might easily be mistaken for a small rush. Some of the lakes in the Waikato and in the Taupo districts also contain another species of quillwort (I. Kirkii).Indeed, it is highly probable these plants are commoner than is generally supposed.