106 NEW ZEALAND PLANTS. 



and inhabited by a fresh-water alga, which, however, does its host 

 no damage, but, like a respectable lodger, probably pays for its accom- 

 modation. 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 niiuht easilv be mis- 



tl 



taken for a small rush. Some of the lakes in the Waikato and in the 

 Taupo districts also contain another species of quillwort (7. Kirkii). 

 Indeed, it is highly probable these plants are commoner than is gene- 

 rally supposed. 



THE POND-WEEDS AND WATER-MILFOILS. 



In most parts of New Zealand one may see, floating on the surface 

 of slow-flowing rivers or calm sheets of water, the oval brown leaves 

 of some species or other of pond-weed (Potamogeton), the commonest 

 of which is P. Cheesemannii. Besides the above leaves there are 

 others which live always submerged, and which differ considerably 

 from the floating ones. These submerged leaves are very thin, erect, 

 more or less ribbon-shaped, and are also extremely numerous. Since 

 there can be no danger of want of water, such leaves are entiiely 

 without any protection on that score ; on the contrary, they are so 

 constructed as to be able to absorb water over their whole surf ad- 

 just like the filmy ferns of Chapter III, and thereby secure at the 

 same time the oxygen which it contains. Their ribbon-like shape is 



*/ t? - 



well adapted to withstand damage from the currents of water, while 

 sufficient extent of leaf-surface is provided by increase in number of 

 leaves. It is also an interesting fact that these submerged leaves are 

 similar to the early seedling ones of the pond- weed, and that this par- 

 ticular shape of leaf is common even amongst the land members of 

 that great division of plants to which Potamogeton belongs. Some 

 of the pond-weeds also never produce floating leaves e.g.. Potamogeton 

 ochreatus and P. pectinatus.* 



* Found frequently in slightly brackish \\ater. 



