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 entirely without any protection on that score; on the contrary, they are so constructed as to be able to absorb water over their whole surface 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 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 particular 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.^*

The water-milfoils (Myriophyllum) differ from the pond-weeds in that they have no floating leaves, but boldly raise their upper portions above the water-surface. They agree, however, in the fact that the aerial leaves differ from the submerged ones. This is the more interesting because no line of demarcation on the erect stem separates the two except the water-surface—that is to say, the same tissue can change its leaf-form according to change of outer circumstances. The water-milfoils are graceful, feathery - looking plants, with the leaves frequently given off, four or more, from the same height round the stem. The submerged leaves are cut into fine segments, a very common occurrence in many water-plants, whereas the aerial leaves are broader and much less cut. Some of the water-milfoils are bog-plants rather than aquatics—e.g., M. Votschii.^*

It is highly probable that all seed-bearing aquatic plants are descended from land plants, which took to the water through competition with rivals better suited than they to their original stations. This is no place to discuss this question, but it may be pointed out that some plants can live equally well on land and in water, and even do not mind being submerged. The water-starworts (Callitriche) are examples.

In many streams the native aquatic vegetation has been ousted by the introduced watercress or the American water-weed (Elodea canadensis).It is interesting to note how much the former varies in both leaf and flower when growing on dry ground by the sides of ditches. Both plants are noxious weeds, and it costs the country much money annually to keep open the watercourses where they flourish.