Valisneria spiralis is a very interesting case. It is a native of European rivers, and the female flower has a long spiral stalk which enables it to float on the surface of the water. The male flowers have no stalks, and grow low down on the plant. They soon, however, detach themselves altogether, rise to the surface, and thus are enabled to fertilise the female flowers among which they float. The spiral stalk of the female flower then contracts and draws it down to the bottom of the water so that the seeds may ripen in safety. Many plants throw or bury their seeds. The sensitive plants close their leaves when touched, and the leaflets of Desmodium gyrans are continually revolving. I have already mentioned that the spores of sea-weeds swim freely in the water by means of cilia. Some microscopic plants do so throughout a great part of their lives. A still lower group, the Myxomycetes, which resemble small, more or less branched, masses of jelly, and live in damp soil, among decaying leaves, under bark and in similar moist situations, are still more remarkably animal like. They are never fixed, but in almost continual movement, due to differences of moisture, warmth, light, or chemical action. If, for instance, a moist body is brought into contact with one of their projections, or "pseudopods," the protoplasm seems to roll itself in that direction, and so the whole organism gradually changes its place. So again, while a solution of salt, carbonate of potash, or saltpetre causes them to withdraw from the danger, an infusion of sugar, or tan, produces a flow of protoplasm towards the source of nourishment. In fact, in the same way it rolls over and round its food, absorbing what is nutritious as it passes along. In cold weather they descend into the soil, and one of them (Oethalium), which lives in tan pits, descends in winter to a depth of several feet. When about to fructify it changes its habits, seeks the light instead of avoiding it, climbs upwards, and produces its fruit above ground. IMPERFECTION OF OUR KNOWLEDGE The total number of living species of plants may be roughly estimated at 500,000, and there is not one, of which we can say that the structure, uses, and life-history are yet fully known to us. Our museums contain large numbers which botanists have not yet had time to describe and name. Even in our own country not a year passes without some additional plant being discovered; as regards the less known regions of the earth not half the species have yet been collected. Among the Lichens and Fungi especially many problems of their life-history, some, indeed, of especial importance to man, still await solution. Our knowledge of the fossil forms, moreover, falls far short even of that of existing species, which, on the other hand, they must have greatly exceeded in number. Every difference of form, structure, and colour has doubtless some cause and explanation, so that the field for research is really inexhaustible. FOOTNOTES: