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:

[19] Thomson.

[20] Lu