Single-celled green plants growing
and dividing (Pleurococcus).
(After Thuret and Bornet.)
Before we examine them, however, look for a minute at a small drop of this greenish film which I have taken from the rain-water taken outside. I have put some under each microscope, and those who can look into them will see the slide almost covered with small round green cells very much like the yeast cells we saw when studying the Fungi, only that instead of being colourless they are a bright green. Some of these cells will I suspect be longer than others, and these long cells will be moving over the slide very rapidly, swimming hither and thither, and you will see, perhaps for the first time, that very low plants can swim about in water. These green cells are, indeed, the simplest of all plants, and are merely bags of living matter which, by the help of the green granules in them, are able to work up water and gases into nourishing food, and so to live, grow, and multiply.
There are many kinds of these single-celled plants in the world. You may find them on damp paths, in almost any rain-water butt, in ponds and ditches, in sparkling waterfalls, along the banks of flowing rivers, and in the cold clear springs on the bleak mountains. Some of them take the form of tangled threads[1] composed of long strings of cells, and these sometimes form long streamers in flowing water, and at other times are gathered together in a shapeless film only to be disentangled under a microscope. Other kinds[2] wave to and fro on the water, forming dense patches of violet, orange-brown, or glossy green scum shining in the bright sunlight, and these flourish equally in the ponds of our gardens and in pools in the Himalaya mountains, 18,000 feet above the sea. Others again[3] seize on every damp patch on tree trunks, rocks, or moist walls, covering them with a green powder formed of single plant cells. Other species of this family turn a bright red colour when the cells are still; and one, under the name of Gory Dew,[4] has often frightened the peasants of Italy, by growing very rapidly over damp walls and then turning the colour of blood. Another[5] forms the "red snow" of the Arctic regions, where it covers wide surfaces of snow with a deep red colour. Others[6] form a shiny jelly over rocks and stones, and these may be found almost everywhere, from the garden path to the warm springs of India, from the marshes of New Zealand up to the shores of the Arctic ocean, and even on the surface of floating icebergs.
The reason why these plants can live in such very different regions is that they do not take their food through roots out of the ground, but suck in water and gases through the thin membrane which covers their cell, and each cell does its own work. So it matters very little to them where they lie, so long as they have moisture and sunlight to help them in their work. Wherever they are, if they have these, they can take in carbonic acid from the air and work up the carbon with other gases which they imbibe with the water, and so make living material. In this way they grow, and as a cell grows larger the covering is stretched and part of the digested food goes to build up more covering membrane, and by and by the cell divides into two and each membrane closes up, so that there are two single-celled plants where there was only one before. This will sometimes go on so fast that a small pond may be covered in a few hours with a green film formed of new cells.
Now we have seen, when studying mushrooms, that the one difference between these green plants and the single-celled Fungi is that while the green cells make their own food, colourless cells can only take it in ready-made, and therefore prey upon all kinds of living matter. This is just what happens in the lichens; and botanists have discovered that these curious growths are really the result of a partnership between single-celled green plants and single-celled fungi. The grey part is a fungus; but when it is examined under the microscope we find it is not a fungus only; a number of green cells can be seen scattered through it, which, when carefully studied, prove to be some species of the green single-celled plants.
Here are two drawings of sections cut through two different lichens, and enormously magnified so that the cells are clearly seen. 1, Fig. 30 is part of a hairy lichen (1, Fig. 28), and 2 is part of a leafy lichen (2, Fig. 28). The hairy lichen as you see has a row of green cells all round the tiny branch, with fungus cells on all sides of them. The leafy lichen, which only presents one surface to the sun and air while the other side is against the tree, has only one layer of green cells near the surface, but protected by the fungus above.
Fig. 30.
Sections of Lichens. (Sachs.)
1, Section of a hairy lichen, Usnea barbata.
2, Section of a leafy lichen, Sticta fuliginosa.
3, Early growth of a lichen.
gc, Green cells. f, Fungus.