How is it, then, that these moss stems, though each independent, grow in such a dense mass? Partly because moss multiplies so rapidly that new stems are always thrusting themselves up to the light, but chiefly because the stems were not always separate, but in very early life sprang from a common source.
If, instead of bringing the moss home and tearing it apart, you went to a spot in the wood where fresh moss was growing, and looked very carefully on the surface of the ground or among the water of a marsh, you would find a spongy green mass below the growing moss, very much like the green scum on a pond. This film, some of which I have brought home, is seen under the microscope to be a mass of tangled green threads (t, Fig. 34, p. 88) like those of the Confervæ (see p. 79), composed of rows of cells, while here and there upon these threads you would find a bud (mb, Fig. 34) rising up into the air.
This tangled mass of green threads, called the protonema, is the first growth, from which the moss stems spring. It has itself originated from a moss-spore; as we shall see by and by. As soon as it has started it grows and spreads very rapidly, drinking in water and air through all its cells and sending up the moss buds which swell and grow, giving out roots below and fine stems above, which soon become crowded with leaves, forming the velvety carpet we call moss. Meanwhile the soft threads below die away, giving up all their nourishment to the moss-stems, and this is why, when you take up the moss, you find each stem separate. But now comes the question, How does each stem live after the nourishing threads below have died? It is true each stem has a few hairy roots, but these are very feeble, and not at all like the roots of higher plants. The fact is, the moss is built up entirely of tender cells, like the green cells in the lichen, or in the film upon the pond. These cells are not shut in behind a thick skin as in the leaves of higher plants, but have every one of them the power to take in water and gases through their tender membrane.
I made last night a rough drawing of the leaf of the feathery moss put under the microscope, but you will see it far better by putting a leaf with a little water on a glass slide under the covering glass and examining it for yourself. You will see that it is composed of a number of oval-shaped cells packed closely together (c Fig. 33), with a few long narrow ones mr in the middle of the leaf forming the midrib. Every cell is as clear and distinct as if it were floating in the water, and the tiny green grains which help it to work up its food are clearly visible.
Fig. 33.
Moss-leaf magnified. (From life.)
Showing the cells c, each of which can take in and work up its own food. mr, Long cells of the mid-rib.
Each of these cells can act as a separate plant, drinking in the water and air it needs, and feeding and growing quite independently of the roots below. Yet at the same time the moss stem has a great advantage over single-celled plants in having root-hairs, and being able to grow upright and expose its leaves to the sun and air.
Now you will no longer wonder that moss grows so fast and so thick, and another curious fact follows from the independence of each cell, namely, that new growths can start from almost any part of the plant. For example, pieces will often break off from the tangled mass or protonema below, and, starting on their own account, form other thread masses. Then, after the moss stems have grown, a new mass of threads may grow from one of the tiny root-hairs of a stem and make a fresh tangle; nay, a thread will sometimes even spring out of a damp moss leaf and make a new beginning, while the moss stems themselves often put forth buds and branches, which grow root-hairs and settle down on their own account.