The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

full, when they are ripe, of a dust of barely visible spores. The whole apparatus, stalk and capsule, is called a sporogonium, or sporegenerator. The spores are shed from the sporogonium and are blown about, much as they are in ferns. When they germinate they give rise at first to a protonema—a branching feltwork of extremely delicate green threads having no very definite arrangement. This elementary, groping web is moss-tissue feeling about, so to speak, for the best and most comfortable situation in which to settle down. Here and there the protonema strikes lucky ; it throws special branches up into the air to become stems and leaves of a primitive kind and sprouts root-like hairs down into the soil. In this way the protonema gives rise

Fig. 110. More animal than vegetable in their restless actwity—two sperms from a Fern (left) and a Moss (right).

to several moss-plants, after which it generally dies, and its offshoots, the moss-plants, lose their organic connection with each other ; they become discrete and separate things although they originated as parts of the same living body. So the moss as we know it is formed with simple roots and stems and leaves, and proceeds to live on water and salts and air and light in the manner that we have already described.

After a time this moss-plant develops sexual organs, very like the sexual organs of a fern prothallus and quite unlike the flowers of a higher plant. They appear in tiny clusters at the ends of the main stem or of its branches. As with the prothallus there are male organs producing active spermatozoa and female organs shaped like

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THE SCIENCE OF LIFE

CHAPTER 5

tiny flasks, with necks down which the spermatozoa swim to the waiting egg-cells. Then the fertilized egg-cell, like the egg-cell of a fern and unlike the egg-cell of a flowering plant, does not become part of a hard seed but begins to grow and develop at once. It grows into the stalk and capsule that we have already noted—the sporogonium. It gives rise to this mass of tissue that sprouts up parasitically from the moss-plant, sucking nourishment from it, but which is nevertheless not a branch but a child of that plant.

Here, again, then, we have an alternation of generations—the sporogonium gives rise to spores, each spore gives rise through a protonema to one or more moss-planis, the moss-plant gives rise to spermatozoa and egg-cells, and the fertilized egg-cell gives rise to a sporogonium again. The chief difference between a moss and a fern is that in the latter the sporebearing phase of the cycle is larger and more elaborately organized than the gamete-bearing phase, while in the former the reverse is the case. But there is another important difference, for mosses lack the “vascular tissue”’ of the stem—the installation of pipes along which fluids are driven from roots to leaves and from leaves to roots. Their transport arrangements are very primitive and inefficient, and for this reason they never develop any impressive architecture; the leaves must stay close to the soil, and a stem that soars for any considerable distance is quite impossible. The tallest and stateliest mosses, found in New Zealand, reach a height of about twenty inches.

Related to the mosses are the liverworts, similar but even humbler in structure. The two together are spoken of as the Bryophyta, or moss-like plants.

Now here is a curious and fascinating series of homologues. We have in the fern-cycle (a) fern-plant, (b) spores, (c) prothallus, (d) spermatozoa and ovum, (e) fertilized ovum and so back to (a) fern-plant. In the moss we have the equivalent of the fernplant in (a) the subordinated sporogonium which produces (2) the moss spores and so leads with the insertion of a little vegetative spreading and breaking up to (c) the mossplant, which is plainly not the equivalent of a fern-plant at all, and so on to (d), (e) and back to (a) the sporogonium. So the fernplant is an exalted sporogonium and the moss-plant is not the equivalent (homologue)