MOSSES simplest of land plants

Mosses are the simplest true land plants which are able to withstand drought and grow in dry habitats, such as rock faces and stone walls. However the vast majority of species are to be found associated with freshwater along stream sides, in bogs or in moist woodland and tropical rainforest. The 14,500 species of moss are typically small, low-growing or mat-forming plants a tew centimetres in height. The Australian genus Daw-sonia may, however, reach 70cm (27.5111) in length. In northern Europe, one of the largest mosses Polytriehum commune may grow to 20cm (9111) in acid bogs, while in New Zealand a length of 150cm (5<)in) has been claimed for the same species growing under water. A number of species such as Fontinalis antipyretica are only found in running or still water and this species may grow to almost 100cm (40111).

Structure and life cycle

Before we go further in studying the typical moss plant and comparing it with the structures seen in higher plants we must be aware of one important difference. This is a result of the phenomenon of alternation of generations, first seen in the algae. In mosses the generation which forms the typical moss plant is the haploid gametophyte. The spore-producing diploid sporophyte is normally comparatively small, grows from the leaves of the gametophyte and is partly parasitic on it. In ferns and flowering plants the obvious plant body represents the sporophyte generation, while the gametophyte has become reduced. This important difference should always be remembered when comparing mosses with higher plants.

Taking the spore as the starting point of the life cycle, this first germinates into a delicate branched filament rather like a filamentous alga. This is known as the protonema. When this has developed sufficiently, buds are produced from which the plant proper grows. This consists of a distinct stem and leaves. The stem, which may be branched, has small rootlike multicellular rhizoids at its base which have a water-absorbing function. A strand of water-conducting tissue is also present in the stem. The leaves are usually only one cell thick except for a distinctive midrib which is seen in some genera including Bryuin and Milium. In Polytrichia)! the midrib occupies much of the leaf and the single-celled blade is reduced to a narrow strip of cells on either side. A cuticle is also present in these genera. Here we see that a simple system for translocation (movement of water and food supplies) has developed, although it is of rather limited efficiency, for if the surrounding air is too dry, wilting will still take place. To compensate for this, water can also be conducted externally in leaf ducts known as capillary channels. Mineral salts are also obtained externally from airborne dust.

An important difference also exists between mosses and higher plants in their resistances to drought. While higher plants survive by their ability to retain water and a turgid cell structure, mosses are able to lose almost all their water, appearing completely dead and dehydrated. However, as soon as water is applied, they quickly reflate. This enables genera such as Tortella, which has no cuticle, to survive on walls which are dry for much of the year. Under experimental conditions some mosses have survived temperatures of io0°C (2i2°F) for short periods.

The main purpose of the gametophyte is to produce the male and female gametes for which specialized structures, the antheridia and archegonia, have evolved. These structures are also characteristic of liverworts, which with mosses make up the bryophytcs. The male gametes (antherozoids) are produced in the antheridia which appear as minute saclike structures, each borne on a short stalk and accompanied by sterile club-shaped paraphyses. The archegonia, on the other hand, are flask- or bottle-shaped, having a narrow neck of cells with a rounded base known as the venter, in which the single egg cell develops. When this is fully developed the cells in the centre of the neck break down, allowing entry of the gametes which swim from the antheridia. One of these fuses with the egg and fertilization is complete. Specialized leaves not only protect the antheridia and archegonia but also hold water to facilitate the swimming of the gametes. As, in some species, plants may be either male or female exclusively, the gametes may have some distance to swim. Water is therefore vital to the reproduction of mosses.

Once fertilized, the egg is able to develop into the sporophyte. This consists of an organ for absorbing moisture from the parent game-tophytc, a stalk or seta, and a terminal capsule in which the spores are produced. As the capsule matures, the seta lengthens slowly usually to about I-2cm (o.4-o.8in). The ripe capsule may be pear-shaped or cylindrical with a distinct cap. This cap is then shed, leaving behind a single or double ring of teeth. These are known as the peristome and function in regulating the release of spores from the capsule. Spore release in wet weather is prevented by swelling of the peristome. In some species the peristome may act by actively flicking out the spores as the peristome rings dry. The cells of the sporophyte, although obtaining water from the gametophyte by means of the absorbing body and a conducting strand, also contain chlorophyll. The sporophyte is therefore at least partly self-sufficient as it can produce its own food. Cuticle and stomata, similar in form and function to higher plants, are also present in some genera.

Sphagnum and Andreaea

Two genera do not fit into the general scheme described above. These are Sphagnum, the ‘bog mosses’, and Andreaea, a small reddish-black moss which forms small patches on rocks in exposed mountainous or arctic regions. Both genera show adaptations to their environment as well as other peculiarities. In both, the protonema is a disc and not a filament. From this Andreaea develops a peculiar rhizoid complex which anchors the plant to the rocks. Sphagnum on the other hand has no need of a strong foothold as it grows in a mass in bogs. No central strands are present in shoots or leaves, instead the outer layer of cells contain pores which allow the direct entry of water and the whole plant acts as a sponge. Both Sphagnum and Andreaea have distinctive capsules. In Sphagnum this is raised on a short stalk of the game-tophyte, is round in shape and has no peristome. When ripe the walls shrink until the cap is blown off by air pressure and the spores released with explosive force. Andreaea has no cap at all. Instead, as the ovoid capsule dries it splits along four longitudinal lines of weakness, rather like a Chinese lantern. If the capsule becomes wet, however, the sections act as valves and the capsule is closed.

Ecology

Although mosses are often overlooked there are a few types of plant community where they form a significant element. In two examples, arctic tundra and in the various types of peat bog, mosses are dominant. The bog mosses are again of particular interest—nine or ten species each with a specific environmental requirement are involved in the various stages of development of highly acidic peat bogs. Here these plants dominate to the exclusion of most other species. This succession involves the gradual building of moss hummocks from bog pools. Specific species are associated with the wettest condition encountered in the pools while others such as Sphagnum papillosum are the main hummock formers. As these hummocks become drier they are colonized by heathland species such as heather, but over a considerable period these become moribund and the hummock breaks down to become a pool once more, while all around new hummocks are being formed. The growth of mosses may even raise the bog above the adjacent land surface to form a vast ‘raised bog’ which is convex in cross-section.

Mosses are of great use to the plant ecologist as indicator species. Certain species are characteristic of acid soil while others are only found in calcareous conditions. Others are indicators of nitrate and phosphate enrichment.

Evolution

Although it is tempting to consider the bryo-phytes in an evolutionary sequence between the algae and ferns, there is little evidence to substantiate this as no living or fossil intermediates — ‘missing links’—have been found. Similarly, no intermediates exist between mosses and liverworts, and fossil evidence suggests that these groups were distinct at the end of the Palaeozoic (over 200 million years ago). There is, however, biochemical and structural evidence to link the green algae (chlorophytes), bryophytes and higher plants. An apparent link with ferns may be found in their gametophyte which superficially resembles a thalloid liverwort and bears antheridia and archegonia. An interesting modern theory suggests that bryophytes should be regarded as simplified descendants of one of the fernlike pteridophytes rather than a more advanced descendant of algae.

The sphagnum bog

Sphagnum starts in bog pools, and after a time other species begin forming hummocks and the bog surface dries out a little. Such hummocks provide footholds for higher plants like the rushes seen here, and also the red-leaved insect-eating sundews just visible. Bogs of this sort are apt to rise steadily, sometimes above the level of the sur- rounding land. If eventually they dry out the sphagnum disappears. After many centuries peat is formed from such bogs.

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