Mosses and liverworts

The mosses and liverworts are a group of simple green plants found throughout the world— they are among the most primitive of land plants. Classified botanically as bryophytes, the species number more than 9,000 for mosses and about 8,000 for liverworts. They require water to complete their life cycles; for this reason most of them frequent damp, shady habitats. Some bryophytes (such as the moss Polytrichum) can, however, withstand periods of drought.

A liverwort thallus (A) has no leaves, stem, or roots. It is simply arranged into upper layers of chloroplast-containing cells and lower, empty ones. Fine rhizoids attach it to the soil. A moss stem (B) has a primitive vascular arrangement of non-nucieated, water-conducting cells, surrounded by nucleated ones which carry organic compounds.

Plant structure

Most bryophytes form cushions or layers of vegetation no more than 6 inches (15 centimeters) high for mosses and less than .5 inch (1.4 centimeters) high for liverworts. But the largest moss, the Australian species Dawsonia, grows up to 3 feet (91 centimeters) tall. All are attached to the ground or to a substratum, such as tree bark, by rootlike threads called rhizoids. In liverworts rhizoids generally comprise one cell each, whereas in mosses they are made up of several cells.

Nearly all bryophyte structures that grow above ground contain chlorophyll, as well as other pigments, and are green; the chlorophyll is located in disk-shaped chloroplasts clustered in certain cells. The moss plant consists of a system of shoots along which leaves are arranged alternately. In most moss species the leaves have a characteristic central midrib, but otherwise vary enormously in shape and size, from the pointed, tooth-edged leaves of Orthotrichum to the broad leaves of Mnium, which have unbroken edges and long hairlike extensions at their tips.

Liverworts are more diverse in appearance than mosses and are described as either thal-loid or leafy. A thalloid liverwort consists of a flat, platelike structure (the thallus) with rhizoids on its underside. The thallus may have a scalloped edge, as in Lunuiaria, or may be deeply divided into lobes, as in Conoce-phaium. In leafy liverworts, which comprise more than 80 per cent of the total number of species, the thallus is divided into leaflike structures arranged up the “stem” in ranks of two or three. Most liverwort leaves have no midrib.

The spore-filled capsules at the tip of the stalks of mosses form the sporophyte generation.

Alternation of generations

Bryophytes are of considerable botanical interest because they are the most primitive plants to display alternation of generations. According to this system, the plant’s life cycle takes place in two stages or generations: the gametophyte generation, which is responsible for producing sex cells, and the sporophyte generation, whose function is to make and disperse large numbers of asexual spores. In flowering plants the gametophyte generation is confined to specific parts of the flower, but in bryophytes it takes the form of the obvious main green “body” of the plant.

Anatomically, bryophyte sex organs vary little between species. Male sex cells (sperm) are made in antheridia, and female sex cells (called eggs) in archegonia. The antheridia are thin-walled sacs, which, when mature, contain sperm, each of which bears a pair of long whiplike flagella. The fully developed arche-gonium containing the egg is usually flaskshaped with a long neck.

For sexual reproduction to take place, a sperm must swim to an egg in an archego-nium and fuse with it. Water is thus essential to the process of fertilization. The journey of the male cells is facilitated by a “spreading agent,” which when released from the anther-idium, lowers the surface tension of the water and thus aids the flow of the sperm. The fertilized egg develops into a sporophyte, which is incapable of an independent existence. Instead it grows as a semiparasite on the gametophyte, although in some bryophytes, such as the unusual Anthoceros, it contains chlorophyll and so is capable of photosynthesis. The typical sporophyte of a moss or liverwort consists of a flattened foot, a long stalk or seta, and a capsule containing thousands of spores. The methods of spore release from the capsule vary, but most depend on the drying of the capsule and an inbuilt spore-ejector mechanism.

Spores that fall on a suitable environment germinate and develop into a new gametophyte generation. As well as reproducing sexually, this gametophyte also has the capacity for asexual reproduction—that is, leaves or other parts broken off the plant may develop into new, independent gametophytes.

The capsules on a mature moss plant release spores. A spore develops into a threaded mesh (pro-tonema), which, from a bud, gives rise to a male or female gametophyte plant. Club-shaped sperm are released from the antheridia of the male plant and swim to the archegonia on the female plant where they fertilize the egg. Each egg develops into an embryo that eventually becomes the spore-producing capsule.

Evolution and ecology

The evolution of bryophytes is not well understood. Botanists are still not certain whether bryophytes evolved from a kind of alga or whether they represent a retrograde evolution from pteridophytes—the ferns and their allies.

Ecologically, bryophytes are important because they can survive in inhospitable conditions and because they are often among the * pioneer species that colonize land that has been laid waste by catastrophes such as fire or earthquake. Once established, bryophyte colonies inhibit soil erosion and promote the retention of soil moisture. In swamps, the Sphagnum moss plays a leading role, building organic soil and eventually making it sufficiently firm and nutrient-rich to support shrub and tree species.

Because many bryophytes have specific nutrient requirements, it is possible to tell whether habitats are acid or alkaline, or contain high levels of nitrogen or phosphorus, by the types of bryophytes that inhabit them. Similarly, bryophyte species can, by their presence or absence, indicate whether there is a high level of sulfur dioxide in the atmosphere.

Cup-shaped structures found commonly on liverworts contain special bodies called gemmae. These structures are organs of vegetative reproduction. The gemmae are distributed to new habitats by splashes of water into the cups, or by attaching to the feet or fur of passing animals. In a suitable environment they develop into a new gametophyte plant.