Bryophytes and Ferns
|Biology - Botany|
Mosses and ferns are frequently seen growing together, particularly in moist, cool places. Many people mistakenly think they are closely related, while they are in fact quite different types of land plants.
Both mosses and ferns were among the first plants to evolve on land. Some similarities as well as some major differences exist in the way these two groups have adapted to the terrestrial environment. The first plants were undoubtedly algae living in water. The best evidence indicates that the green algae were the progenitors of the land plants, probably one similar to this present-day Fritschiella.
Plants in water are surrounded by a medium that supplies many of their basic requirements. Water contains oxygen, carbon dioxide, and minerals necessary for survival and growth. Light penetrates the surface of the water, and the buoyant plants are able to carry on photosynthesis. But most importantly, the cells have a constant supply of water with no exposure to drying air. On land the problems are considerably different. An alga simply thrown on the ground would quickly dry out. With no means of slowing desiccation and no way to absorb water from the soil, the cells would soon die.
For plants to live on land they must be protected from drying out. This protection is provided by a waxy layer, or cuticle, found on the surface of all land plants. The cuticle greatly reduces the evaporation of water from the moist internal tissues. To aid in the exchange of gases through the cuticle, land plants have openings called stomata. Guard cells, which can change the size of the stomata, regulate the exchange of gases and water vapour between the internal tissues and the environment. The cuticle and guard cells retard water loss but do not prevent it. Consequently, water must be taken up by the plant from the soil. At the same time, the plant cannot grow underground because there is no light for photosynthesis.
What all this means is that any plant more than a few cells in length or breadth must develop a plant body in which there is a division of labour: an above-ground part in which photosynthesis takes place and a below-ground part for the absorption of water. There must also be transport between the parts, so that all the cells have water moved upward from the soil and carbohydrates moved downward from the leaves.
There are also problems in reproduction associated with life on land. Drying out of gametes and spores is one problem. Also, water can no longer serve as a major avenue for the transmission of gametes. The mosses and ferns represent different solutions to these various problems of life on land. Let us examine their structure and reproduction in this light, considering the mosses first.
Mosses are members of the division Bryophyta of the kingdom Plantae. Also found in the Bryophyta are the liverworts and the hornworts. As these are less widely known than the mosses, we will concentrate on the mosses. Most mosses are small plants, only millimetres or, at most, centimetres tall. They frequently grow as mats of small, leafy shoots in relatively moist locations. These leafy shoots are the gametophyte, or haploid, generation. The leaves may be only a single cell thick, and the stem is also very simple, lacking true vascular cells characteristic of the xylem or phloem.
Water moves by surface tension up the exterior of the plant and is absorbed through the leaves. This type of water movement is possible because of the small size of the plant. The shoots develop from algal-like filaments of cells, known as protonemata, which are produced by the germination of a spore. Special cells of the protonemata enlarge and undergo cell division, developing into a mature shoot. One protonema gives rise to more than one shoot, and the initiation of shoots does not inhibit further protonemal growth. This is the origin of moss mats, which may grow rapidly and persist over long periods of time.
The protonema of a single spore has been known to cover an area forty centimetres in diameter in several months' time. When the shoot matures, antheridia that will produce sperm and archegonia that will produce eggs develop. Depending on the species, these may form on the same shoot or different shoots. Antheridia are composed of relatively small, densely cytoplasmic cells that form the sperms. These cells occupy the central core of the antheridium and are surrounded by a single layer of non-reproductive, or sterile, cells. Several hundred sperms may form in each antheridium through the process of mitosis. The sperms have two flagella and are highly modified cells.
At maturity, the chromosomes are tightly condensed in a ribbon-shaped nucleus that determines the shape of the sperm. Besides a single, long mitochondrion, the plasma membrane, and the flagella, little else is present in the sperm cell. In the archegonium the sterile cells, again one cell layer thick, are in the form of a flask or bottle. A column of cells differentiates in the centre of the bottle. The bottom cell is the egg, while the ones occupying the neck are called the neck-canal cells.
When the egg is mature, these cells degenerate, and the sterile cells at the top of the neck separate, forming a clear canal through which the sperms swim to the egg. Sperms are released only when water is available for them to swim in. They are actively attracted to the opening in the neck of the archegonium and swim to it. They then swim down the neck to the egg, and although several sperms may reach the egg, only one will enter it.
The fusion of the egg and sperm results in the formation of the zygote. The zygote, being diploid, is the first cell of the sporophyte generation. The zygote divides in place, surrounded by cells of the archegonium. As the embryo develops, one of the first parts formed is the foot. This is believed to help in the transfer of food from the gametophyte to the young embryo. As the embryo grows, cells of the jacket of the archegonium divide. Eventually, the upper part of the archegonium breaks away from the bottom and is carried upward by the developing sporophyte. This structure, known as the calyptra, forms a tight protective cover over the tip of the young sporophyte and prevents the sporophyte from developing prematurely.
The mature sporophyte consists of a foot, seta, and capsule. The seta is usually long and slender, several centimetres in length. The capsule is one to several millimetres long and is involved in the production and discharge of the spores. Cells occupying the centre of the capsule differentiate into the spore mother cells, or sporocytes. These cells undergo meiosis, each producing four haploid spores. An elaborate wall forms around each spore. This wall is very resistant to degradation and protects the cytoplasm and nucleus of the spore. The tip of the capsule is covered by a lid, or operculum. When the spores mature, the operculum falls away, revealing a ring of teeth-like units, the peristome. The tips of the teeth move back and forth in response to changes in moisture content of the air. They close when they are wet or the air is humid. In dry, windy periods they open, and the spores are released and carried long distances by the wind.
With the germination of the spore, the moss life cycle is complete. It is a life cycle in which the gametophyte is the independent generation. It is the gametophyte that we notice when we see a moss plant. The sporophyte is dependent on the gametophyte for all or most of its food. Without the gametophyte, the sporophyte would die. Free water is necessary for the sperm to reach the egg. The spore is adapted to conditions on the land, but the sperm swim as they do in the algae. The gametophyte is successful on land, but it is largely confined to regions of readily available water and no direct competition with other plants, and never reaches large sizes.
In the mosses we see one approach to adaptation to life on land. In the ferns we can study another, more successful solution to the same problem. Ferns are members of the division Tracheophyta, the vascular plants. Members of this division have true vascular tissue and include the gymnosperms and angiosperms as well as the ferns. Ferns are some of the commonest land plants. Most are relatively small plants with large compound leaves a half to one metre in length. They grow on the forest floor and along the edges of fields and on the banks of streams. Because of their ability to live in reduced amounts of light, they make excellent house plants. Some have adapted to life in the water and are floating plants with reduced stems. They frequently form coverings on ponds and streams. Still other ferns have a true tree form. These are tree ferns found growing in New Zealand, where they form forests of plants 10 metres or more in height.
The most common body form found in ferns, however, is that of an underground stem or rhizome, large compound leaves, and small wiry roots. The leaves of ferns vary enormously in size and shape. The fern leaf most familiar to us is a large compound leaf. It consists of a stemlike stipe and a blade, which may be divided into pinnae that may, in turn, be divided in a number of ways. When such compound leaves are formed, frequently underground, the stipe and pinnae are tightly coiled. The leaf uncoils as the cells on the lower side elongate more rapidly than those on the upper side. The coiled frond, called a fiddlehead, is an interesting and frequently beautiful object as it uncoils in the spring.
Not all ferns have elaborate compound leaves. Some 10 percent of the species have simple blades not divided into pinnae. The compound leaf is considered more primitive than the simple, bladed leaves. Stems of most ferns are either underground or short, erect structures near ground level. In a few species, as we have seen, the erect stems may be barrel-shaped or quite tall, as in the tree ferns. The tip of the stem is surrounded by the developing leaves. Frequently, several years' growth of leaves form underground around the stem tip.
The rhizomes of ferns play an important role in the spread of the plant. Growing through the soil, they are less subject to drying out or being eaten by animals. By branching, they can spread rapidly into an area and provide stiff competition to grasses and other plants. Ferns have small, inconspicuous roots arising from the stem near the leaves. The differences between the mosses and the ferns are much more profound than the size and organisation of the parts.
We said that when we looked at a moss, we were seeing the gametophyte generation. When we look at a fern, we are seeing the sporophyte generation, or diploid generation. Moreover, we are seeing a plant that is truly adapted to the land. The stem, leaves, and roots of a fern contain true vascular tissue, tissue specifically designed to conduct water, minerals, and organic materials throughout the plant. It is these tissues that permit the ferns and other vascular plants to grow to sizes more than a few millimetres high and to live in the vast variety of environments that they do.
In all the vascular land plants, two types of tissue systems are present: xylem that conducts water and minerals and phloem that conducts organic materials, such as carbohydrates, through the plant. In the mosses we see land plants that are small enough that they can survive in favoured locations without special tissues for internal transport of materials from one part to another. In the ferns we see plants with elaborate tissue systems that carry water and minerals from the soil, and carbohydrates and other organic materials from the leaves, to all parts of the plant. Such a system permits the true exploitation of the land as an environment for plant growth.
But what of reproduction? How is this accomplished in ferns, and how is it influenced by life on land? Let us start with the sporophyte generation and the production of spores. The haploid spores are produced in sporangia located on the leaves; either ordinary vegetative leaves or highly differentiated leaves. The sporangia are often grouped together in clusters, the sori. Covering the sori in many ferns is a thin outgrowth of the leaf known as an indusium. The spore mother cells present in the centre of the sporangia undergo meiosis, and each produces four haploid spores. These form elaborate and highly resistant walls around themselves. They are light and can survive long periods of dryness.
A fern sporangium is shaped like an ancient helmet, with a ribbed back composed of a row of cells with specially thickened walls. The inward curving of these cells causes the sporangium to rupture, and the spores are carried back into the top portion of the sporangium. Next, a rapid, vigorous motion returns the cells to their original shape and throws the spores in the air where they may be carried for miles by the wind. The spores germinate rapidly on moist soil, and a small gametophyte, or haploid plant, develops. This is known as the prothallus and frequently forms a heart-shaped plate of cells. Antheridia that produce the sperms and archegonia that produce the eggs develop on the lower surface of the prothallus. The antheridia form on the older parts while the archegonia form on the new portions near the apical notch.
The form of the antheridia in ferns is similar to that of the mosses, and the functions are the same. Fern sperm are multiflagellate and swim actively. Fern archegonia are also similar in form and function to those of the mosses. The egg is at the base of a column of cells that degenerate to form a path down which the sperm swim. A sperm fuses with the egg and the zygote results. This re-establishes the sporophytic, or diploid, generation. The zygote soon begins to divide to form an embryo. A root and shoot apex, a first leaf, and a foot differentiate in the embryo. For a short time the sporophyte draws nourishment from the gametophyte through the foot, but it soon becomes independent. Thus, in ferns, the sporophyte is the large plant that is familiar to us while the gametophyte is small and rarely seen.
When the life cycle of mosses and ferns are compared, we see some striking differences and some striking similarities. The similarities include the form and function of the antheridia and archegonia, the presence of motile sperm requiring free water for their passage to the egg. Both mosses and ferns produce haploid spores that can be carried long distances and survive extreme environmental conditions. The differences include the relative size, longevity, and importance of the sporophyte and gametophyte generations. In mosses the gametophyte is dominant. In ferns the sporophyte is dominant. But the biggest difference is in the nature of the sporophyte itself. In the mosses the sporophyte is short lived and simple in structure, having no vascular tissue and little photosynthetic capacity. In the ferns the sporophyte is a complex, long-lived structure with well-developed vascular tissue. It is because of the well-developed sporophyte that the ferns and all other vascular plants have been able to so successfully invade the land.
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