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25.4A: Seedless Vascular Plants - Biology

25.4A: Seedless Vascular Plants - Biology


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Seedless vascular plants, which reproduce and spread through spores, are plants that contain vascular tissue, but do not flower or seed.

Learning Objectives

  • Evaluate the evolution of seedless vascular plants

Key Points

  • The life cycle of seedless vascular plants alternates between a diploid sporophyte and a haploid gametophyte phase.
  • Seedless vascular plants reproduce through unicellular, haploid spores instead of seeds; the lightweight spores allow for easy dispersion in the wind.
  • Seedless vascular plants require water for sperm motility during reproduction and, thus, are often found in moist environments.

Key Terms

  • gametophyte: a plant (or the haploid phase in its life cycle) that produces gametes by mitosis in order to produce a zygote
  • sporophyte: a plant (or the diploid phase in its life cycle) that produces spores by meiosis in order to produce gametophytes
  • tracheophyte: any plant possessing vascular tissue (xylem and phloem), including ferns, conifers, and flowering plants

Seedless Vascular Plants

The vascular plants, or tracheophytes, are the dominant and most conspicuous group of land plants. They contain tissue that transports water and other substances throughout the plant. More than 260,000 species of tracheophytes represent more than 90 percent of the earth’s vegetation. By the late Devonian period, plants had evolved vascular tissue, well-defined leaves, and root systems. With these advantages, plants increased in height and size and were able to spread to all habitats.

Seedless vascular plants are plants that contain vascular tissue, but do not produce flowers or seeds. In seedless vascular plants, such as ferns and horsetails, the plants reproduce using haploid, unicellular spores instead of seeds. The spores are very lightweight (unlike many seeds), which allows for their easy dispersion in the wind and for the plants to spread to new habitats. Although seedless vascular plants have evolved to spread to all types of habitats, they still depend on water during fertilization, as the sperm must swim on a layer of moisture to reach the egg. This step in reproduction explains why ferns and their relatives are more abundant in damp environments, including marshes and rainforests. The life cycle of seedless vascular plants is an alternation of generations, where the diploid sporophyte alternates with the haploid gametophyte phase. The diploid sporophyte is the dominant phase of the life cycle, while the gametophyte is an inconspicuous, but still-independent, organism. Throughout plant evolution, there is a clear reversal of roles in the dominant phase of the life cycle.


Ferns and Other Seedless Vascular Plants

By the late Devonian period, plants had evolved vascular tissue, well-defined leaves, and root systems. With these advantages, plants increased in height and size. During the Carboniferous period, swamp forests of club mosses and horsetails—some specimens reaching heights of more than 30 m (100 ft)—covered most of the land. These forests gave rise to the extensive coal deposits that gave the Carboniferous its name. In seedless vascular plants, the sporophyte became the dominant phase of the lifecycle.

Water is still required for fertilization of seedless vascular plants, and most favor a moist environment. Modern-day seedless tracheophytes include club mosses, horsetails, ferns, and whisk ferns.


Vascular Tissue: Xylem and Phloem

The first fossils that show the presence of vascular tissue date to the Silurian period, about 430 million years ago. The simplest arrangement of conductive cells shows a pattern of xylem at the center surrounded by phloem. Xylem is the tissue responsible for the storage and long-distance transport of water and nutrients, as well as the transfer of water-soluble growth factors from the organs of synthesis to the target organs. The tissue consists of conducting cells, known as tracheids, and supportive filler tissue, called parenchyma. Xylem conductive cells incorporate the compound lignin into their walls, and are thus described as lignified. Lignin itself is a complex polymer that is impermeable to water and confers mechanical strength to vascular tissue. With their rigid cell walls, the xylem cells provide support to the plant and allow it to achieve impressive heights. Tall plants have a selective advantage by being able to reach unfiltered sunlight and disperse their spores or seeds further away, thus expanding their range. By growing higher than other plants, tall trees cast their shadow on shorter plants and limit competition for water and precious nutrients in the soil.

Phloem is the second type of vascular tissue it transports sugars, proteins, and other solutes throughout the plant. Phloem cells are divided into sieve elements (conducting cells) and cells that support the sieve elements. Together, xylem and phloem tissues form the vascular system of plants.


Roots: Support for the Plant

Roots are not well-preserved in the fossil record. Nevertheless, it seems that roots appeared later in evolution than vascular tissue. The development of an extensive network of roots represented a significant new feature of vascular plants. Thin rhizoids attached bryophytes to the substrate, but these rather flimsy filaments did not provide a strong anchor for the plant nor did they absorb substantial amounts of water and nutrients. In contrast, roots, with their prominent vascular tissue system, transfer water and minerals from the soil to the rest of the plant. The extensive network of roots that penetrates deep into the soil to reach sources of water also stabilizes plants by acting as a ballast or anchor. The majority of roots establish a symbiotic relationship with fungi, forming mutualistic mycorrhizae, which benefit the plant by greatly increasing the surface area for absorption of water, soil minerals, and nutrients.


Mosses

The cushion-like green layers covering a stream or damp landscapes are mosses. Mosses are present in cold mountain peaks as well. They are soft plants with no true roots, stems, and leaves. They directly take up water and nutrients right away to their green parts. Spore formation in mosses: Spore formation in mosses is similar to that of reproduction in ferns.

Shoots of mosses are classified as male and female

The male part produces sperms and the female shoot generates eggs

For sperms to form, mosses need damp and wet conditions. Under favorable conditions, the sperm swims its way towards eggs using its flagella

On the fusion of these cells, spores are formed which later gets collected in the stalk


Section Summary

The seedless vascular plants show several features important to living on land: vascular tissue, roots, and leaves. Vascular systems consist of xylem tissue, which transports water and minerals, and phloem tissue, which transports sugars and proteins. With the development of the vascular system, leaves appeared to act as large photosynthetic organs, and roots to access water from the ground. Small uncomplicated leaves are termed microphylls. Large leaves with vein patterns are termed megaphylls. Modified leaves that bear sporangia are called sporophylls. Some sporophylls are arranged in cone structures called strobili.

The support and conductive properties of vascular tissues have allowed the sporophyte generation of vascular plants to become increasingly dominant. The seedless vascular plants include club mosses, which are the most primitive whisk ferns, which lost leaves and roots by reductive evolution and horsetails and ferns. Ferns are the most advanced group of seedless vascular plants. They are distinguished by large leaves called fronds and small sporangia-containing structures called sori, which are found on the underside of the fronds.

Both mosses and ferns play an essential role in the balance of the ecosystems. Mosses are pioneering species that colonize bare or devastated environments and make it possible for succession to occur. They contribute to the enrichment of the soil and provide shelter and nutrients for animals in hostile environments. Mosses are important biological indicators of environmental pollution. Ferns are important for providing natural habitats, as soil stabilizers, and as decorative plants. Both mosses and ferns are part of traditional medical practice. In addition to culinary, medical, and decorative purposes, mosses and ferns can be used as fuels, and ancient seedless plants were important contributors to the fossil fuel deposits that we now use as an energy resource.


Characteristics

These plants are some of the oldest land plants on Earth, first originating in the Silurian Period around 40 million years ago. During the Carboniferous Era, they were so abundant that their remains accumulated faster than they could decompose, resulting in the massive reserves of coal that we use today. At that time, they grew as large as 98 feet tall, though such large species became extinct later.

Seedless vascular plants are characterized by the presence of true roots, stems, and leaves, though sometimes these parts cannot be clearly distinguished from each other. In a few cases, the leaves and roots arise from underground stems, called rhizomes, which also store food. The plants show prominent stomata which can’t close, and a vascular cuticle which covers the whole body, thus preventing water loss.

They contain vascular tissues which transport substances from one part of the plant to the other. They are:

Xylem: They transport water and minerals from the soil to the other parts of the plant. They consist of a network of lignified, tube-like, dead cells, that along with transport, provide structural support to the plant.

Phloem: Phloem consists of a tubular chain of living cells that transports sugar, amino acids, and other food materials prepared in the leaves during photosynthesis to other plant parts.

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These vascular tissues helped the plants grow much taller and wider, as compared to non-vascular plants, as they transport nutrients and food all through the plant’s body. Growing taller is an evolutionary advantage, as the plant could reach the canopy height to obtain sunlight, while blocking out the light from reaching its competitors. This larger size also triggered evolutionary changes in other organisms, like arthropods and vertebrates, by providing them with more diverse food sources.

These plants propagate by releasing spores, which differ from seeds in several aspects. The multicellular seeds are a result of sexual reproduction, while spores are unicellular and a method of asexual reproduction. Seeds are hardier and carry more nutrients for the developing embryo, while spores, being smaller, carry less food. Seeds can be dispersed in various ways, like by animals, water, and wind, and germinate wherever they contact soil. Spores can only be dispersed by the wind, and require moist soils to germinate.

The life cycle of seedless vascular plants shows an ‘Alternation of Generations’. First, the sporangium releases many spores, formed by meiosis, which germinate in moist soil to form a haploid gametophyte. This gametophyte contains the antheridia (male sex organ) and the archegonia (female sex organ) together on one plant. The antheridia releases sperms which travel to the archegonia to fertilize the egg. The sperm and egg unite during fertilization to form a diploid zygote. This zygote develops into an embryo, that finally grows into a mature diploid sporophyte which can again release spores. So, the life cycle of each plant alternates between the diploid sporophyte and the haploid gametophyte, which is called alternation of generations.

The sporophyte is the dominant stage of the plant, which may be between a few millimeters to several meters in height. The mature sporophyte and the gametophyte are independent of each other, because they photosynthesize to survive. However, during the initial stage of development, the sporophyte absorbs nutrition from the gametophyte. Moreover, during the initial stage of development of the gametophyte, it forms associations with fungi for partial absorption of nutrients.

Another major difference between seed-bearing and seedless vascular plants is, in the former, reproductive tissues like flowers are aerial in nature and grow high above the ground. On the other hand, in the latter, the gametophyte, which contains the sexual organs (antheridia and archegonia), grows close to the ground. The long-lived sporophyte grows on the gametophyte, which is between a few millimeters to a meter in size.

Seedless vascular plants occur most commonly in the tropical rain forests and moist temperate forests, since the sperm produced in the gametophyte is flagellated and requires moisture to swim towards the egg. They are found along the banks of rivers, lakes, streams, and ponds.


Characteristics of Plants

Almost all plants live on land and have adapted to the conditions on land through the development of a waxy cuticle to prevent drying out, structures to absorb and transport water throughout their bodies (the bryophytes are an exception), and rigid internal support to remain erect without the buoyancy available in water. This rigidity is provided in large part by the cell wall, which is composed of cellulose , a complex carbohydrate , and lignin , a phenolic compound that stiffens the cellulose fibers.

The plant life cycle has two distinct multicellular phases: a haploid phase (in which chromosomes are present only as single copies) and a diploid phase (in which chromosomes are present in pairs). The haploid organism produces gametes that fuse to form an embryo, which develops into the diploid organism. The diploid organism produces haploid spores that germinate to form the haploid organism. This Ȫlternation of generations" is found only in plants and some algae.

Almost all plants photosynthesize, using the sun's energy to power the production of sugar from carbon dioxide and water. Photosynthesis occurs in chloroplasts, membrane-bound organelles that contain the green pigment chlorophyll. Chloroplasts are descended from free-living photosynthetic bacteria that became symbiotic partners of ancient single-celled plant ancestors. Evidence of the chloroplast's bacterial origin is found in the presence of deoxyribonucleic acid (DNA) within it, as well as its size and structure.

The photosynthetic production of sugars by plants is the basis for all terrestrial food chains. Photosynthesis also produces oxygen, needed by animals, fungi, and other organisms (including plants themselves) to release the stored energy in those sugars.


Seedless Vascular Plants

The vascular plants, or tracheophytes, are the dominant and most conspicuous group of land plants. More than 260,000 species of tracheophytes represent more than 90 percent of Earth’s vegetation. Several evolutionary innovations explain their success and their ability to spread to all habitats.

Bryophytes may have been successful at the transition from an aquatic habitat to land, but they are still dependent on water for reproduction, and must absorb moisture and nutrients through the gametophyte surface. The lack of roots for absorbing water and minerals from the soil, as well as a lack of lignin-reinforced conducting cells, limit bryophytes to small sizes. Although they may survive in reasonably dry conditions, they cannot reproduce and expand their habitat range in the absence of water. Vascular plants, on the other hand, can achieve enormous heights, thus competing successfully for light. Photosynthetic organs become leaves, and pipe-like cells or vascular tissues transport water, minerals, and fixed carbon organic compounds throughout the organism.

Throughout plant evolution, there is a progressive increase in the dominance of the sporophyte generation. In seedless vascular plants, the diploid sporophyte is the dominant phase of the life cycle. The gametophyte is now less conspicuous, but still independent of the sporophyte. Seedless vascular plants still depend on water during fertilization, as the flagellated sperm must swim on a layer of moisture to reach the egg. This step in reproduction explains why ferns and their relatives are more abundant in damp environments.


Examples of a Nonvascular Plant

Liverwort

Where moss grows in small branching structures, and many organisms get packed in a larger mat or bundle, liverwort grows as small, individual leaf-like structure. The thallus, as it is called, is the dominant gametophyte. The thallus will produce specialized organs, to house the sporophyte. Liverwort and hornwort are almost indistinguishable, besides some differences in their thallus and the structure of their sporophytes. However, genetic evidence has revealed that liverwort and hornwort, while both a nonvascular plant group, are unrelated enough to deserve two separate divisions. A typical liverwort can be seen below.

Hornwort

Commonly mistaken as liverwort, hornwort is a closely related group of nonvascular plant species. Like mosses and liverworts, hornworts exist as a dominant gametophyte form. Hornworts, because of the way that they combine their chloroplasts with other organelles, are thought to be more closely related to certain species of algae than other land plants. While hornwort, liverwort, and mosses used to all belong to the Bryophyta, the hornworts and liverworts have been given their own divisions. This reflects the finding that the groups are not closely related enough to be considered the same group. In the picture below you can see a hornwort. Note that while it looks like liverwort, you can easily see the horn-like structures. These structures house the sporophyte generation, creating spores.

Algae

Not all algae is considered a nonvascular plant. Typically, only those algae found in the clade Viridiplantae are considered nonvascular plants. However, the evolutionary relationships between algae and land plants are not entirely clear. It is sometimes assumed that nonvascular algae led to nonvascular land plants, which led to vascular land plants. This theory, however, is not necessarily supported by the genetic and paleological evidence. However, some algae do have specific tissues, some of which are even specialized for water transport. An alternate theory supposes that some algae developed into vascular plants, where other algae became the modern nonvascular plant.

1. A scientist is trying to grow a nonvascular plant which is 20 feet tall. Why is this unlikely?
A. Sounds reasonable enough!
B. Water pressure
C. A nonvascular plant cannot retain water

2. The following is a list of features of vascular plants. Which of the features is shared by a nonvascular plant?
A. Organized, multicellular structure and the ability to photosynthesize
B. An organized internal system for transporting water
C. Specialized corridors of cells for transporting sugar

3. Is a nonvascular plant species LESS EVOLVED than a vascular plant species?
A. No
B. Yes
C. It depends…



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