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10.6: Introduction to Seedless Plants - Biology

10.6: Introduction to Seedless Plants - Biology


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Classify seedless plants

An incredible variety of seedless plants populates the terrestrial landscape. Their decomposition created large deposits of coal that we mine today.

Current evolutionary thought holds that all plants—green algae as well as land dwellers—are monophyletic; that is, they are descendants of a single common ancestor. The evolutionary transition from water to land imposed severe constraints on plants. They had to develop strategies to avoid drying out, to disperse reproductive cells in air, for structural support, and for capturing and filtering sunlight. While seed plants developed adaptations that allowed them to populate even the most arid habitats on Earth, full independence from water did not happen in all plants. Most seedless plants still require a moist environment.

What You’ll Learn to Do

  • Describe the timeline of plant evolution and the impact of land plants on other living things
  • Describe the traits shared by green algae and land plants
  • Identify the main characteristics of bryophytes
  • Differentiate between vascular and non-vascular plants
  • Identify the main characteristics of seedless vascular plants

Learning Activities

The learning activities for this section include the following:

  • Early Plant Life
  • Green Algae: Precursors of Land Plants
  • Bryophytes
  • Seedless Vascular Plants
  • Self Check: Plant Structures

Introduction

Seedless plants, like these horsetails (Equisetum sp.), thrive in damp, shaded environments under a tree canopy where dryness is rare. (credit: modification of work by Jerry Kirkhart)

An incredible variety of seedless plants populates the terrestrial landscape. Mosses may grow on a tree trunk, and horsetails may display their jointed stems and spindly leaves across the forest floor. Today, seedless plants represent only a small fraction of the plants in our environment yet, three hundred million years ago, seedless plants dominated the landscape and grew in the enormous swampy forests of the Carboniferous period. Their decomposition created large deposits of coal that we mine today.

Current evolutionary thought holds that all plants—green algae as well as land dwellers—are monophyletic that is, they are descendants of a single common ancestor. The evolutionary transition from water to land imposed severe constraints on plants. They had to develop strategies to avoid drying out, to disperse reproductive cells in air, for structural support, and for capturing and filtering sunlight. While seed plants developed adaptations that allowed them to populate even the most arid habitats on Earth, full independence from water did not happen in all plants. Most seedless plants still require a moist environment.


Answer to Problem 1VCQ

Correct answer:

The correct answer is option (d) The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. Cohesin proteins break down and the sister chromatids separate. The nucleus reforms and the cell divides.

Explanation of Solution

Explanation/justification for the correct answer:

Option (d): during early metaphase, the kinetochore attaches to the spindle fibers. The sister chromatids of the dividing cells line up at the metaphase plate, after the alignment, the spindle fibers attach to the kinetochore. The sister chromatids are bind together with the help of cohesin. After the alignment of the sister chromatids at the metaphase plate, the cohesin breaks and the sister chromatid separate and move to the opposite poles. A nucleus is formed on each opposite pole and then the cytoplasmic division results in the formation of two new cells.

Explanation for the incorrect answer:

Option (a): In this option, the separation of sister chromatids is placed after the formation of a nucleus, but the formation of the nucleus takes place after the sister chromatids are separated and move to opposite poles. So, it is an incorrect option.

Option (b): In this option, the separation of the sister chromatids is placed before the lining of the sister chromatids on the metaphase plate. The cohesin protein breaks after the sister chromatids are lined up on the metaphase plate, which causes the breaking of the sister chromatids. So, it is an incorrect option.

Option (c): In this option, it is said that the kinetochore is attached to the cohesin protein, but the kinetochore actually gets attached to the spindle fibers. So, it is an incorrect answer.

During metaphase, the mitotic spindle binds to the kinetochore and causes the alignment of sister chromatids on metaphase plate and the cohesin breaks resulting in separation of the sister chromatids. The sister chromatids are then pulled to the opposite pole, there the new nucleus is formed followed by cytokinesis and in this way two new cells from one existing cell are formed. Hence, option (d) is the correct answer.

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Introduction

Seedless plants, like these horsetails (Equisetum sp.), thrive in damp, shaded environments under a tree canopy where dryness is rare. (credit: modification of work by Jerry Kirkhart)

An incredible variety of seedless plants populates the terrestrial landscape. Mosses may grow on a tree trunk, and horsetails may display their jointed stems and spindly leaves across the forest floor. Today, seedless plants represent only a small fraction of the plants in our environment yet, 300 million years ago, seedless plants dominated the landscape and grew in the enormous swampy forests of the Carboniferous period. Their decomposition created large deposits of coal that we mine today.

Current evolutionary thought holds that all plants—some green algae as well as land plants—are monophyletic that is, they are descendants of a single common ancestor. The evolutionary transition from water to land imposed severe constraints on plants. They had to develop strategies to avoid drying out, to disperse reproductive cells in air, for structural support, and for capturing and filtering sunlight. While seed plants have developed adaptations that allow them to populate even the most arid habitats on Earth, full independence from water did not happen in all plants. Most seedless plants still require a moist environment for reproduction.


Introduction

Seedless plants, like these horsetails (Equisetum sp.), thrive in damp, shaded environments under a tree canopy where dryness is rare. (credit: modification of work by Jerry Kirkhart)

An incredible variety of seedless plants populates the terrestrial landscape. Mosses may grow on a tree trunk, and horsetails may display their jointed stems and spindly leaves across the forest floor. Today, seedless plants represent only a small fraction of the plants in our environment yet, 300 million years ago, seedless plants dominated the landscape and grew in the enormous swampy forests of the Carboniferous period. Their decomposition created large deposits of coal that we mine today.

Current evolutionary thought holds that all plants—some green algae as well as land plants—are monophyletic that is, they are descendants of a single common ancestor. The evolutionary transition from water to land imposed severe constraints on plants. They had to develop strategies to avoid drying out, to disperse reproductive cells in air, for structural support, and for capturing and filtering sunlight. While seed plants have developed adaptations that allow them to populate even the most arid habitats on Earth, full independence from water did not happen in all plants. Most seedless plants still require a moist environment for reproduction.


Chapter 18 Introduction – Seedless Plants


An incredible variety of seedless plants populates the terrestrial landscape. Mosses may grow on a tree trunk, and horsetails may display their jointed stems and spindly leaves across the forest floor. Today, seedless plants represent only a small fraction of the plants in our environment yet, 300 million years ago, seedless plants dominated the landscape and grew in the enormous swampy forests of the Carboniferous period. Their decomposition created large deposits of coal that we mine today.

Current evolutionary thought holds that all plants—some green algae as well as land plants—are monophyletic that is, they are descendants of a single common ancestor. The evolutionary transition from water to land imposed severe constraints on plants. They had to develop strategies to avoid drying out, to disperse reproductive cells in air, for structural support, and for capturing and filtering sunlight. While seed plants have developed adaptations that allow them to populate even the most arid habitats on Earth, full independence from water did not happen in all plants. Most seedless plants still require a moist environment for reproduction.


10.6: Introduction to Seedless Plants - Biology

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There are three major groupings covering the plant life on Earth&mdashnonvascular plants, seedless vascular plants, and seed plants.

The seedless vascular plants were the first to evolve specialized vascular systems&mdashan adaptation that helped them become the first tall plants on Earth. Today, seedless vascular plants are represented by lycophytes and monilophytes.

Lycophytes include clubmosses, spikemosses, and quillworts. Notably, none of the lycophytes are true mosses, which are nonvascular plants. Monilophytes include ferns, horsetails, and whisk ferns and their relatives.

Like all plants, seedless vascular plants display an alternation of generations in their life cycle, as shown here using a fern plant as an example.

This means that they spend part of their life cycle as a haploid gametophyte, and the other part as a diploid sporophyte.

Like nonvascular plants, seedless vascular plants reproduce using spores, rather than seeds. The spores are haploid, and are dispersed by structures called sori, clustered on the underside of the leaves.

The sori themselves contain many sporangia. Upon reaching maturity, these sporangia open, dispersing the haploid spores. The spores then grow via mitosis to form the haploid gametophyte.

At the gametophyte stage - which is typically very small and found on or just below the soil surface - haploid gametes are formed by mitosis. A single gametophyte is bisexual and develops two different structures - the antheridia and archegonia - that produce gametes in male and female forms respectively.

Like the nonvascular plants, the male sperm gamete is flagellated and requires water to travel to the female gamete, following a chemical attractant to find the egg.

Because the gametes in a single gametophyte will be genetically identical due to their haploid origin, crosses typically occur between different gametophytes. Ferns can prevent any self-fertilization by having their antheridia and archegonia mature at different times.

Finally, the fertilized egg will grow a new diploid sporophyte from the diploid zygote of the gametophyte, completing the life cycle.

Like seed plants, seedless vascular plants have life cycles dominated by sporophytes. However, unlike either of the other major plant lineages, their smaller gametophytes can live independently&mdashmeaning they do not provide nourishment to the sporophyte, or require it from the sporophyte.

Arguably the key feature of seedless vascular plants is their specialized network of vascular tissue, akin to that of the seed plants. This adaptation allowed them to transport water, nutrients, and other organic materials, and to attain greater sizes&mdashwhich distinguished them from their nonvascular relatives.

34.3: Seedless Vascular Plants

Seedless Vascular Plants Were the First Tall Plants on Earth

Today, seedless vascular plants are represented by monilophytes and lycophytes. Ferns&mdashthe most common seedless vascular plants&mdashare monilophytes. Whisk ferns (and their relatives) and horsetails are also monilophytes. Lycophytes include club mosses, spikemosses, and quillworts&mdashnone of which are true mosses.

Unlike nonvascular plants, vascular plants&mdashincluding seedless vascular plants&mdashhave an extensive network of vascular tissue comprised of xylem and phloem. Most seedless vascular plants also have true roots and leaves. Furthermore, the life cycles of seedless vascular plants are dominated by diploid spore-producing sporophytes, rather than gametophytes.

However, like nonvascular plants, seedless vascular plants reproduce with spores rather than seeds. Seedless vascular plants are also typically more reproductively successful in moist environments because their sperm require a film of water to reach the eggs.

The Life Cycle of Seedless Vascular Plants

Like animals, seedless vascular plants (and other plants) alternate between meiosis and fertilization during reproduction. Meiosis is a cell division process that produces haploid cells&mdashwhich contain one complete set of chromosomes&mdashfrom a diploid cell&mdashwhich contains two complete sets of chromosomes. Fertilization, by contrast, produces a diploid cell called a zygote through the fusion of haploid cells called gametes&mdashsperm and eggs.

In most animals, only the diploid stage is multicellular, and gametes are the only haploid cells. Plants, however, alternate between haploid and diploid stages that are both multicellular this is called alternation of generations. Alternation of generations is a feature of all sexually reproducing plants, but the relative size and prominence of the haploid and diploid stages differ among plants.

In seedless vascular plants (as well as seed plants), the diploid stage of the life cycle&mdashthe sporophyte&mdashis dominant. For example, what most people recognize as a fern is the large, independent fern sporophyte. Sporophytes produce haploid cells called spores through meiosis.

A spore can germinate and develop into a gametophyte&mdashthe haploid stage of the life cycle&mdashthrough mitosis. Gametophytes produce egg and sperm cells through mitosis (unlike animals, which produce gametes through meiosis). Most seedless vascular plants produce one type of spore that gives rise to a bisexual gametophyte. The gametophytes are smaller and less structurally complex than the sporophytes, but they can photosynthesize and do not depend on the sporophyte for nourishment or protection.

Egg and sperm cells fuse through fertilization, forming a diploid zygote. The zygote divides through mitosis to generate the familiar, fronded fern sporophyte&mdashcontinuing the cycle.

Jones, Victor A.s., and Liam Dolan. 2012. "The Evolution of Root Hairs and Rhizoids." Annals of Botany 110 (2): 205&ndash12. [Source]

Pittermann, Jarmila, Craig Brodersen, and James E. Watkins. 2013. "The Physiological Resilience of Fern Sporophytes and Gametophytes: Advances in Water Relations Offer New Insights into an Old Lineage." Frontiers in Plant Science 4. [Source]

Sigel, Erin M., Eric Schuettpelz, Kathleen M. Pryer, and Joshua P. Der. 2018. "Overlapping Patterns of Gene Expression Between Gametophyte and Sporophyte Phases in the Fern Polypodium Amorphum (Polypodiales)." Frontiers in Plant Science 9 (September). [Source]


Section Summary

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, there appeared leaves to act as large photosynthetic organs, and roots to access water from the ground. Small uncomplicated leaves are microphylls. Large leaves with vein patterns are megaphylls. Modified leaves that bear sporangia are sporophylls. Some sporophylls are arranged in cone structures called strobili.

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.

Mosses play an essential role in the balance of the ecosystems they are pioneering species that colonize bare or devastated environments and make it possible for a succession to occur. They contribute to the enrichment of the soil and provide shelter and nutrients for animals in hostile environments. Mosses and ferns can be used as fuels and serve culinary, medical, and decorative purposes.


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