Seedless Vascular Plants

Seedless Vascular Plants

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 Takeaways

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.

image

Life cycle of a fern: This life cycle of a fern shows alternation of generations with a dominant sporophyte stage.

Vascular Tissue: Xylem and Phloem

Xylem and phloem form the vascular system of plants to transport water and other substances throughout the plant.

Learning Objectives

Describe the functions of plant vascular tissue

Key Takeaways

Key Points

  • Xylem transports and stores water and water-soluble nutrients in vascular plants.
  • Phloem is responsible for transporting sugars, proteins, and other organic molecules in plants.
  • Vascular plants are able to grow higher than other plants due to the rigidity of xylem cells, which support the plant.

Key Terms

  • xylem: a vascular tissue in land plants primarily responsible for the distribution of water and minerals taken up by the roots; also the primary component of wood
  • phloem: a vascular tissue in land plants primarily responsible for the distribution of sugars and nutrients manufactured in the shoot
  • tracheid: elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts

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. Together, xylem and phloem tissues form the vascular system of plants.

image

Xylem and phloem: Xylem and phloem tissue make up the transport cells of stems. The direction of water and sugar transportation through each tissue is shown by the arrows.

Xylem is the tissue responsible for supporting the plant as well as for the storage and long-distance transport of water and nutrients, including the transfer of water-soluble growth factors from the organs of synthesis to the target organs. The tissue consists of vessel elements, conducting cells, known as tracheids, and supportive filler tissue, called parenchyma. These cells are joined end-to-end to form long tubes. Vessels and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. It is the thick walls of the tracheids that provide support for 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. The tracheids do not have end openings like the vessels do, but their ends overlap with each other, with pairs of pits present. The pit pairs allow water to pass horizontally from cell to cell.

image

Tracheids and vessel elements: Tracheids (top) and vessel elements (bottom) are the water conducting cells of xylem tissue.

Phloem tissue is responsible for translocation, which is the transport of soluble organic substances, for example, sugar. The substances travel along sieve elements, but other types of cells are also present: the companion cells, parenchyma cells, and fibers. The end walls, unlike vessel members in xylem, do not have large openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates. Despite the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. The activity of the sieve tubes is controlled by companion cells through plasmadesmata.

The Evolution of Roots in Seedless Plants

Roots support plants by anchoring them to soil, absorbing water and minerals, and storing products of photosynthesis.

Learning Objectives

Explain how roots provide support for plants

Key Takeaways

Key Points

  • There are two main types of root systems: tap root systems consist of one main root that grows down vertically with smaller lateral roots growing off of the main root, while fibrous root systems form a dense network of roots near the soil surface.
  • Roots can be modified to store food or starches and to provide additional support for plants; many vegetables, such as carrots, are modified roots.
  • A zone of cell division, a zone of elongation, and a zone of maturation and differentiation make up a root tip, where the root cells divide, grow, and differentiate into specialized cells.
  • The vascular system of roots is surrounded by an epidermis, which regulates materials that enter the root’s vascular system.

Key Terms

  • endodermis: in a plant stem or root, a cylinder of cells that separates the outer cortex from the central core and controls the flow of water and minerals within the plant
  • suberin: a waxy material found in bark that can repel water
  • pericycle: in a plant root, the cylinder of plant tissue between the endodermis and phloem

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. Roots provided seed plants with three major functions: anchoring the plant to the soil, absorbing water and minerals and transporting them upwards, and storing the products of photosynthesis. Importantly, roots are modified to absorb moisture and exchange gases. In addition, while most roots are underground, some plants have adventitious roots, which emerge above the ground from the shoot.

Types of Root Systems

There are mainly two types of root systems. Dicots (flowering plants with two embryonic seed leaves) have a tap root system while monocots (flowering plants with one embryonic seed leaf) have a fibrous root system. A tap root system has a main root that grows down vertically from which many smaller lateral roots arise. Dandelions are a good example; their tap roots usually break off when trying to pull these weeds; they can regrow another shoot from the remaining root.

image

Root types: (a) Tap root systems have a main root that grows down, while (b) fibrous root systems consist of many small roots.

A tap root system penetrates deep into the soil. In contrast, a fibrous root system is located closer to the soil surface, forming a dense network of roots that also helps prevent soil erosion (lawn grasses are a good example, as are wheat, rice, and corn). In addition, some plants actually have a combination of tap root and fibrous roots. Plants that grow in dry areas often have deep root systems, whereas plants growing in areas with abundant water tend to have shallower root systems.

Root Growth and Anatomy

image

Zones on a root tip: A longitudinal view of the root reveals the zones of cell division, elongation, and maturation. Cell division occurs in the apical meristem.

Root growth begins with seed germination. When the plant embryo emerges from the seed, the radicle of the embryo forms the root system. The tip of the root is protected by the root cap, a structure exclusive to roots and unlike any other plant structure. The root cap is continuously replaced because it gets damaged easily as the root pushes through soil. The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation and differentiation. The zone of cell division is closest to the root tip; it is made up of the actively-dividing cells of the root meristem. The zone of elongation is where the newly-formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into special cell types. All three zones are in the first centimeter or so of the root tip.

image

Modified roots: Many vegetables are modified roots, such as radishes and carrots, which store energy in the form of starches and sugars.

The vascular tissue in the root is arranged in the inner portion of the root, which is called the vascular cylinder. A layer of cells, known as the endodermis, separates the vascular tissue from the ground tissue in the outer portion of the root. The endodermis is exclusive to roots, serving as a checkpoint for materials entering the root’s vascular system. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded. The outermost cell layer of the root’s vascular tissue is the pericycle, an area that can give rise to lateral roots. In dicot roots, the xylem and phloem of the stele are arranged alternately in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith.

Root Modifications

Root structures may be modified for specific purposes. For example, some roots are bulbous and store starch. Aerial roots and prop roots are two forms of aboveground roots that provide additional support to anchor the plant. Tap roots, such as carrots, turnips, and beets, are examples of roots that are modified for food storage.

Ferns and Other Seedless Vascular Plants

Ferns, club mosses, horsetails, and whisk ferns are seedless vascular plants that reproduce with spores and are found in moist environments.

Learning Objectives

Identify types of seedless vascular plants

Key Takeaways

Key Points

  • Club mosses, which are the earliest form of seedless vascular plants, are lycophytes that contain a stem and microphylls.
  • Horsetails are often found in marshes and are characterized by jointed hollow stems with whorled leaves.
  • Photosynthesis occurs in the stems of whisk ferns, which lack roots and leaves.
  • Most ferns have branching roots and form large compound leaves, or fronds, that perform photosynthesis and carry the reproductive organs of the plant.

Key Terms

  • sorus: a cluster of sporangia associated with a fern leaf
  • lycophyte: a tracheophyte subdivision of the Kingdom Plantae; the oldest extant (living) vascular plant division at around 410 million years old
  • sporangia: enclosures in which spores are formed

Ferns and Other Seedless Vascular Plants

Water is required for fertilization of seedless vascular plants; most favor a moist environment. Modern-day seedless tracheophytes include lycophytes and monilophytes.

Phylum Lycopodiophyta: Club Mosses

The club mosses, or phylum Lycopodiophyta, are the earliest group of seedless vascular plants. They dominated the landscape of the Carboniferous, growing into tall trees and forming large swamp forests. Today’s club mosses are diminutive, evergreen plants consisting of a stem (which may be branched) and microphylls (leaves with a single unbranched vein). The phylum Lycopodiophyta consists of close to 1,200 species, including the quillworts (Isoetales), the club mosses (Lycopodiales), and spike mosses (Selaginellales), none of which are true mosses or bryophytes.

Lycophytes follow the pattern of alternation of generations seen in the bryophytes, except that the sporophyte is the major stage of the life cycle. The gametophytes do not depend on the sporophyte for nutrients. Some gametophytes develop underground and form mycorrhizal associations with fungi. In club mosses, the sporophyte gives rise to sporophylls arranged in strobili, cone-like structures that give the class its name. Lycophytes can be homosporous or heterosporous.

image

Strobili of club mosses: In some club mosses such as Lycopodium clavatum, sporangia are arranged in clusters called strobili.

Phylum Monilophyta: Class Equisetopsida (Horsetails)

Horsetails, whisk ferns, and ferns belong to the phylum Monilophyta, with horsetails placed in the Class Equisetopsida. The single extant genus Equisetum is the survivor of a large group of plants, which produced large trees, shrubs, and vines in the swamp forests in the Carboniferous. The plants are usually found in damp environments and marshes.

The stem of a horsetail is characterized by the presence of joints or nodes, hence the old name Arthrophyta (arthro- = “joint”; -phyta = “plant”). Leaves and branches come out as whorls from the evenly-spaced joints. The needle-shaped leaves do not contribute greatly to photosynthesis, the majority of which takes place in the green stem.

image

Leaves of a horsetail: The whorls of green structures at the joints are actually stems. The leaves are barely noticeable as brown rings just above each joint. Horsetails were once used as scrubbing brushes and so were called scouring rushes.

Silica collects in the epidermal cells, contributing to the stiffness of horsetail plants. Underground stems known as rhizomes anchor the plants to the ground. Modern-day horsetails are homosporous and produce bisexual gametophytes.

Phylum Monilophyta: Class Psilotopsida (Whisk Ferns)

While most ferns form large leaves and branching roots, the whisk ferns, Class Psilotopsida, lack both roots and leaves, which were probably lost by reduction. Photosynthesis takes place in their green stems; small yellow knobs form at the tip of the branch stem and contain the sporangia. Whisk ferns were considered an early pterophytes. However, recent comparative DNA analysis suggests that this group may have lost both leaves and roots through evolution and is more closely related to ferns.

Phylum Monilophyta: Class Polypodiopsida (Ferns)

With their large fronds, ferns are the most-readily recognizable seedless vascular plants. More than 20,000 species of ferns live in environments ranging from tropics to temperate forests. Although some species survive in dry environments, most ferns are restricted to moist, shaded places. Ferns made their appearance in the fossil record during the Devonian period and expanded during the Carboniferous.

The dominant stage of the life cycle of a fern is the sporophyte, which typically consists of large compound leaves called fronds. Fronds fulfill a double role; they are photosynthetic organs that also carry reproductive structure. The stem may be buried underground as a rhizome from which adventitious roots grow to absorb water and nutrients from the soil, or they may grow above ground as a trunk in tree ferns. Adventitious organs are those that grow in unusual places, such as roots growing from the side of a stem. Most ferns produce the same type of spores and are, therefore, homosporous. The diploid sporophyte is the most conspicuous stage of the life cycle. On the underside of its mature fronds, sori (singular, sorus) form as small clusters where sporangia develop. Sporangia in a sorus produce spores by meiosis and release them into the air. Those that land on a suitable substrate germinate and form a heart-shaped gametophyte, which is attached to the ground by thin filamentous rhizoids. The inconspicuous gametophyte harbors both sex gametangia. Flagellated sperm are released and swim on a wet surface to where the egg is fertilized. The newly-formed zygote grows into a sporophyte that emerges from the gametophyte, growing by mitosis into the next generation sporophyte.

image

Sori on a fern frond: Sori appear as small bumps on the underside of a fern frond.

The Importance of Seedless Vascular Plants

Seedless vascular plants provide many benefits to life in ecosystems, including food and shelter and, to humans, fuel and medicine.

Learning Objectives

Explain the beneficial roles of seedless vascular plants

Key Takeaways

Key Points

  • Mosses and liverworts provide food and shelter for other organisms in otherwise barren or hostile environments.
  • The level of pollution in an environment can be determined by the disappearance of mosses, which absorb the pollutants with moisture through their entire surfaces.
  • Dried peat moss is used as a renewable resource for fuel.
  • Ferns prevent soil erosion, promote topsoil formation, restore nitrogen to aquatic habitats by harboring cyanobacteria, make good house plants, and have been used as food and for medicinal remedies.
  • Coal, a major fuel source and contributor to global warming, was deposited by the seedless vascular plants of the Carboniferous period.

Key Terms

  • bioindicator: any species that acts as a biological indicator of the health of an environment
  • pharmacopoeia: an official book describing medicines or other pharmacological substances, especially their use, preparation, and regulation
  • sphagnum: any of various widely-distributed mosses, of the genus Sphagnum, which slowly decompose to form peat; often used for fuel

The Importance of Seedless Vascular Plants

Mosses and liverworts are often the first macroscopic organisms to colonize an area, both in a primary succession (where bare land is settled for the first time by living organisms) or in a secondary succession (where soil remains intact after a catastrophic event wipes out many existing species ). Their spores are carried by the wind, birds, or insects. Once mosses and liverworts are established, they provide food and shelter for other species. In a hostile environment, such as the tundra where the soil is frozen, bryophytes grow well because they do not have roots and can dry and rehydrate rapidly once water is again available. Mosses are at the base of the food chain in the tundra biome. Many species, from small insects to musk oxen and reindeer, depend on mosses for food. In turn, predators feed on the herbivores, which are the primary consumers. Some reports indicate that bryophytes make the soil more amenable to colonization by other plants. Because they establish symbiotic relationships with nitrogen-fixing cyanobacteria, mosses replenish the soil with nitrogen.

At the end of the nineteenth century, scientists observed that lichens and mosses were becoming increasingly rare in urban and suburban areas. Since bryophytes have neither a root system for absorption of water and nutrients, nor a cuticle layer that protects them from desiccation, pollutants in rainwater readily penetrate their tissues; they absorb moisture and nutrients through their entire exposed surfaces. Therefore, pollutants dissolved in rainwater penetrate plant tissues readily and have a larger impact on mosses than on other plants. The disappearance of mosses can be considered a bioindicator for the level of pollution in the environment.

Ferns contribute to the environment by promoting the weathering of rock, accelerating the formation of topsoil, and slowing down erosion by spreading rhizomes in the soil. The water ferns of the genus Azolla harbor nitrogen-fixing cyanobacteria and restore this important nutrient to aquatic habitats.

Seedless plants have historically played a role in human life through uses as tools, fuel, and medicine. Dried peat moss, Sphagnum, is commonly used as fuel in some parts of Europe and is considered a renewable resource. Sphagnum bogs are cultivated with cranberry and blueberry bushes. The ability of Sphagnum to hold moisture makes the moss a common soil conditioner. Florists use blocks of Sphagnum to maintain moisture for floral arrangements.

image

Plants as a renewable resource for fuel: Sphagnum acutifolium is dried peat moss and can be used as fuel.

The attractive fronds of ferns make them a favorite ornamental plant. Because they thrive in low light, they are well suited as house plants. More importantly, fiddleheads are a traditional spring food of Native Americans in the Pacific Northwest and are popular as a side dish in French cuisine. The licorice fern, Polypodium glycyrrhiza, is part of the diet of the Pacific Northwest coastal tribes, owing in part to the sweetness of its rhizomes. It has a faint licorice taste and serves as a sweetener. The rhizome also figures in the pharmacopoeia of Native Americans for its medicinal properties and is used as a remedy for sore throat.

image

Fiddlehead ferns as food: A chicken dish with fiddlehead ferns as a side is shown. Native Americans traditionally cook fiddleheads with meals during the spring.

By far the greatest impact of seedless vascular plants on human life, however, comes from their extinct progenitors. The tall club mosses, horsetails, and tree-like ferns that flourished in the swampy forests of the Carboniferous period gave rise to large deposits of coal throughout the world. Coal provided an abundant source of energy during the Industrial Revolution, which had tremendous consequences on human societies, including rapid technological progress and growth of large cities, as well as the degradation of the environment. Coal is still a prime source of energy and also a major contributor to global warming.

image

Carboniferous period plants: This drawing depicts the tall mosses and tree-like ferns of the Carboniferous period that deposited the large amounts of coal throughout the world.