Seedless Plants by Owen Borville 10.31.2024 Biology Lesson 25
The Kingdom Plantae contains mosses, ferns, conifers, and flowering plants. Some plants are seedless and some plants have seeds. Algae have characteristics of plants but are usually not classified with plants. Algae can produce their own food by photosynthesis using chlorophyll and chloroplast organelles in their structures. However, algae do not have roots, stems, and leaves. Evolutionists believe that algae are a link between single celled organisms and plants. Green algae, diatoms, dinoflagellates, red algae, and cyanobacteria all photosynthesize. However, some algae do not photosynthesize (heterotrophic algae, which are colorless and commonly parasitic). Many algae types are microscopic, but not all (seaweed). Microscopic algae can become visible during an algae bloom, when algae abundantly accumulate in water.
Alternation of generations is a life cycle in which plants alternate between two distinct stages, or generations, during their life cycle: the haploid (single chromosome) gametophyte and the diploid (chromosome pair) sporophyte. In some plants and organisms, the haploid stage of the life cycle is dominant and in other organisms, the diploid stage is dominant. All plants have alternation of generations life cycle. Embryophytes are a subkingdom (Embryophyta) of plants in which the embryo is retained within maternal tissue and which include the bryophytes and tracheophytes.
The sporophyte of seedless plants is diploid and results from syngamy (fusion) of two gametes. The sporophyte bears the sporangia, or vessel for spores. Inside the sporangia, the diploid sporocyte mother cells produce haploid spores by meiosis, where the chromosome number is reduced from 2n to 1n. The spores are released into the environment. When the haploid spore germinates, it generates a gametophyte by mitosis. The gametophyte supports the zygote formed from the fusion of gametes and the young sporophyte.
Some plants only produce one type of spore (homosporous) and produces male and female gametes. Non-vascular plants are homosporous. Plants that produce two types of spores are called heterosporous. Most seeded plants and some seedless vascular plants are heterosporous. The spores of seedless plants are surrounded by thick cell walls containing a tough polymer known as sporopollenin, also found in the walls of pollen grains. Sporopollenin is strongly resistant to chemical and biological degradation.
Gametangia (singular, gametangium) are structures observed on multicellular haploid gametophytes. In the gametangia, precursor cells give rise to gametes by mitosis. The male gametangium (antheridium) releases sperm. Seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment to the archegonium: the female gametangium. The embryo develops inside the archegonium as the sporophyte. Gametangia are prominent in seedless plants, but are absent or rudimentary in seed plants.
Shoots and roots of plants increase in length through rapid cell division in a tissue called the apical meristem, which is a small mitotically active zone of cells found at the shoot tip or root tip. The apical meristem is made of undifferentiated cells that continue to grow through the life of the plant. Meristemic cells promote all of the specialized tissues in the organism.
Extinct species of plants are found in the fossil record, and are not living today, while extant species are still living today. Extinct species can have different features than today's living plants.
Green algae and land plants are classified into the subphylum Strepophyta (the green plants). Charophytes are freshwater green algae, while Embryophytes are land plants.
Land plants can be classified as non-vascular or vascular, vascular being a tissue formed of specialized cells for the transport of water and nutrients. Liverworts, mosses, and hornworts are seedless, non-vascular plants.
Lycophytes and monilophytes are referred to as seedless vascular plants. Lycophytes include club mosses, quillworts, and spike mosses. Monilophytes include ferns, whisk ferns, and horsetails.
The seed plants, or spermatophytes, form the largest group of all existing plants. The seed plants include gymnosperms, including conifers, which produce naked seeds, and the flowering plants called angiosperms, where the center of the flower structure protects the seeds.
Streptophytes are land plants and green algae. Green algae fall into two major groups, the chlorophytes and the charophytes. These differ in molecular structure.
Green algae reproduce both asexually, by fragmentation or dispersal of spores, or sexually, by producing gametes that fuse during fertilization. Some green algae undergo mitosis after fertilization and some do not. Charophyta is a group of freshwater green algae that include the Charales, the Zygnematales, and the Coleochaetales orders. The Charales live in freshwater and have unpleasant smell. Large cells form the thallus, the main stem while branches also extend from the stem. The Coleochaetes are branched or disclike multicellular forms and they reproduce asexually and sexually with a haplontic life cycle. Zygnematales include Spirogyra and the desmids.
Bryophytes live in mostly moist habitats, but some live in dry habitats like deserts and tundra. Bryophytes are non-vascular, non-tracheophyte, and the main organs belong to the haploid organism or gametophyte. Male gametes swim with flagellum in water to fertilize. The sporophyte that develops from the embryo is barely noticeable. The sporangium, the multicellular sexual reproductive structure in which meiosis produces haploid spores, is present in bryophytes like most land plants. The bryophytes are divided into three phyla: the liverworts or Marchantiophyta, the hornworts or Anthocerotophyta, and the mosses or true Bryophyta.
Liverworts live on every land habitat on earth with thousands of species. Lobate liverworts form a flat thallus (body) that resemble liver lobes. Leafy liverworts have leaf-like features attached to a stalk. The life cycle of a lobate liverwort: The sperm swims into the archegonium and fertilizes the egg. The embryo grows into a slender stalk called a seta and meiosis produces spores in a mature sporophyte. The spore grow into a thallus with rhizoids.
Hornworts have a characteristic narrow, pipe-like sporophyte and live on land near a source of moisture. Stomata are air pores that open and close in hornworts. Meristem cells at the base allow the hornwort to grow taller. Hornworts have symbiotic relationships with cyanobacteria that fix nitrogen in the environment.
Hornwort life cycle follows alternation of generations. The gametophyte grows as flat thalli on the soil with male and female gametangia. Flagellated sperm swim to the archegonia and fertilize eggs, producing an embryo. The embryo zygote develops into a long and slender sporophyte that eventually splits open down the side, releasing spores. Thin branched cells called pseudoelaters surround the spores and help propel them farther in the environment. The haploid spores germinate and give rise to the next generation of gametophytes.
Mosses are the most abundant of the non-vascular plants with ten thousand species identified and habitats in tundra and tropical forests. In tundra, shallow rhizoids allow mosses to stick to a surface without penetrating the frozen soil. Mosses benefit the environment by helping slow down erosion, store moisture and soil nutrients, provide shelter for small animals, and food for larger animals. Mosses are sensitive to air pollution.
Mosses form diminutive gametophytes and are green, flat structures with a simple midrib and no stomata or vascular tissue, attached to a central stalk in a spiral. The sporophyte has stomata, allowing water and nutrients. The rest of the moss body can absorb water and nutrients.
Moss life cycle: follows the pattern of alternation of generations: The haploid gametophyte germinates from a haploid spore and forms the first protonema, single-celled filaments that grasp the ground. Cells similar to an apical meristem divide and produce a gametophore, with a photosynthetic stem and foliage-like structures. Male and female gametangia develop at the tip of separate gametophores. The antheridia (male organs) produce many sperm, whereas the archegonia (the female organs) each form a single egg at the base (venter) of a flask-shaped structure. The archegonium produces attractant substances and at fertilization, the sperm swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.
The moss sporophyte is dependent on the gametophyte for nutrients. The slender seta (plural, setae) contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium or capsule. Spore mother cells in the sporangium undergo meiosis to produce haploid spores. The sporophyte has several features that protect the developing spores and aid in their dispersal. The calyptra, derived from the walls of the archegonium, covers the sporangium. A structure called the operculum is at the tip of the spore capsule. The calyptra and operculum fall off when the spores are ready for dispersal. The peristome, tissue around the mouth of the capsule, is made of triangular, close-fitting units like little “teeth.” The peristome opens and closes, depending on moisture levels, and periodically releases spores.
The vascular plants, or tracheophytes, are the dominant group of land plants on Earth with 260,000 species and represent 90 percent of Earth's vegetation.
Vascular tissue contains xylem in the center and is 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: It is impermeable to water and confers mechanical strength on vascular tissue. With their rigid cell walls, the xylem cells provide support to the plant and allow it to achieve impressive heights, such as with tall trees as they develop an advantage in attracting light and nutrients from their height and size.
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.
The origin of roots in vascular plants is difficult to explain because they are not well preserved in the fossil record. Roots are much longer and more developed than rhizoids. Roots support the plant, providing a strong anchor, absorb water and nutrients from the soil, establish symbiotic relationships with fungi and allowing for even more absorption of water and nutrients. The appearance of developed roots does not seem to have an evolutionary line that can be traced.
Leaves in vascular plants help in photosynthesis as they capture more sunlight with their larger surface area. Leaves are another plant feature that does not seem to have an evolutionary link. Two types of leaf morphology are microphylls and megaphylls. Microphylls have a single, unbranched vein and the vascular tissue runs through the center of the leaf. Megaphylls are larger leaves with a pattern of multiple veins.
Pine cones, mature fronds of ferns, and flowers are all sporophylls, leaves that were modified structurally to bear sporangia. The strobili of pine cones are cone-like structures that contain sporangia.
Club mosses, or Lycophyta, are diminutive, evergreen plants consisting of a stem and microphylls. Quillworts, club mosses, and spike mosses are all part of Lycophyta and none are true mosses or bryophytes.
Lycophytes follow the pattern of alternation of generations like the bryophytes, except that the sporophyte is the major stage of the life cycle. Lycophytes can have spores of the same size or different sizes and can have symbiotic relationships with fungi.
Horsetails belong to phylum Monilophyta and class Equisetopsida, which also include the ferns and whisk ferns and are found in wet environments like marshes. The stem of a horsetail has joints or nodes where the needle-shaped leaves and branches come out. Photosynthesis occurs mainly in the thick stem. Silica in the epidermal cells makes this plant stiff. Horsetails are homosporous.
Whisk Ferns: Phylum Monilophyta: Class Psilotopsida. Whisk ferns are not like most ferns in that they do not have leaves and roots. Photosynthesis takes place in their branching stems. The tips of the branches contain small yellow knobs with sporangia, which produce spores that develop into male and female gametangia.
True Ferns: Phylum Monilophyta: Class Polypodiopsida. True ferns have large fronds and are very recognizable among seedless vascular plants. Some 20,000 species of true ferns live in temperate and tropical forest environments. The dominant stage in the life cycle of the fern is the sporophyte, which are the large compound leaves called fronds. Some fronds are broadly lobed and others are finely lobed. The fronds are photosynthetic and reproductive organs. The stem may be buried underground from which roots grow, or they may grow above ground as a trunk in tree ferns. Some roots grow from the side of the stem.
The underside of the fern frond are groups of sporangia called sori and are homosporous. In the life cycle of ferns, spores are produced by meiosis and are released into the air from the sporangium. Those that land on a suitable substrate germinate and form a heart-shaped gametophyte, or prothallus, which is attached to the ground by thin filamentous rhizoids. Gametophytes produce both antheridia and archegonia. Like the sperm cells of other pterophytes, fern sperm have multiple flagella and must swim to the archegonium, which releases a chemoattractant to guide them. The zygote develops into a fern sporophyte, which emerges from the archegonium of the gametophyte. Maturation of antheridia and archegonia at different times encourages cross-fertilization.
The importance of seedless plants:
Mosses and liverworts provide food and shelter for other plant species. Bryophytes grow well in tundra because they do not have deep roots and can dry and rehydrate quickly. Mosses are a food source for animals in the tundra. Mosses partner with cyanobacteria to replenish the soil with nutrients. Peat moss is used as fuel and in landscaping. Ferns are a popular ornamental plant.
The Kingdom Plantae contains mosses, ferns, conifers, and flowering plants. Some plants are seedless and some plants have seeds. Algae have characteristics of plants but are usually not classified with plants. Algae can produce their own food by photosynthesis using chlorophyll and chloroplast organelles in their structures. However, algae do not have roots, stems, and leaves. Evolutionists believe that algae are a link between single celled organisms and plants. Green algae, diatoms, dinoflagellates, red algae, and cyanobacteria all photosynthesize. However, some algae do not photosynthesize (heterotrophic algae, which are colorless and commonly parasitic). Many algae types are microscopic, but not all (seaweed). Microscopic algae can become visible during an algae bloom, when algae abundantly accumulate in water.
Alternation of generations is a life cycle in which plants alternate between two distinct stages, or generations, during their life cycle: the haploid (single chromosome) gametophyte and the diploid (chromosome pair) sporophyte. In some plants and organisms, the haploid stage of the life cycle is dominant and in other organisms, the diploid stage is dominant. All plants have alternation of generations life cycle. Embryophytes are a subkingdom (Embryophyta) of plants in which the embryo is retained within maternal tissue and which include the bryophytes and tracheophytes.
The sporophyte of seedless plants is diploid and results from syngamy (fusion) of two gametes. The sporophyte bears the sporangia, or vessel for spores. Inside the sporangia, the diploid sporocyte mother cells produce haploid spores by meiosis, where the chromosome number is reduced from 2n to 1n. The spores are released into the environment. When the haploid spore germinates, it generates a gametophyte by mitosis. The gametophyte supports the zygote formed from the fusion of gametes and the young sporophyte.
Some plants only produce one type of spore (homosporous) and produces male and female gametes. Non-vascular plants are homosporous. Plants that produce two types of spores are called heterosporous. Most seeded plants and some seedless vascular plants are heterosporous. The spores of seedless plants are surrounded by thick cell walls containing a tough polymer known as sporopollenin, also found in the walls of pollen grains. Sporopollenin is strongly resistant to chemical and biological degradation.
Gametangia (singular, gametangium) are structures observed on multicellular haploid gametophytes. In the gametangia, precursor cells give rise to gametes by mitosis. The male gametangium (antheridium) releases sperm. Seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment to the archegonium: the female gametangium. The embryo develops inside the archegonium as the sporophyte. Gametangia are prominent in seedless plants, but are absent or rudimentary in seed plants.
Shoots and roots of plants increase in length through rapid cell division in a tissue called the apical meristem, which is a small mitotically active zone of cells found at the shoot tip or root tip. The apical meristem is made of undifferentiated cells that continue to grow through the life of the plant. Meristemic cells promote all of the specialized tissues in the organism.
Extinct species of plants are found in the fossil record, and are not living today, while extant species are still living today. Extinct species can have different features than today's living plants.
Green algae and land plants are classified into the subphylum Strepophyta (the green plants). Charophytes are freshwater green algae, while Embryophytes are land plants.
Land plants can be classified as non-vascular or vascular, vascular being a tissue formed of specialized cells for the transport of water and nutrients. Liverworts, mosses, and hornworts are seedless, non-vascular plants.
Lycophytes and monilophytes are referred to as seedless vascular plants. Lycophytes include club mosses, quillworts, and spike mosses. Monilophytes include ferns, whisk ferns, and horsetails.
The seed plants, or spermatophytes, form the largest group of all existing plants. The seed plants include gymnosperms, including conifers, which produce naked seeds, and the flowering plants called angiosperms, where the center of the flower structure protects the seeds.
Streptophytes are land plants and green algae. Green algae fall into two major groups, the chlorophytes and the charophytes. These differ in molecular structure.
Green algae reproduce both asexually, by fragmentation or dispersal of spores, or sexually, by producing gametes that fuse during fertilization. Some green algae undergo mitosis after fertilization and some do not. Charophyta is a group of freshwater green algae that include the Charales, the Zygnematales, and the Coleochaetales orders. The Charales live in freshwater and have unpleasant smell. Large cells form the thallus, the main stem while branches also extend from the stem. The Coleochaetes are branched or disclike multicellular forms and they reproduce asexually and sexually with a haplontic life cycle. Zygnematales include Spirogyra and the desmids.
Bryophytes live in mostly moist habitats, but some live in dry habitats like deserts and tundra. Bryophytes are non-vascular, non-tracheophyte, and the main organs belong to the haploid organism or gametophyte. Male gametes swim with flagellum in water to fertilize. The sporophyte that develops from the embryo is barely noticeable. The sporangium, the multicellular sexual reproductive structure in which meiosis produces haploid spores, is present in bryophytes like most land plants. The bryophytes are divided into three phyla: the liverworts or Marchantiophyta, the hornworts or Anthocerotophyta, and the mosses or true Bryophyta.
Liverworts live on every land habitat on earth with thousands of species. Lobate liverworts form a flat thallus (body) that resemble liver lobes. Leafy liverworts have leaf-like features attached to a stalk. The life cycle of a lobate liverwort: The sperm swims into the archegonium and fertilizes the egg. The embryo grows into a slender stalk called a seta and meiosis produces spores in a mature sporophyte. The spore grow into a thallus with rhizoids.
Hornworts have a characteristic narrow, pipe-like sporophyte and live on land near a source of moisture. Stomata are air pores that open and close in hornworts. Meristem cells at the base allow the hornwort to grow taller. Hornworts have symbiotic relationships with cyanobacteria that fix nitrogen in the environment.
Hornwort life cycle follows alternation of generations. The gametophyte grows as flat thalli on the soil with male and female gametangia. Flagellated sperm swim to the archegonia and fertilize eggs, producing an embryo. The embryo zygote develops into a long and slender sporophyte that eventually splits open down the side, releasing spores. Thin branched cells called pseudoelaters surround the spores and help propel them farther in the environment. The haploid spores germinate and give rise to the next generation of gametophytes.
Mosses are the most abundant of the non-vascular plants with ten thousand species identified and habitats in tundra and tropical forests. In tundra, shallow rhizoids allow mosses to stick to a surface without penetrating the frozen soil. Mosses benefit the environment by helping slow down erosion, store moisture and soil nutrients, provide shelter for small animals, and food for larger animals. Mosses are sensitive to air pollution.
Mosses form diminutive gametophytes and are green, flat structures with a simple midrib and no stomata or vascular tissue, attached to a central stalk in a spiral. The sporophyte has stomata, allowing water and nutrients. The rest of the moss body can absorb water and nutrients.
Moss life cycle: follows the pattern of alternation of generations: The haploid gametophyte germinates from a haploid spore and forms the first protonema, single-celled filaments that grasp the ground. Cells similar to an apical meristem divide and produce a gametophore, with a photosynthetic stem and foliage-like structures. Male and female gametangia develop at the tip of separate gametophores. The antheridia (male organs) produce many sperm, whereas the archegonia (the female organs) each form a single egg at the base (venter) of a flask-shaped structure. The archegonium produces attractant substances and at fertilization, the sperm swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.
The moss sporophyte is dependent on the gametophyte for nutrients. The slender seta (plural, setae) contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium or capsule. Spore mother cells in the sporangium undergo meiosis to produce haploid spores. The sporophyte has several features that protect the developing spores and aid in their dispersal. The calyptra, derived from the walls of the archegonium, covers the sporangium. A structure called the operculum is at the tip of the spore capsule. The calyptra and operculum fall off when the spores are ready for dispersal. The peristome, tissue around the mouth of the capsule, is made of triangular, close-fitting units like little “teeth.” The peristome opens and closes, depending on moisture levels, and periodically releases spores.
The vascular plants, or tracheophytes, are the dominant group of land plants on Earth with 260,000 species and represent 90 percent of Earth's vegetation.
Vascular tissue contains xylem in the center and is 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: It is impermeable to water and confers mechanical strength on vascular tissue. With their rigid cell walls, the xylem cells provide support to the plant and allow it to achieve impressive heights, such as with tall trees as they develop an advantage in attracting light and nutrients from their height and size.
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.
The origin of roots in vascular plants is difficult to explain because they are not well preserved in the fossil record. Roots are much longer and more developed than rhizoids. Roots support the plant, providing a strong anchor, absorb water and nutrients from the soil, establish symbiotic relationships with fungi and allowing for even more absorption of water and nutrients. The appearance of developed roots does not seem to have an evolutionary line that can be traced.
Leaves in vascular plants help in photosynthesis as they capture more sunlight with their larger surface area. Leaves are another plant feature that does not seem to have an evolutionary link. Two types of leaf morphology are microphylls and megaphylls. Microphylls have a single, unbranched vein and the vascular tissue runs through the center of the leaf. Megaphylls are larger leaves with a pattern of multiple veins.
Pine cones, mature fronds of ferns, and flowers are all sporophylls, leaves that were modified structurally to bear sporangia. The strobili of pine cones are cone-like structures that contain sporangia.
Club mosses, or Lycophyta, are diminutive, evergreen plants consisting of a stem and microphylls. Quillworts, club mosses, and spike mosses are all part of Lycophyta and none are true mosses or bryophytes.
Lycophytes follow the pattern of alternation of generations like the bryophytes, except that the sporophyte is the major stage of the life cycle. Lycophytes can have spores of the same size or different sizes and can have symbiotic relationships with fungi.
Horsetails belong to phylum Monilophyta and class Equisetopsida, which also include the ferns and whisk ferns and are found in wet environments like marshes. The stem of a horsetail has joints or nodes where the needle-shaped leaves and branches come out. Photosynthesis occurs mainly in the thick stem. Silica in the epidermal cells makes this plant stiff. Horsetails are homosporous.
Whisk Ferns: Phylum Monilophyta: Class Psilotopsida. Whisk ferns are not like most ferns in that they do not have leaves and roots. Photosynthesis takes place in their branching stems. The tips of the branches contain small yellow knobs with sporangia, which produce spores that develop into male and female gametangia.
True Ferns: Phylum Monilophyta: Class Polypodiopsida. True ferns have large fronds and are very recognizable among seedless vascular plants. Some 20,000 species of true ferns live in temperate and tropical forest environments. The dominant stage in the life cycle of the fern is the sporophyte, which are the large compound leaves called fronds. Some fronds are broadly lobed and others are finely lobed. The fronds are photosynthetic and reproductive organs. The stem may be buried underground from which roots grow, or they may grow above ground as a trunk in tree ferns. Some roots grow from the side of the stem.
The underside of the fern frond are groups of sporangia called sori and are homosporous. In the life cycle of ferns, spores are produced by meiosis and are released into the air from the sporangium. Those that land on a suitable substrate germinate and form a heart-shaped gametophyte, or prothallus, which is attached to the ground by thin filamentous rhizoids. Gametophytes produce both antheridia and archegonia. Like the sperm cells of other pterophytes, fern sperm have multiple flagella and must swim to the archegonium, which releases a chemoattractant to guide them. The zygote develops into a fern sporophyte, which emerges from the archegonium of the gametophyte. Maturation of antheridia and archegonia at different times encourages cross-fertilization.
The importance of seedless plants:
Mosses and liverworts provide food and shelter for other plant species. Bryophytes grow well in tundra because they do not have deep roots and can dry and rehydrate quickly. Mosses are a food source for animals in the tundra. Mosses partner with cyanobacteria to replenish the soil with nutrients. Peat moss is used as fuel and in landscaping. Ferns are a popular ornamental plant.