Introduction to the Plant Kingdom - 1 The Plant Kingdom comprises a large and varied group of organisms that have the following characteristics in common. All plants are: Eukaryotic Photosynthetic Multicellular with tissue development Sexually reproducing Who are the Plants? The general classification of plants uses the following features to divide plants into four broad groups and a number of phyla: Presence or absence of vascular tissue Dispersal by spores or by seeds Seed plants are distinguished further by the protection of the seed.
Introduction to the Plant Kingdom - 2 The Plant Phyla
Introduction to the Plant Kingdom - 3 The Life History of all plants involves an alternation of a haploid phase (the Gametophyte) that produces gametes in protective structures called gametangia, with a diploid phase (the Sporophyte) that produces sporangia. To start our discussion of plants, we will review the alternation of generations, first presented in Biology 201. Variations in life history are essential to classification in the Plant Kingdom. Plant Life History - The Alternation of Generations Sperm (n) + Egg (n) Zygote (2n) Growth by mitosis produces: Embryo (2n) Sporophyte (2n) Sporangium (2n)* Meiosis in Sporangium produces: Spore (n) Growth by mitosis produces: Gametophyte (n) Antheridium (n) Archegonium (n) Sperm (n) Egg (n * Variations in Life History Patterns Sporangia: Homosporous or Heterosporous Prevalence (predominance) of sporophyte or gametophyte. More complex plants = more sporophyte dominance Independence or dependence of Sporophyte or Gametophyte on alternate generation. More advanced plants have a dependent gametophyte.
Introduction to the Plant Kingdom - 4 Evolutionary Trends in the Plant Kingdom Most plants are terrestrial. Plants evolved from green algae ancestors (Charophyceae) that successfully inhabited land. As we have discussed, plants must obtain their nutrients from both air and soil, and much of their structure relates to the need to maximize obtaining nutrients and water, minimizing water loss and protecting the plant from damage. To this end, to survive on land, plants evolved: Protective surfaces to minimize desiccation - cuticle Gas exchange to circumvent the cuticle stomata Increase in photosynthetic surface area - leaves Increase in dimension with conducting and support tissues Reproductive strategies that work without water transport of gametes All plants disperse to new locations via single-celled spores or by seeds. Spore walls contain sporopollenin, a protective coating that provides protection against decay and dehydration. Seeds are protected by a seed coat, a multicellular layer of thickwalled resistant cells.
Introduction to the Plant Kingdom - 5 Non-Vascular Plants The non-vascular plants include the mosses, hornworts and liverworts. Because they lack vascular tissue they are small in vertical stature, and typically grow in clumps or masses. Most lack a cuticle and many are capable of withstanding long periods of desiccation. They attach to their substrate with rhizoids, but absorb water and minerals through all surface cells. Some have air pores for diffusion of gases. Some Bryophytes have a thallus (flattened-shaped) structure and others are tiny, three-dimensional with an axis and leaves, called phyllodes, attached. They have apical meristems. All Bryophytes disperse by spores and water is required to transport the motile sperm for fertilization. The gametophyte generation is dominant. The three non-vascular Hepatophyta Anthocerophyta Bryophyta plant phyla are: Vascular Plants Vascular plants exhibit a number of advances that promote survival on land. Protection of above-ground body surfaces with a cuticle or surfaces tissues with cell walls with impermeable layers minimizes water loss, often mentioned in Biology as a significant problem for plants. The evolution of conducting tissue provided a means for plants to become large in dimension and volume. Vascular tissue provides support for the plant as well as conducting water and solutes throughout the plant body. Dependence on diffusion of water and nutrients affects plant size, as seen in the Bryophytes. The vascular plants all have a dominant sporophyte generation. They produce multiple numbers of sporangia on each sporophyte and the spore-dispersing vascular plants can disperse many airborne spores which means plants can occupy more locations rapidly. Transport of sperm in many plants requires water. Motile sperm requiring water for transport limits plants ability to sexually reproduce. Gametophytes are greatly reduced in size as well as duration relative to the sporophyte in the vascular plants. As vascular plants become even more complex, sperm are retained in the male gametophyte, the pollen grain, freeing sexual reproduction from water dependence. In the seed plants, the seed (embryo sporophyte surrounded by nutrients and seed coat) replaces the haploid single-celled spore as the dispersal unit. These adaptations to the terrestrial environment have made vascular plants the predominant vegetation in most parts of the world. While abundant, Bryophytes are rarely predominant.
Introduction to the Plant Kingdom - 6 To summarize, Vascular Plants have: Body plan along a supporting axis (the stem) Vascular tissue Xylem promotes (allows) increase in size Sporophyte generation independent and assimilative Gametophyte often dependent on the sporophyte Cutin and/or suberin produced on aerial parts to minimize dehydration Generally large surface area/volume ratio Allows land survival with less surface H 2 O Classification of the Vascular Plants There are seven (or nine) extant vascular plant phyla and at several phyla comprised of extinct vascular plants. We use the fossil record of the extinct vascular plants to trace our plant ancestors. Moreover, long-extinct precursors of today s vascular plants are the source of most of the world s coal deposits. Evolutionary Features of Vascular Plants
Introduction to the Plant Kingdom - 7 Review of Features used in Classifying Vascular Plants Vascularization Protostele, siphonostele and eustele vascular patterns in stems The protostele is a solid core of stem vascular tissue from which leaves diverge without leaving a stem vascular gap. The siphonostele has a central pith surrounded by a cylinder of vascular tissue. In some plants that have siphonosteles the vascular strands that go to leaves (the leaf traces) result in a leaf gap of parenchyma cells. The eustele, found in most seed plants, is comprised of discreet vascular bundles surrounding the pith. Leaf trace gaps are found in the eustele. Microphylls vs. Megaphylls for leaves Microphylls contain one vascular strand, probably evolving from small projections, or enations located along stem tissue. Megaphylls have a branching system of veins, and probably evolved from side branches of stems, in a webbing process, in which tissue containing chlorophyll filled in spaces between nearby branches. Microphylls Megaphylls
Introduction to the Plant Kingdom - 8 Dispersal Method Spore (single cell) vs. seed Sperm transport H 2 O vs. pollen grain Leaf trace patterns and nodes in protostele and siphonstele Types of Sporangia Plants that produce one type of sporangium are called homosporous. Other plants produce two different sporangia, megasporangia and microsporangia and are heterosporous. A plant that is homosporous has one type of sporangium. However homosporous plants can produce one type of spore that grows into a gametophyte that contains both male and female reproductive structures (the antheridium and archegonium, respectively) or a homosporous plant can produce two types of spores in its sporangia. When two types of spores are produced, one grows into male gametophytes and the second grows into female gametophytes. Heterosporous plants always have separate sporangia. The microsporangia produce only microspores that grow into male gametophytes, producing only antheridia. Megasporangia produce megaspores that grow into female gametophytes, producing only archegonia. The megasporangia and microsporangia may be found on the same sporophyte individual, or on separate sporophyte individuals.
Introduction to the Plant Kingdom - 9 The Vascular Plant Phyla Spore-dispersing Vascular Plant Phyla Rhyniophyta Extinct Zosterophyllophyta - Extinct Trimerophytophyta - Extinct Lycophyta (Lycopodiophyta) Lycopodiae Selaginellae Isoetae Pteridophyta Equisetales (Equisetales) Psilotales (Psilotales) Ferns (Pterophyta) Ophioglossales Eusporangiate Marattiales Eusporangiate Filicales Homosporous and Leptosporangiate Salviniales Heterosporous and Leptosporangiate Marsileales Heterosporous and Leptosporangiate Seed-dispersing Vascular Plant Phyla Progymnospermophyta - Extinct Pteridospermales Cordaitales Bennettitales The Gymnosperm Phyla Cycadophyta Ginkgophyta Coniferophyta Gnetophyta The Angiosperms Anthophyta Monocotyledones Eudicotyledones Plus Magnoliidae, including several orders Nymphaeales Water Lilies Illiciales Star Anise Amborellales Archaefructales Earliest Anthophyte Fossil
Introduction to the Plant Kingdom - 10 Generalized Life History of the Vascular Plants Spore (n) Gametophyte (n) Antheridium (n) Archegonium (n) Sperm (n) Egg (n) (motile) Zygote (2n) Growth by mitosis produces a multicellular Sporophyte (2n) Sporangia (2n) If Homosporous If Heterosporous Sporangium Megasporangium Microsporangium Meiosis Meiosis Meiosis Spores (n) Megaspores (n) Microspores (n) Growth by mitosis from single-celled spore produces a multicellular Megagametophyte Microgametophyte (Often retained in the Sporophyte) Two One Separate Gametophyte Gametophytes with both Antheridia (Male and Female) and Archegonia present