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The Evolution and Diversity of Plants

Rhodophyta

Two sister taxa, Rhodophyta, or "Red Algae" and the Green Plants (sometimes called "Viridaeplantae") form the more derived group in the Archaeplastida clade.

Rhodophytes were once phylogenetically linked to cyanobacteria because of similarity in two groups of phycobilliprotein pigments, phycocyanins and phycoerythrins, which the algae use as accessory photosynthetic pigments. Whether these pigments are analogous or homologous in the red algae and cyanobacteria is not certain. But it is possible that at least a billion years ago, an ancestral, heterotrophic eukaryote took up a symbiotic relationship with a cyanobacterium-like prokaryote.

That early ancestral lineage is believed to have given rise to today's Rhodophyta and Green Plants.

  • What do you suppose might be the significance of having a red pigment (phycoerythrin) as a photosynthetic accessory pigment?
  • Hint: what wavelengths would such a pigment (1) absorb and (2) reflect.
  • And which of those wavelengths penetrates deeper into water?

    Rhodophyta is the sister taxon to Green Plants, and both groups are included in the putatively monophyletic taxon Archaeplastida, formerly known as Plantae.

    Green Plants

    Green plants include all organisms commonly known as "green algae" and all land plants. Their common ancestry is reflected in these synapomorphies, unique to green algae and land plants...

    Remember that the above characters are synapomorphies with respect to the taxa which branched off earlier on the phylogenetic tree.

    These same characters are considered to be symplesiomorphies with respect to the green algae and plants considered as a group.

    These characters are thus not informative in determining further clades within the green plants.

    The synapomorphies of land plants relative to their green algae relatives include:

    Many green algae exhibit a progression of complexity (e.g., the volvocine line of evolution). This should not be taken to represent a progression towards multicellular land plants. It merely shows that many algae appear to be genetically predisposed to have a division of labor among their cells. But this predisposition could be the reason that multicellular plants eventually did evolve

    .

    Early Cousins of Land Plants: Zygnematales

    The Zygnematales are (haploid) green algae that undergo sexual reproduction via conjugation: haploid vegetative cells in the algal adult form non-flagellated gametes. Complementary (+ and - types) gametes fuse to form a zygote. There are many variations on the conjugation theme, but one example is nicely shown in Spirogyra:

    Each zygote undergoes meiosis to produce four new haploid propagules, which are released from the cell wall of the former vegetative cell to grow into new, haploid filaments of algae.

    Early Cousins of Land Plants: Charales

    The Charales, aslo known as stoneworts or brittleworts, are common in slow-moving, often eutrophic freshwater habitats, and are found on every continent except Antarctica.

    The organism's body (a.k.a. thallus; a plant body not differentiated into roots, stems and leaves) consists of large cells that may be several centimeters long. The algae may branch at nodes made up of smaller cells.

    As in land plants, growth occurs at the apex of each tip of the thallus. Translucent rhizoids, rootlike structures that are not absorptive, but primarily for anchoring the organism, sprout from the bottom of the plant.

    Stoneworts have multicellular male and female sex organs (male = antheridium; female = oogonium), which grow at the nodes.

    The name of these plants comes from the crust of white calcium carbonate seen in some species, which gives them a crunchy feel. Some species have a strong, musky smell, and are sometimes given the nickname "skunkweed" or "musk grass".

    Chara is a species of particular interest in southern Florida, as it is a problematic invasive exotic that chokes canals and waterways throughout our area.

    Early Cousins of Land Plants: Coleochaetales

    There are fifteen extant species of Coleochaete, of interest to systematists because they appear to be the closest living relatives of land plants. What characters do they share? Coleochaetes are abundant, often found attached to the leaves of aquatic flowering plants and other firm substrates in fresh water. Land Plants, Zygnematales, Coleochaetales and Charales share important synapomorphies:

    Land Plants: Evolutionary History and Diversity

    Land plants all produce embryos that develop inside the tissues of a maternal plant. This is where the taxon gets its name. All embryophytes are primitively land-dwelling, though some are not entirely free of a need for water in the environment, especially for reproduction.

    Why would a terrestrial existence be an advantage for a plant? Why didn't they just stay in the water?

  • The earliest plant ancestors are known from Ordovician fossils (little more than small bits of vascular tissue)
  • Lignin-like fossil compounds have been found in Silurian fossils
  • The earliest plants were likely endomycorhizzal, and had symbiotic zygomycete (i.e., members of Phylum Zygomycota, the Black Bread Molds) fungal partners.
  • The earliest definite plant fossil is 430 million years old, dating from the Silurian.

    Embryophyta: The Land Plants

    What makes land plants different from other members of Viridaeplantae?
    Several synapomorphies link all land plants together and distinguish them from their algal relatives:

    Items above that appear in [brackets] are found in all plants, but may also appear in a few highly derived green algae and/or charophytes.

    Embryophyta: An Overview

  • The Bryophytes - Non-vascular Plants
    These include the Marchantiomorpha (liverworts), Anthocerophyta (hornworts), and Bryophyta (true mosses). Bryophytes lack an internal vascular system for transport: no xylem, no phloem.

    In Bryophytes, the haploid gametophyte phase is the dominant life cycle phase, living more than one season.
    The diploid sporophyte generation is small and ephemeral.

  • In bryophytes, the gametophyte is the dominant life-cycle phase, living year after year.

  • The sporophyte is small and ephemeral, living only long enough to produce spores, and then die.

  • Bryophytes lack complex vascular tissue, though some of the more derived mosses have traces of conducting tissues.

  • Because bryophytes lack true vascular tissue, their leaf-, stem- and rootlike structures are NOT true leaves, stems and roots.

  • The body is thus known as a thallus, the term used to describe a plant (or fungal) "body".

    Liverwort gametophytes:

  • The male (left) bears stalks called antheridiophores that bear the sperm-generating antheridia

  • The female (right) bears stalks called archegoniophores that bear the ovum-bearingarchegonia

    .

  • When the environment is wet, the antheridia release sperm, which swim up the archegoniophore and into the archegonium.

  • Fertilization takes place inside the archegonium, and the resulting zygote then grows into a sporophyte, in situ.

  • Still attached to the archegoniophore, the sporophyte grows a single sporangium, within which meiosis takes place.

  • The resulting spores are released into the environment, where the lucky ones will germinate and grow into gametophytes (either male or female).

  • The sporophyte and archegoniphore then wither and die, leaving the gametophyte thallus to grow and prosper. Here's the whole cycle...

    Mosses and Hornworts have very similar life cycles, though the plants look somewhat different.
    In all Bryophytes, the gametophyte is the long-living stage, whereas the sporophyte is ephemeral.
    The situation is completely reversed in the...

  • Tracheophyta - The Vascular Plants As the name implies, these plants have an internal transport system consisting of In Tracheophytes, the diploid sporophyte phase is dominant, living more than one season.
    The haploid gametophyte generation is small and ephemeral.
    Recall this from the fern life cycle:

    The life cycle is similar in all tracheophytes, though--of course--the life cycle stages and the plants themselves look different. But the sporophytes, gametophytes and all associated structures of all the plants are homologous, no matter how different they might appear.

    An Overview of Plant Taxa

  • Angiosperms - The Flowering Plants)