The charophytes: Link between Algae and Plants? This is a paraphyletic assemblage that is still in biosystematic flux. Some charophytes appear to be more closely related to plants than to other "charophytes".

The species of greatest interest to plant systematists is...

Coleochaete orbicularis - an aquatic epiphyte (a plant which lives on the surface of another plant, but is not parasitic) that shows several synapomorphies w/ plants.

• has parenchyma-like cells

• cell plate & phragmatoplasts (complex of endoplasmic reticulum and microtubules which faciltate mitosis/cell plate formation) are almost identical to those of plants

• zygote develops into a sporophyte while surrounded by female gametophyte's archegonial tissues

• lignin-like compound present in tissues

• cellose content of cell wall is 20-26% (much higher than algae, and similar to that found in early plants)

• peroxisome enzyme composition is the same as that in plants

  (the peroxisome is an intracellular body which enzymatically adds H to Oxygen waste product, creating H2O2. Other enzymes then degrade this peroxide into harmless byproducts)

• sperm structure more similar to that of plants than to that of algae.

• RNA analysis suggests that charophytes and plants are sister taxa.

• Weird sidelight: charophytes do not show alternation of generations.

  Does this mean that plant/algae alternation of generations is a homoplasy? Or that charophytes have secondarily lost the alternation of generations seen in many green algae? No one knows.

Recall the synapomorphies that link all plants together and distinguish them from green algae:

• true tissues
• waxy cuticle (to prevent desiccation)
• stomates
• (gametangia)
• alternation of generations with heteromorphy
• (oogamy)

  (note: oogamy is a special case of heterogamy--in which two gametes are physically distinguishable--in which the ovum is relatively large and sedentary and the sperm is small and motile)

• (the zygote develops inside the maternal tissues)

A quick "Tiptoe through the Taxa" to orient ourselves:

• Superphylum Bryophyta - The Non-vascular Plants
  In Bryophytes, the haploid gametophyte phase is "dominant," living more than one season;
  the diploid sporophyte generation is small and ephemeral.
  • Phylum Hepatophyta - the liverworts
  • Phylum Anthocerophyta - the hornworts
  • Phylum Bryophyta - the mosses

• Superphlum Tracheophyta - The Vascular Plants In Tracheophytes, the diploid sporophyte phase is "dominant," living more than one season;
  the haploid gametophyte generation is small and ephemeral.
  • The Seedless Vascular Plants
    • Phylum Psilophyta - the whisk ferns
    • Phylum Lycophyta - the club "mosses"
    • Phylum Sphenophyta - the horsetails
    • Phylum Pterophyta - the true ferns

  • The Seed-producing Vascular Plants (or just "seed plants")
    • Gymnosperms (the naked seed plants)
      • Phylum Cycadophyta - the cycads ("sago palms")
      • Phylum Ginkgophyta - maidenhair trees
      • Phylum Coniferophyta - coniferophyta
      • Phylum Gnetophyta - the gnetophytes

    • Angiosperms (the flowering plants)
      • Phylum Anthophyta
        (a.k.a. Magnoliophyta; (Magnolia is the most primitive of all flowering plants)
        • Dicotyledonae
          (dicots - probably paraphyletic)
        • Monocotyledonae
          (monocots - a monophyletic lineage branched from one of the many dicot taxa)

NameNote: The suffix "wort" is from the ancient Anglo Saxon word wyrt meaning "herb".

PhylogenyNote: Liverworts (Hepatophyta) may not be monophyletic with the rest of the plants. Many of their characters (including the stomate-like structures that don't open and close) appear to be convergent-- not homlogous with other plant structures.

Anthocerophytes--the hornworts--are believed to be the closest living relatives of the charophytes. They have stomates that actually open and close, as do those of true plants.

PLANT TISSUES

Plants became truly land dwelling with the advent of:

1. spores with durable, protective walls (you'll find out what a spore is soon enough)

2. thickened waxy cuticle (relative to bryophyte cuticle)

3. stomates that open and close

4. lignin (an important component of wood that confers compressional strength):

The diagram on the left shows monomers (sugars) from which water can be removed, and the monomers linked to form a complex, sheetlike structure, lignin, on the right. (Obviously, there are many different lignins, each with different substituted benzene rings and cross-linkages).
Few microorganisms can break down lignins, and it is impermeable to most anaerobic decay processes. Lignin--like most organic compounds--does break \ down in the presence of oxygen, but this happens slowly.

Lignin and cellulose are the main structural components of plants, conferring both compressional strength and flexibility.

5. progression of alternation of generations so that the gametophyte is the small, ephemeral stage, and the sporophyte the large, persistent stage.

PLANT ORGANS There are only three:

• root
• stem
• leaf

Plant organs are generally defined by the presence of more than one type of tissue. So before we embark on our study of plant organs, let's have a look at their components.

PLANT TISSUES A Tissue is an aggregation of cells coordinated to perform a particular function or set of functions.

Tissues may be

• simple (composed of only one type of cell)
  • parenchyma
  • collenchyma
  • sclerenchyma

• complex (composed of more than one type of cell)
  • dermal (protective covering)
  • vascular (conducting tissue)
    • xylem (conducts water and dissolved minerals)
    • phloem (conducts water and dissolved organics)
  • ground (bulk of the body; primarily parenchyma, collenchyma & sclerenchyma)

  Let's meet the tissues in more detail.

• parenchyma
  
  (when parenchyma cells have a high density of chloroplasts, they are sometimes referred to as "chlorenchyma" cells)
• collenchyma
• sclerenchyma

• XYLEM
  • conducting cells (tracheids; vessel elements)
    
    Vessel elements are more derived than tracheids. They form long, continuous tubes when stacked on top of each other, whereas tracheids "dovetail" onto each other, and have smaller lumens.

    At maturity, xylem conducting cells are hollow and dead.

  • ground cells (parenchyma, collenchyma, sclerenchyma)

• PHLOEM
  • sieve cells - pores small, uniform; dovetail end to end
  • sieve tube members (= elements) - pores on some walls are larger than sieve cell pores, and are clustered to form SIEVE PLATES. These are most commonly found on the end walls, and are areas of contact between adjacent sieve tube members. These are more derived than sieve cells.

    At maturity, the conducting cells of phloem contain a protoplast (unlike xylem, which is dead and hollow at maturity): but it has NO MAJOR ORGANELLES.

  • albuminous cells (gymnosperms & more primitive taxa)
  • companion cells (angiosperms)
  • transfer cells (high surface area for chemical transfers)
  • ground cells (parenchyma, sclerenchyma, collenchyma)

  More about companion cells and albuminous cells...

• In angiosperms
  each sieve tube member is associated with a COMPANION CELL, which is a fully equipped parenchyma cell that performs all necessary metabolic functions for both cells.
  • The two are joined by numerous plasmodesmata, creating a large surface area for transport between the two cell membranes.

  • The two are derived from the same progenitor (mother) cell.
    

• In gymnosperms
  each sieve cell is associated with an albuminous cell.
  • This performs the same function as the companion cell in angiosperms, but it is NOT derived from the same progenitor cell as the sieve cell it serves.

• guard cells (bordering the stomates)
• trichomes (cell with a long, hairlike extension)
  
  The botanical terms
  • glabrous: hairless
  • pubescent: fuzzy
  • hirsute: hairy
  all refer to the presence or absence of foliar trichomes.
  e.g. - root hairs, surface hairs on leaves & stems, special absorbing trichomes in epiphyte foliage, salt-secreting cells
• gland cells - produce a wide variety of compounds, from nectar to stinging compounds (associated with trichomes) to...just about anything.

PERIDERM
This is a secondary epidermis, produced by the cork cambium. It consists of

• phellum (cork) which is heavily impregnated with waxy suberin.
• phloem
• cork cambium