Diversity and Evolution of Woody and Seed Plants

• Lignophytes


  (Woody Plants) a monophyletic lineage of the vascular plants that share the derived features of a vascular cambium, which gives rise to wood, and a cork cambium, which produces cork
• Vascular Cambium


  • A sheath or hollow cylinder of cells that develops within the stems and roots as a continuous layer, between the xylem and phloem in extant, eustelic spermatophytes
  • The cells of the vascular cambium divide mostly in a tangential plane, resulting initially in two layers of cells
  • One layer remains as the vascular cambium and continues to divide indefinitely, the other layer eventually differentiates into either secondary xylem (wood) or secondary phloem
• Generally, much more secondary xylem is produced than secondary phloem
• As secondary tissue is formed, the inner cylinder of wood expands
• Many woody plants have regular growth periods, e.g., forming annual rings of wood
• Cork Cambium


  • It is similar to a vascular cambium, only it differentiates near the periphery of the stem or root axis
  • The cork cambium and its derivatives constitute the periderm (referred to as the outer bark)
  • The outermost layer of the periderm is cork
• Cork Cells


  They contain a waxy polymer called suberin (similar to cutin) that is quite resistant to water loss
• Major Evolutionary Novelty


  • The vascular cambium and cork cambium were a major evolutionary novelty
  • Secondary xylem, or wood, functions in structural support, enabling the plant to grow tall and acquire massive systems of lateral branches
  • The vascular cambium was a precursor to the formation of intricately branched shrubs or trees with tall overstory canopies – a significant ecological adaptation
  • Cork produced by the cork cambium functions as a thick layer of cells that protects the delicate vascular cambium and secondary phloem from mechanical damage, predation, and desiccation
• Wood anatomy can be quite complex. The details of cellular structure are important characters used in the classification and identification of woody plants. Wood anatomical features may also be used to study the past, a specialty known as dendrochronology
• Spermatophytes (Seed Plants)


  A monophyletic lineage within the lignophytes
• Major Evolutionary Novelty of Spermatophytes


  • The seed, defined as an embryo, which is an immature diploid sporophyte developing from the zygote, surrounded by nutritive tissue and enveloped by a seed coat
  • The embryo generally consists of: Radicle - an immature root, Epicotyl - a shoot apical meristem, Cotyledons - one or more young seed leaves, Hypocotyl - the transition region between root and stem
  • Ovule - an immature seed, prior to fertilization
• Seed Evolution


  1. Heterospory

  2. Endospory

  3. Reduction of megaspore number to one

  4. Retention of the megaspore

  5. Evolution of the integument
• Heterospory


  The formation of two types of haploid spores within two types of sporangia: large, fewer-numbered megaspores, which develop via meiosis in the megasporangium, and small, more numerous microspores, the products of meiosis in the microsporangium
• Endospory


  The complete development, in this case, the female gametophyte within the original spore wall
• Reduction of Megaspore Number to One

  1. First, there evolved a reduction in the number of cells within the megasporangium that undergo meiosis (each termed a megasporocyte or megaspore mother cell) was reduced to one. After meiosis, the single diploid megasporocyte gives rise to four haploid megaspores.

  2. Second, of the four haploid megaspores produced by meiosis, three consistently abort, leaving only one functional megaspore. This single megaspore also undergoes a great increase in size, correlated with the increased availability of space and resources in the megasporangium.
• Retention of the Megaspore

  • Instead of the megaspore being released from the sporangium (the ancestral condition, as occurs in all homosporous nonseed plants), in seed plants it is retained within the megasporangium.
  • This was accompanied by a reduction in thickness of the megaspore wall.
• Evolution of the Integument

  1. Most likely, the final event in seed evolution was the envelopment of the megasporangium by tissue, called the integument.

  2. The integument grows from the base of the megasporangium (which is often called a nucellus when surrounded by an integument) and surrounds it, except at the distal end.

  3. Fossil evidence suggests that integuments may have evolved first as separate lobes. In all extant seed plants, however, the integument develops as a continuous sheath and completely surrounds the nucellus except for a small pore at the distal end called the micropyle.
• Micropyle


  • The site of entry of pollen grains (or in angiosperms, of pollen tubes), which effect fertilization of the egg.
  • It also functions in the mechanics of pollination droplet formation and resorption.
• Pollen Grain


  • Technically, an immature, endosporic male gametophyte.
  • They are termed immature male gametophytes because, at the time of their release, they have not fully differentiated.
• Pollen grains of seed plants are extremely reduced male gametophytes, consisting of only a few cells.
• Pollen Grain Transport


  1. After being released from the microsporangium, pollen must be transported to the micropyle of the ovule (or, in angiosperms, to the stigmatic tissue of the carpel) in order to ultimately effect fertilization.

  2. The ancestral means of pollen transport was wind dispersal, in combination with an ovule pollination droplet.
• Pollen Tube Development


  1. After being transported to the ovule (or stigmatic tissue), the male gametophyte completes development by undergoing additional mitotic divisions and differentiation.

  2. The male gametophyte grows an exosporic pollen tube, which functions as a haustorial organ, obtaining nutrition by absorption from the surrounding sporophytic tissue.
• Siphonogamy


  The formation of pollen tubes is termed siphonogamy (siphono, tube + gamos, marriage).
• Pollen Tube


  • The pollen tubes, which may become branched in some taxa, function as a haustorial organ, growing into and feeding from the megasporangial (nucellar) tissue.
  • Pollen tubes also function to deliver the sperm cells, directly or indirectly, to the egg of the ovule.
• Pollination Droplet


  • A droplet of liquid that is secreted by the young ovule through the micropyle.
  • The pollination droplet functions in transporting pollen grains through the micropyle.
  • The pollination droplet functions by resorption, which pulls pollen grains that have contacted the droplet into the pollination chamber.
• It is unknown whether a pollination droplet was present in the earliest seed plants. However, the presence of a pollination droplet in many nonflowering seed plants suggests that its occurrence may be ancestral for at least extant seed plant lineages.
• Ovule & Seed Development

  1. After pollination, the megasporocyte develops within the megasporangium of the ovule.

  2. The megasporocyte is a single cell that undergoes meiosis, producing a tetrad of four haploid megaspores, which in most extant seed plants are arranged in a straight line, or linearly.

  3. The three megaspores that are distal (away from the ovule base) abort; only the proximal megaspore (near the ovule base) continues to develop.

  4. In the pollination chamber, the resorbed pollen grains develop into mature male gametophytes and form pollen tubes, which grow into the tissue of the megasporangium.

  5. The functional megaspore greatly expands, accompanied by numerous mitotic divisions, to form the endosporic female gametophyte.

  6. In the seeds of gymnosperms, archegonia differentiate at the apex of the female gametophyte.
• Megasporocyte


  A single cell that undergoes meiosis, producing a tetrad of four haploid megaspores
• Megasporangium


  The structure within the ovule where the megasporocyte develops
• Ovule & Seed Development

  1. Megasporocyte develops within megasporangium

  2. Megasporocyte undergoes meiosis, producing tetrad of four haploid megaspores

  3. Three distal megaspores abort, only proximal megaspore continues to develop

  4. Functional megaspore expands and divides to form endosporic female gametophyte
• Archegonia


  Structures that differentiate at the apex of the female gametophyte in gymnosperm seeds, containing a large egg cell and neck cells
• Ovule & Seed Development

  1. Pollen grains develop into male gametophytes and form pollen tubes that grow into megasporangial tissue

  2. Sperm cells released into archegonial chamber and fertilize egg cell
• Zygote


  Diploid cell formed by the fusion of sperm and egg, which undergoes divisions and differentiation to form the embryo
• Seed


  • Provides protection from mechanical damage, desiccation, and predation
  • Functions as the dispersal unit of sexual reproduction
• Seed dispersal mechanisms


  • Fleshy outer seed coat layer for animal dispersal
  • Seed coat differentiation into wings for wind dispersal
• Seed coat dormancy


  Mechanisms that ensure germination only under ideal conditions
• Nutritive tissue


  Surrounds the embryo and provides energy for the young seedling
• In seed plants, the female gametophyte remains attached to and nutritionally dependent upon the sporophyte, the reverse of the condition in bryophytes
• Eustele


  Primary stem vasculature consisting of a single ring of discrete vascular bundles, with xylem and phloem positioned radially and protoxylem endarch
• Archeopteris


  • Extinct lignophyte that lacked seeds, with wood like a conifer but leaves like a fern
  • Some species were heterosporous