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Spermatopsida: The Seed Plants

The seed plants are the dominant plant life forms on the planet today, and the arose from numerous branchings of the plant clade. Collectively, the seed plants are classified in Spermatopsida.

Five phyla of seed plants still exist today.

Of these, the first four above are gymnosperms, and the last comprises the angiosperms. In this lecture, we'll concentrate on the gymnosperms.


The Gymnosperms: Plants with Seeds, but no Flowers or Fruit

The gymnosperms (from the Greek gymnos, meaning "naked" and sperm, meaning "seed") are the first seed plants. The seed conferred a major selective advantage because it not only protects the growing sporophyte embryo from the environment (the seedless plant sporophyte embryo is exposed and helpless in comparison), but also provides nutrients and, in many cases, a means by which the new sporophtyes can be dispersed very far from the parent organism.

The Magical Seed

Unlike the seedless tracheophytes, which are mostly homosporous, all gymnosperms are heterosporous, producing male microspores and female megaspores. Seeds amplify heterospory, with megaspores developing inside an ovule which will eventually become a seed.

Over the course of seed plant evolution, the following events each conferred a selective advantage. They led to the evolution of the ovule as we know it today.

We don't know which of these events occurred first, but fossil evidence suggests that the integuments evolved from fleshy lobes of sporophyte tissue (the cupule) that originally surrounded the megasporangium. Their gradual fusion left only the micropyle as an opening.
The earliest cupule is seen in the late Devonian plant Elkinsia polymorpha:

And later fossils show a progression of cupule fusion that probably led to today's ovule:


Progymnosperms

Phylum Progymnospermophyta is an extinct taxon of woody plants that had characteristics intermediate between the seedless tracheophytes and the seed plants. Like the seedless plants, they Like seed plants, they
  • produced true wood made of secondary xylem (fossils look like conifer wood!)
  • produced secondary phloem
  • had a bifacial (i.e., growing xylem in one direction and phloem in the other) vascular cambium
  • had flattened structures that apparently were true leaves
  • had a protostele
  • a few species were heterosporous These plants formed large trees (some 17 meters tall!), and were found in dense forests. Some have called them the "first modern trees". They likely share a common ancestor with all modern seed plants.


    Extinct Gymnosperms

    Recall that fossil plants taxa are almost always considered to be morphotaxa, since we cannot examine them closely enough to determine ultrastructure and DNA sequences that would reveal true evolutionary relationships. One such group is the "seed ferns" (Pteriodpsermales), and another is the Cordaitales (coniferlike seed plants). The former is almost certainly polyphyletic. Both groups reached their peak of dominance in the Carboniferous.

    "Coal Age" plants are of great ecological importance, especially today with mounting evidence of anthropogenic climate change.

  • Today, photosynthesis takes up about 100 billion metric tons of CO2 per year.
  • This is only about 10% of total atmospheric CO2
  • Respiration returns about the same amount of CO2 to the atmosphere as what is removed via photosynthesis. (The difference is about 1 part in 10,000)
  • This slight loss of CO2 is due to dead, organic matter being buried and put out of reach of decomposition.
  • Plant matter so buried becomes peat.
  • If peat is further buried under many layers of sedimentary rock, and comes under pressure, it can transform into coal.
  • Certain periods in earth's history saw much greater deposition of "fossil fuel" than others, and one is the aptly named Carboniferous. Carbon-based organic matter was present in huge quantities in the Carboniferous: Five groups of plants dominated the lands at this time. Three of them--the lycophytes, equisetophytes and ferns--were seedless vascular plants, and the other two weee the seed ferns and the cordaites.


    Extant Gymnosperms

    Recall the four phyla of gymnosperms that still grace our planet today:

    Let's meet them.


    Coniferophyta - The Conifers

    The most familiar gymnosperms are the conifers. Pine, spruce, fir, redwoods, cedar, cypress, juniper...the list goes on. But one thing they share is a life cycle that's a bit more complex than that of the seedless tracheophytes. A few important notes:

    The Diversity of Conifers

    Seventy genera include the approximately 630 species included in the Coniferophyta. The largest living organisms ever known, the Giant Redwoods (Sequoia sempervirens) belong to this group, as do the oldest living things, the Bristlecone Pines (Pinus longaeva).

    And as we tumble farther into autumn and the winter holiday season, it's interesting to note that the Christmas Tree had its origin as a phallic symbol adopted from the seasonal fertility rites of the pagan Germanic and Roman people of ancient Europe.

    The approximately 80 species of pines dominate the biome known as coniferous forest or taiga. Here they dominate because of the slight advantage they have of being able to photosynthesize--albeit at a very low rate--even during the harsh winter months, which deciduous flowering trees cannot.

    Pine Leaves: Unique Arrangement

    In a pine seedling, the leaves (long, thin modified leaves often called "needles") are borne singly, and whorl around the branches.

    As the tree matures, the pine starts to produce the needles in bunches. These needle clusters are called fascicles, and may have from 1-8 needles (depending on species) held together at the base by a small collar of scalelike leaves. The needles (leaves) are born on an extremely short shoot within the fascicle base, in which growth of the meristem has been suspended. The fascicle is actually an extremely short branch with determinate growth.

    The leaves are evolved to allow the pine to survive and thrive in dry, often very cold climates. The cuticle is very thick, and stomates are usually recessed into grooves that protect them from wind and desiccation. Leaves, stems and roots are all impregnated with resin, which not only deters insects, but also has a lower freezing point than water.

    Although most conifers are known as "evergreens," they do shed their needles and change them out every 2-4 years, though usually not all at once.

    Pine Stems and Wood

    The stem of conifers is typical of the woody stems we already have studied, and all conifers produce secondary growth via a vascular cambium.

    In the timber industry, the wood of conifers is known as softwood, and the wood of angiosperm trees is known as hardwood. Comprised primarily of tracheids, not the more highly lignified vessel elements, conifer wood is indeed less rigid than anthophyte wood.

    Life Cycle of the Pine

    The life cycle of Pinus is a good example, and is very similar to that of all other gymnosperms.

    In pines, this process takes two years.

    Male and female cones are distinctive in appearance, with male cones lasting only a few months (just long enough for pollination), and female cones taking two years to mature.

    Other familiar and economically important conifers include the firs, larches, spruces, redwoods, yews, cypresses, and junipers.

    The toxic Yew is a source of important chemicals of use for treating tumors. Its seeds are not borne in strobili, but rather surrounded by a fleshy cup called an aril, which attracts birds, which disperse the seeds.

    Text reading assignment of note: Pay special attention to the material on Family Araucariaceae, and the "living fossil" Wollemia nobilis on pages 424-426.


    Cycadophyta - The Cycads

    The common name "Sago Palm" is misleading, because these are not palms at all, but ancient gymnosperms that have been around since before the dinosaurs. There are 11 extant genera, and most do very well in southern Florida's mild climate.


    Above, our native "coontie", Zamia pumila and its endangered butterfly (for which it is a larval host plant), the Atala Eumaeus atala.

    Interesting Life Cycle Note:

    Cycad pollen tubes are haustorial. That is, they are modified for penetration of tissues and absorption of nutrients. Some botanists suggest that the pollen tube evolved from a structure that was originally meant to allow the microgametophyte to collect nutrients from the sporophyte for sperm production, and that the structure only later became a sperm delivery apparatus.


    Ginkgophyta - The Ginkgo Trees

    The only living species of this mostly extinct phylum is the famous Ginkgo biloba. Its fan-shaped leaves are easily recognized and distinct among conifers.


    Virtually extinct in the wild in their native China, these are widely cultivated as ornamentals, and are very resistant to smog.

    Life Cycle Note:
    Like those of cycads, Ginkgo pollen tubes are haustorial. This suggests that both cycads and ginkgos are more primitive in this respect than pines or gnetophytes.


    Gnetophyta - The Gnetophytes

    Believed to be the closest relatives to the flowering plants, there are only three living genera:
  • Ephedra
  • Gnetum
  • Welwitschia

    The only Gnetophyte native to the U.S. is Ephedra viridis, commonly known as "Mormon Tea" or "Joint Fir."

    Ephedra (sinica) is perhaps most famous recently because of its powerfully stimulant alkaloid, Ephedrine, which was marketed as a weight-loss aid until people started dying of heart attacks after taking it.

    Male and female strobili are borne on separate branches on these monoecious plants.

    Genus Gnetum has members with several morphological similarities to angiosperms, such as

    Gnetophytes and Angiosperms very likely share a common ancestor. Molecular data support this monophyly. So next time, we'll meet the Big, Successful Cousins, the Angiosperms.