Chapter 24. Plantae: Gymnosperms (emphasizing the conifers)

• Seeds and seed plants (gymnosperms and angiosperms)
• Seed = matured ovule.
• Seed development: in megasporangium meiosis produces megaspores, one of which develops into a megagametophyte (female gametophyte).  Fertilization of the egg produces a zygote which develops into an embryo.  The seed consists of the embryo, nutritive tissue (derived from remains of megasporangium), and seed coat (derived from integument of ovule).
• Gymnosperms = "naked seed" plants, so called because the seed is exposed on a modified leaf (scale).
• Angiosperms = "enclosed seed" plants so called because the seed is enclosed in a modified leaf (carpel).
• Gymnosperms (900 species)
• Arose in Devonian along with all other major vascular plant groups (except angiosperms), achieved dominance during the Mesozoic (along with the dinosaurs), largely eclipsed by angiosperms (which today number at least 250,000 species) in Cenozoic.
• Four gymnosperm divisions:
• Pinophyta,  the conifers.  This is the largest group of gymnosperms; the other divisions are "relicts".
• Cycadophyta, cycads.
• Ginkgophyta, one species (Ginkgo biloba).
• Gnetophyta, three genera.
• Pinophyta (Conifers:  700 species in 7 families)
• Largely restricted to high altitudes, high latitudes.   Also in nutrient-poor soils, e.g. in American Southeast.
• Leaves are generally evergreen and small and have many adaptations to drought (seasonal or physiological).  Needles can appear singly on stem (e.g. spruces, junipers) or in bundles (e.g. in pines). The pine needle bundle = a short shoot [I will diagram this].
• Conifer forest ecosystems -- some typical characteristics:
• Tend to be low in plant species diversity compared to angiosperm-dominated forests.
• Develop on acid soils, further acidify the soils due to litter chemistry.
• Acid soils - fungi outcompete bacteria, are the major decomposers.
• Evergreen leaves must have effective chemical defenses, and conifer foliage is rich in defensive secondary compounds.  Consequently, conifer forests have low diversity of grazing animals and their predators.
• Generally species poor understory flora (it is always shady in a confer forest understory).
• Conifer stem anatomy
• Conifers have well-developed secondary thickening due to a vascular cambium.
• Conifer wood is composed of tracheids.  There are no vessels.
• Often xylem rays are present for axial transport.
• Many conifers produce resin ducts.  Resin = complex aromatic compounds, constitute defense against boring insects.  Rosin, pitch and pine tar are commercial products derived from resin.
• Life cycle of a pine (Section 24.5, Figure 24.13).
• The "tree you see" is the sporophyte, the gametophytes are microscopic.
• Life cycle is heterosporous:
• Megaspores are produced in megasporangia borne on scales arranged into a strobilus ("female cone", Figure 24.13a).
• Microspores are produced in microsporangia borne on scales arranged into a strobilus ("male cone", Figure 24.13d).
• Meiosis produces:
• Microspores which develop into microgametophytes (male gametophytes, Figure 24.13i).  The male gametophytes are dispersed as pollen grains.
• Megaspores which develop into megagametophytes (female gametophytes, Figure 24.13k).  The female gametophytes remain enclosed in (and are dependent on) maternal sporophyte tissue.
• Pollen grains and pollination.
• Pollen grain = immature male gametophyte, contain only two nuclei Figure 24.13h).
• Pollination = wind carries pollen grains from male cones to female cones.
• Fertilization
• Megagametophyte develops a few archegonia, each with a single egg.
• Germinating pollen grain produces a pollen tube which grows toward egg, sperms are released, fertilization produces zygote.
• Seed dispersal
• Seed = dormant embryo, food reserves, protective coat.
• Dispersal can be by wind or by the agency of forgetful mammals (squirrels, etc.).
• Some pines require a hot forest fire for the cones to open, allowing the seeds to disperse.