Chapter 25.  Plantae:  Angiosperms

• Angiosperms: the flowering plants.  These are seed plants (like the gymnosperms), but in angiosperms the ovules (later seeds) are enclosed in an ovary composed of one or more carpels (which are modified leaves).  In terms of species diversity and vegetation type coverage, the angiosperms are the dominant plant group today.
• The flower characterizes the angiosperms.
• Flower = a reproductive short shoot.  Leaves develop, but internodes fail to elongate.
• Sepals, petals, stamens are interpreted as modified leaves.
• Stamens and carpels are interpreted as sporophylls (i.e. modified leaves that bear sporangia).
• The interpretation of the flower as a modified shoot is supported by the developmental similarities between shoots and flowers (see Figure 13.8) and the leafy appearance of carpels in primitive angiosperms (see Figure 25.3).
• Angiosperm origins and evolution: "an abominable mystery" (Darwin).
• First angiosperm fossils appear in early Cretaceous (ca. 130MYA).  The first angiosperms evolved from a gymnosperm ancestor.
• There must have been considerable development during the Cretaceous.  This is supported by "microfossil" record (pollen grains).
• Angiosperms began to assume dominance in Tertiary (65MYA). Cretaceous/Tertiary boundary shows a shift from a dinosaur/gymnosperm biota to a mammal/angiosperm biota.
• Angiosperms make poor fossils (except for their pollen grains), but an upland origin could also explain the paucity of the fossil record.  Upland habitat diversity may have favored evolutionary innovation.
• Why did the angiosperms supplant the gymnosperms?
• Angiosperm innovations: flower (more effective pollen transfer), better vascular system (improved translocation), fruit (more effective seed dispersal).
• "Seedling hypothesis" (Bond): compared to gymnosperms, angiosperms are fast-growing and reach reproductive maturity earlier.  Wherever competition among seedlings and young plants is important, angiosperms win.  Gymnosperms win where environmental conditions slow growth, e.g. cold, nutrient poor habitats.
• Angiosperm life cycle (See Figure 25.7, 13.2, 13,7).
• Extreme reduction of gametophyte generation: only 8 nuclei in megagametophyte, 2 nuclei in microgametophyte.  No archegonia or antheridia produced.
• Ovule (later seed) protected in ovary which matures to become a fruit.  Animals may aid in fruit/seed dispersal.
• Flowers and animal dispersal of pollen: animals are more efficient vectors of pollen than wind, thus, less pollen must be produced to ensure the male function of reproduction.
• Double fertilization: one sperm fertilizes the egg to produce a zygote, the other sperm fertilizes the polar nuclei to produce a cell which develops into endosperm. Energy rich endosperm tissue develops only after fertilization.
• Vegetation changes during the Cenozoic
• Active continental drift brought areas into new climate zones, splitting and coalescence of continents brought about major climatic change.
• John Day basin (Oregon, p. 428): very detailed example of rapid change in vegetation, species composition. We can detect changes in species composition, plant morphology, deciduous/evergreen, leaf morphology, etc. Note - the changes are not due to new species evolving but to species replacements.
• Quaternary changes in vegetation were (and continue to be) very rapid - Figure 25.6.
• Angiosperms and agriculture: nearly all crop plants are angiosperms.
• 15,000-10,000 YA: a shift from hunting-and-gathering to agriculture.  This shift took place independently in several places around the world and occurred after the last glacial retreat.
• Most crops (especially grains) are derived from wild annual ancestors.  By selection, traits prized by humans were developed, e.g.,
• Larger seeds/fruits.
• Elimination of special germination requirements.
• Loss of dispersal mechanisms.
• Synchronization of development (germination, maturation).
• Loss of defensive compounds.
• Weeds evolved along with crop varieties.  These are "pioneer" plants adapted to temporary, disturbed habitats (e.g. fields).  Alien weeds have displaced native vegetation in many places of the world (e.g. California grasslands, Hawaii).
• Plants, geography and biodiversity.
• Plant geography - a science stimulated by the voyages of exploration and colonization, 1500-1850, approximately.
• Some interesting (and as yet unexplained) facts:
• Some plant groups are very widespread, others are very localized.  Example: Poaceae (the grass family) occur on all continents, Cactaceae (cactus family) are restricted to the Americas.
• Some plant groups are species rich, others are species poor.  Example: Asteraceae (sunflower family) has 21,000 species, Lemnaceae (duckweed family) has 30 species.
• The major pattern of  plant species diversity is that it decreases as latitude increases.  Equatorial tropical forest are the most diverse, diversity decreases toward the poles.
• There are "hot spots" of species diversity.  Examples: California, Amazon Basin, Himalayan foothills, Cape of South Africa.  These are (or should be) priority areas for conservation efforts.
• Evolution can produce similar plant adaptations in similar climates even though the species may be unrelated.  Examples:
• Deserts of the Americas, Africa, Australia
• Tropical rain forests
• "Mediterranean" climate shrub vegetation (Mediterranean basin, California, Chile, South Africa, Australia).