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Angiosperm= “enclosed seed”; seeds are covered by fruit tissue
- Dominant plant group today
- Approximately 250,000 species
- What features contribute to the success of the angiosperms?
- Complex vascular tissue
- Xylem composed of tracheids (slow system of transporting water) and vessel elements (pipes transporting water)
- More efficient transport system for water and nutrients
- Phloem composed of sieve tubes and companion cells
- More efficient transport system for photosynthate
- Double fertilization
- · Two sperm in the pollen grain
- · One fertilizes the egg à zygote (2n)
- · The other fertilizes polar nuclei (2n) à endosperm (3n) (endosperm is nourishment)
- The most reliable distinguishing characteristic of the angiosperms
- Enclosed ovules and seeds
- Ovules and seeds are contained in carpels (modified leaves) rather than exposed on scales (Carpel = Pistil)
- The carpel also prevents self-pollination or self-fertilization
- Flowers
- In addition to gamete production, flowers produce nectar and promote pollen transfer
- In the process of nectar foraging, animals become pollen vectors
- Some relationships are highly specialized
- Fruit
- The ovary of a flower develops into a fruit after fertilization
- Fruits promote seed dispersal
- Ex: American Robin (Scientific name = Turdus Migratorius : hint hint)
- There are two major clades of angiosperms:
- The monocots and eudicots include the great majority of angiosperms
- Angiosperms make profound contributions to ecosystem services
- Among many others benefits, they are key primary producers (taking up CO2 and releasing O2 in the process)
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“Naked seeded”; seeds are not covered by fruit tissue
- The four gymnosperm clades bear little resemblance to one another
- Cycad – Encephalartos sp. – 300 species—Palm-like --- Tropics/subtopics
- Ginkgo – Gingko biloba – 1 species--- Tree – China (native)
- Gnetophytes—Welwitschia sp. – 90 species –Variable form –wide range
- Conifers –Hemlock (Tsuga sp.) – 700 species – Trees (cones) ---High altitudes and latitudes
- Male and female cones are the reproductive structures
- Megastrobilus: the larger female seed-bearing cone
- Microstrobilus: the smaller male pollen bearing cone
- The see is naked, but not the embryo
- Conifer diversity may not be that great, but…
- The oldest known living organism on Earth is a conifer
- The largest plant on Earth is a conifer
- The largest biome on Earth is a coniferous forest type [Northern coniferous forest (taiga)]
- However, angiosperms are the plants group that dominates today
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Why are seed plants the dominant vegetation today?
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Key features:
- · Secondary growth
- The accumulation of xylem (wood) which increases the diameter
- Provides support to grow taller (primary growth)
- Facilitates light absorption and seed dissemination
- Note: not all seed plant are woody
- · Gametophytes are nourished and protected by the sporophyte
- Evolutionary trend: change in size and autonomy of the generations*
- Seed plants are heterosporous; two different spore sizes
- Microspores produce microgametophytes (pollen) which produce sperm
- Pollen ( microgametophyte) dispersed by wind (or animals)
- Megaspores give rise to megagmetophytes which produces eggs, which are not shed
- Key features:
- · Secondary growth
- · Gametophytes are nourished and protected by the sporophyte
- · Seeds
- A well protected dispersal and/or resting stage
- Seed coat protects the embryo
- Stored energy supports early growth
- Protection of gametes and embryos is the hallmark of seed plants
- Seed plants are divided into gymnosperms and angiosperms
- Angiosperms have flowers – and gymnosperms don’t
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