Lecture 9 Angiosperms I.pdf
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most recently evolved seed plants Angiosperm evolution Angiosperm classification and features Angiosperm life cycle Pollination Flowers – general structure flowering plants oldest fossil evidence from about 140 mill yr ago within 60 mill yr had become dominant plant group over 250,00...
most recently evolved seed plants Angiosperm evolution Angiosperm classification and features Angiosperm life cycle Pollination Flowers – general structure flowering plants oldest fossil evidence from about 140 mill yr ago within 60 mill yr had become dominant plant group over 250,000 species today extremely diverse forms co-evolved with animals for pollination, mycorrhizae continues life cycle trend – gametophyte reduced (even further) & dependent on sporophyte Change in relationship between sporophyte & gametophyte Traditionally: ◦ one angiosperm phylum = Magnoliophyta or Anthophyta with classes Liliopsida (monocots) and Magnoliopsida (dicots) Still under investigation Six clades described two largest clades: monocotyledons (monocots) & eudicotyledons (eudicots) third largest: magnolids informal international group of systematic botanists try to establish a consensus on taxonomy of flowering plants (angiosperms) use recent/emerging knowledge about plant relationships from phylogenetic studies Developed APG III system; work continues https://www.mobot.org/MOBOT/research/APweb/ flowers ovules & seeds enclosed (ovary & fruit, resp.) double fertilization endosperm in seed (triploid nutritive tissue) xylem with vessel elements & fibres as well as tracheids phloem with companion cells as well as sieve tubes Angiosperm life cycle (source: Encyclopaedia Britannica) OVULE(S) in OVARY young (immature) ovule = megasporangium surrounded by two integuments ◦ megasporangium (nucellus) + megasporocyte (2n) →meiosis→ 4 megaspore (n) (one functional) ◦ megaspore →mitosis→ megagametophyte (8 nuclei) = ‘embryo sac’ (n) so older (mature) ovule = megagametophyte (embryo sac) surrounded by integuments (sometimes also remains of nucellus) Diploid megaspore mother cell divides meiotically to yield four megaspores, three of which degenerate. Nucleus of remaining megaspore divides three times by mitosis to produce 8-celled female gametophyte (embryo sac). Two of these nuclei migrate toward centre of embryo sac to form the polar nuclei. These may fuse into a diploid nucleus. Three nuclei are positioned at ‘inner’ end = antipodal cells, three at mycropylar end = two synergids and the female gamete, egg. Development of female gametophyte pollen sac or microsporangium in ANTHER (normally 4 per anther) ◦ microsporangium contains outer nutritive layer (tapetum) + many microsporocytes (2n) →meiosis→ microspores (n) = young pollen (all functional) ◦ microspore → mitosis once → young microgametophyte (2 nuclei) = mature pollen germinating pollen tube (3 nuclei) = mature microgametophyte Pollen (10 to 250 μm diam.) has following features: ◼ ◼ ◼ Thick outer wall of pollen grain, called exine, is elaborately sculpted and used in taxonomy. Exine - of sporopollenin, composite of substances (similar to cutin and suberin), one of most durable natural substances - enables pollen to remain unchanged for thousands of years. Intine – inner wall, thin, of cellulose and pectin; forms the wall of pollen tube (passes through gap/hole in exine). Each microspore nucleus in pollen grain undergoes mitosis once to form two-celled structure. One cell, generative nucleus, further divides mitotically to produce two male gametes – sperm. So, the microgametophyte is just three nuclei within the pollen/pollen tube (normally pollen has formed pollen tube by second division). One sperm fertilizes the female gamete (egg) in ovule to form the zygote, the other fuses with polar nuclei to form the triploid endosperm cell: DOUBLE FERTILIZATION Angiosperm pollination & fertilization Transfer of pollen from anther to stigma of flowers Self-Pollination ◦ Within same flower or between flowers on same plant Cross-Pollination ◦ between anther of one plant and stigma of another plant of same species ◦ (if between different species → hybrid) Elaborate systems evolved with animal-pollinators in many angiosperm species Abiotic agents: non-living, like wind or water 1. ◦ Anemophily: pollination by wind ◦ Hydrophily: pollination by water Biotic agents: animals 2. Zoophily: pollination by animals such as birds, bats, insects, e.g. ◦ Entomophily: by insects Ornithophily: by birds A flower is usually adapted for its pollinator Floral morphology is directly related to pollinator type. The specialized adaptations of a flower for a pollinator type are collectively known as pollination syndromes or systems. Plants differ in degree of their specialization for a particular pollinator: specialists or generalists Wind pollinated - grass Plant ‘rewards’ insect with nectar or pollen or both, occasionally aromatic oils, resins. Insect pollinated – specialist Bucket orchid and male euglossine bees http://www.theorchidcolumn.com https://nothinbiology.org Specialized shoot Develops from flower bud borne on shoot: ◦ main stem (for annuals, biennials, some perennials) ◦ branches (most perennials) Limited growth cluster of flowers = inflorescence or single = solitary Composed of several parts: ◦ stalk (of inflorescence = peduncle; of individual flower = pedicel) ◦ receptacle: expanded, terminal end of flower stalk, on which floral parts arise ◦ sepals, petals: protection, support, attraction ◦ androecium (stamens, male) ◦ and gynoecium (carpels, female) reproduction maybe: nectaries (etc.) - attraction of / reward for pollinators bract(s): leafy appendages at base of stalk (etc.) Carpel produces egg(s) Parts of a ‘typical’ flower Separate ‘circles’ or whorls - members of successive whorls attached in alternate positions - from outside towards centre: two ‘nonessential’ (perianth) sepals (calyx) petals (corolla) two ‘essential’ (reproductive) stamens (androecium) carpels (gynoecium) Calyx: Corolla: 5 sepals 5 petals Androecium: two whorls each of 10 stamens Gynoecium: many separate carpels on convex, conicalshaped receptacle http://www.countrysideinfo.co.uk/flowe r.htm 1) Calyx: Outermost layer or whorl: sepals Protective function Green Encircle & protect the immature flower Later open to expose the mature flower 2) Corolla: petals Attract pollinators Many forms male reproductive whorl stamen comprised of filament anther anther (microsporophyll) bears pollen sacs (microsporangia), usually four, containing microspore mother cells (microsporocytes, 2n) producing pollen grains (microspores, n) may be in two whorls The female reproductive whorl carpel(s) megasporophyll(s) (often fused together) ◦ contains the ovule(s) (megasporangium developing into mega-gametophyte)
