Chapter 30 notes Plant Diversity II Evolution of Seed Plants Concept 30 1 Seeds and Pollen Grains are Key Adaptations for Life on Land All seed plants have reduced gametopytes heterospory ovules and pollen ADVANTAGES OF REDUCED POLLEN Mosses and other bryophytes have life cycles dominated by gametophytes whereas ferns and other seedless vascular plants have sporophyte dominant life cycles OVULATION AND PRODUCTION OF EGGS Although a few species of seedless plants are heterosporous seed plantsre unique in retaining the megasporangium within the parent sporophyte A layer of sporophyte tissue called INTEGUMENT envelopes snd protects the megasporangium Gymnosperm megasporangium are surrounded by one integument whereas those in angiosperms have two integuments Whole structure megasporangium megaspore and their integument s are called an OVULE produces one or more eggs POLLEN AND PRODUCTION OF SPERM A microspore develops into a pollen grain that consists of a male gametophyte enclosed within the pollen wall Sporopollenin in the pollen wall protects the pollen grain as it is transported by wind or by hitchhiking on an animal POLLINATION transfer of pollen to the part of the seeds that has the ovules EVOLUTIONARY ADVANTAGE OF SEEDS If sperm fertilizes the egg of a seed plant the zygote grows into a sporophyte embryo A seed can remain dormant for days months or even years after being released from the parent plant whereas spores have much shorter lifetimes Seeds have a supply of stored food Extant seed plants form two sister clades gymnosperms and angiosperms GYMNOSPERMS have naked seeds on exposed on modified leaves sporophylls that usually form cones Strobili Conifers ANGIOSPERMS seeds are enclosed in chambers that mature into fruits THE LIFE CYCLE OF A PINE 3 key reproductive adaptations miniaturization of their gametophytes the advent if the seed as a resistant dispersible stage in the life cycle The pine tree is the sporophyte Small pollen cones and large ovulate cones Concept 30 2 Gymnosperms Bear Naked Seeds Typically on Cones Concept 30 3 the Reproductive Adaptations of Angiosperms Include Flowers and Fruits Commonly known as flowering plants angiosperms are seed plants with the In pollen cones cells called microsporocytes undergo meiosis producing haploid microspores Each microspore develops into a pollen grain containing a male gametophyte Ovulate cones megasporocytes undergo meiosis and produce haploid megaspores inside the ovule Surviving megaspores develop into female gametopytes which are retained within the sporangia EARLY SEED PLANTS AND THE RISE OF GYMNOSPERMS Gymnosperms thrived as the climate dried because they have key terrestrial adaptations found in all seed plants such as seeds and pollen Some particularly well suited to arid conditions because of the thick cuticles and relatively small surface areas of their needle shaped leaves GYMNOSPERM DIVERSITY Remain an important part of Earth s flora Of the ten plant phyla four are gymnosperms Cycadophyta Ginkophyta Gnetophyta and Coniferophyta reproductive structures called fruits and flowers Angiosperm seeds contained in fruits CHARACTERISTICS OF ANGIOSPERMS Flower is unique to the angiosperm specialized for sexual reproduction Pollination is more directed than the wind dependent pollination of most gymnosperms However SOME angiosperms are wind pollinated FRUITS As seeds develop from ovules after fertilization the ovary wall thickens and the ovary matures into a fruit Fruits protect seeds and aid in their dispersal Some dry fruits split open at maturity to release seeds Fruits with dry outer coverings grains of maize rice wheat and other grasses Seeds of many angiosperms are carried by animals THE ANGIOSPERM LIFE CYCLE Each ovule which develops in the ovary contains the female gametophyte also known as an EMBRYO SAC CROSS POLLINATION in angiosperms the transfer of pollen from another flower on one plant to the stigma of a flower on another plant of the same species After reaching the ovary the pollen tube penetrates through the MICROPYLE a pore in the integuments of the ovule and discharges two sperm cells into the female gametophyte embryo sac One sperm fertilizes the egg producing a diploid zygote the other sperm fuses with the two nuclei in the large central cell of the female gametophyte producing a triploid cell DOUBLE FERTILIZATION one fertilization event produces a zygote and the other produces a triploid cell Unique to angiosperms Following double fertilization ovule matures into a seed Zygote develops into a sporophyte embryo with a rudimentary root and one or two seed leaves called COTYLEDONS Triploid central cell of the female gametophyte develops into ENDOSPERM tissue rich in starch and other food reserves that nourish the developing embryo ANGIOSPERM DIVERSITY MONOCOTS species with one cotyledon DICOTS species with two cotyledons
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