Bio Crash Course

Plants

29-2

29.1 Evolutionary history of plants

• Overview– Multicellular, photosynthetic eukaryotes– Believed to have evolved from a freshwater green alga

• Plants and green algae contain chlorophyll a and b• Both store carbohydrates as starch• Both have cell walls of cellulose

– 4 evolutionary events in evolution of plants• Development of embryo protection• Development of vascular tissue• Development of seeds• Development of flowers

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Evolutionary history of plants cont’d.

• Alternation of generations– Characteristic life cycle of plants

• Two multicellular stages, each producing the other– One is haploid- gametophyte – The other is diploid- sporophyte

• Sporophyte produces haploid spores by meiosis– Each spore develops into a gametophyte

• Gametophyte produces gametes by mitosis– After fertilization, the embryo develops into a new sporophyte

• In primitive plants the gametophyte is dominant, while in more advanced plants the sporophyte is dominant– Only the sporophyte develops vascular tissue

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Alternation of generations

• Fig. 29.2

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29.2 Nonvascular plants

• Overview– Lack vascular tissue-limits size and habitat– Do not have true roots, stems, or leaves– Gametophyte is the dominant generation– Sperm must swim to egg for fertilization– Sporophyte is attached to the gametophyte and

derives nourishment from it– 3 separate divisions on nonvascular plants• Mosses, liverworts, hornworts

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Nonvascular plants cont’d.

• Mosses– Reproduce both sexually and asexually

• Asexually by fragmentation• Sexually by production of gametes in archegonia and antheridia

– Gametophyte has 2 stages• Protonema-branching filaments of cells• Develops leafy shoots at intervals along protonema• Rhizoids anchor moss to substrate

– Dependent sporophyte• Consists of foot, stalk, and capsule or sporangium

– Produces wind-borne spores

– Life cycle is illustrated on the following slide

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Nonvascular plants cont’d.

• Adaptations and uses of nonvascular plants– Can live on bare rock, fences, cracks of sidewalks• Selective advantage to being small and simple

– Help convert rocks to soil– Peat moss• Used as fuel• Holds water-used in gardens to improve soil

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29.3 Seedless vascular plants

• Overview– Ferns and fern allies– Have vascular tissue- xylem and phloem– True roots, stems, and leaves in most– Sporophyte is the predominant life cycle stage• Stage with vascular tissue so can exploit more habitats• Advantage to being diploid- 2 copies of each allele

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Seedless vascular plants cont’d.

• Ferns and their allies– Produce wind-borne spores for dispersal– Spores germinate forming a small gametophyte

that is independent from the sporophyte for its nutrition

– Swimming sperm travel to archegonium to fertilize egg

– Seedless vascular plants formed the swamp forests of Carboniferous period• Compressed to form coal

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Seedless vascular plants cont’d.

• Ferns• Most abundant in warm, moist, tropical regions• Leaves are called fronds

– Immature leaves are called fiddleheads– Grow from rhizome

• Dominant sporophyte stage– Produces wind-blown spores

• Spore germinates and forms small gametophyte– Independent from the sporophyte– Swimming sperm produced by antheridia– Fertilization occurs in archegonia

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Seedless vascular plants cont’d.

• Ferns cont’d.– Adaptations

• True roots, stems, leaves• Gametophyte lacks vascular tissue so is water dependent

– Sperm also need water film to swim

• Sporophytes can spread to drier areas by vegetative reproduction– Rhizomes

– Uses of ferns• Floral decorations• Ornamental landscape plants• Some species used as food

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29.4 Seed plants

• Overview– Gymnosperms and angiosperms are seed plants– Seed contains a sporophyte embryo and stored food

• Allows survival until conditions are favorable for germination

– Gymnosperms-ovule not completely enclosed by diploid tissue

– Angiosperms-ovule completely enclosed within diploid sporophyte tissue (ovary) which becomes a fruit

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Seed plants cont’d.

• Gymnosperms- conifers, gnetophytes, gingkophytes, and cycads– We will use the conifers as our example

• Pines, spruces, firs, cedars, hemlocks, redwoods, cypresses• Cone-bearers

– Exhibit heterospory• Two types of spores- produce two types of gametophytes, male and

female• Pollen grains- male gametophyte• Pollination-deposition of pollen on a female gametophyte• Pollen tube-sperm pass through pollen tube to reach ovule

– No water required as it is in previous groups• Female gametophyte develops within ovule • Follow the pine life cycle on the following slide

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Pine life cycle

• Fig. 29.12

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Seed plants cont’d.

• Adaptations and uses of conifers– Adapted to cold, dry weather– Pollen cones and seed cones are adaptations to

land– Needle-shaped leaves have small surface area-

decreases water loss• Also have a thick cuticle and recessed stomata

– Wood is used in construction and for making paper– Resins are used for production of chemicals

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Seed plants cont’d.

• Angiosperms-flowering plants– Wide range of habitats– Two classes

• Monocotyledones- monocots• Eucotyledones-eudicots

– Classes are distinguished by• Number of cotyledons• Number of flower parts• Pattern of leaf venation• Arrangement of vascular bundles• Type of root system

– These characteristics are summarized on the following slide

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Seed plants cont’d.

• Angiosperms cont’d.– The flower

• Flower parts– Receptacle-tip of stalk that bears flowers– Sepals-modified leaves that protect bud– Petals-modified leaves, may be colorful, collectively called the

corolla– Stamens-male reproductive structures

» Anther-pollen production» Filament

– Carpels-female reproductive structures» Stigma-for reception of pollen» Style» Ovary-ovule production

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Seed plants cont’d.

• Flowering plant life cycle– Illustrated on the following slide– Microsporogenesis produces microspores

• Microsporocyte in anther undergoes meiosis- produces 4 haploid microspores-each becomes a pollen grain (male gametophyte)– Inside pollen grain are 2 cells-1 is a tube cell and the other is a sperm cell

– Megasporogenesis produces megaspores• Megasporocyte divides by meiosis- produces 4 haploid

megaspores-3 will disintegrate leaving 1 functional megaspore– Megaspore divides by mitosis 4 times but in the last division 1 cell doesn’t

undergo cytokinesis» Produces an embryo sac (female gametophyte) with 6 haploid cells

and 1 large central cell with 2 nuclei

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Flowering plant life cycle

• Fig. 29.17

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Seed plants cont’d.

• Life cycle of flowering plants cont’d.– Pollination and double fertilization

• Pollen is deposited by wind, insects, birds, etc. on stigma• Pollen grain opens and the tube cell digests a pollen tube

down the style• Sperm cell divides once by mitosis to produce 2 sperm• Sperm travel down pollen tube to the embryo sac• 1 sperm fertilizes the ovule (1 of the 6 haploid cells in the

embryo sac) and the other 5 break down• The other sperm fertilizes the central cell which becomes the

triploid endosperm– Endosperm is food source within the seed

9-21

9.1 Plant organs

• Plant organs– Roots– Stems– Leaves

• Roots– Anchor plant to soil and give support– Absorb water and minerals– Increased surface area for absorption from root hairs

• Epidermal extensions

– Produce growth hormones– Food storage in herbaceous perrenials

9-22

Plant organs cont’d.

• Stems– Provide structural support for leaves– Contain vascular elements that transport substances

through plant body• Water and minerals from roots to leaves• Products of photosynthesis from leaves to other organs for

storage

– Photosynthetic in some varieties– Water reservoir in succulents– Node- point of attachment of leaf to stem– Internode- regions between nodes

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Plant organs cont’d.

• Leaves– Major synthetic organs in most varieties– Blade- wide portion of leaf– Petiole- stalk that attaches leaf to stem– Axillary bud- bud in upper angle between petiole and

stem– Some leaves are specialized for protection, food storage,

or prey capture (carnivorous plants)– Deciduous leaves- lost in cold weather– Abcission zone- layer of cells at base of petiole that

weakens and allows leaf to drop off

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9.2 Plant tissues

• Meristem tissue– Embryonic tissue that allows plant to grow throughout life– Apical meristem- in tips of stems and roots– 3 types of primary meristems

• Protoderm-gives rise to epidermal tissue• Ground meristem-gives rise to ground tissue• Procambium- gives rise to vascular tissue

• Epidermal tissue– Covers surfaces of plant organs– In areas exposed to air, epidermis is covered with waxy cuticle

• Protect against water loss and disease

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Plant tissues cont’d.

• Epidermal tissue, cont’d.– Epidermal cells of roots have cytoplasmic extensions called

root hairs to increase surface area for absorption– Epidermal cells of leaves and stems may produce hairs for

protection– Some epidermal cells are modified as glands for secretion of

protective substances– The lower epidermis of eudicots and both surfaces of

monocots have epidermal cells called guard cells• Surround stomata and regulate opening and closing for gas exchange

– Epidermis in woody plants is replaced by cork• Protective against fungi, bacteria, and animals

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Plant tissues cont’d.

• Ground tissue– Forms bulk of plant body– Contains parenchyma, collenchyma, and sclerenchyma cells

• Parenchyma- found in all plant organs and may contain chloroplasts or plastids that store products of photosynthesis

• Collenchyma- thick walled cells which form bundles beneath epidermis and give flexible support

• Sclerenchyma- cells with thick secondary walls with lignin for structural support; most of these cells are nonliving

– Two types of sclerenchymal cells• Fibers-long, slender; ex: hemp fibers• Sclereids-shorter and varied in shape; ex: nut shells, seed coats, and

the “grit” in pears

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Plant tissues cont’d.

• Vascular tissue– Specialized for transport of substances through plant body– Two types of vascular tissue- xylem and phloem– Xylem- transports water and minerals

• Two types of conducting cells, both nonliving sclerenchymal cells- tracheids and vessel elements

• Vessel elements-large, elongated with perforated end plates: align to form a continuous tube

• Tracheids-shorter elongated cells with pits• Vascular rays- groups of parenchymal cells that lie outside of

tracheids to conduct water and minerals across the width of the plant

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Plant tissues cont’d.

• Vascular tissue, cont’d.– Phloem- transports organic nutrients through plant body

• Sieve-tube members form a continuous tube with perforated end walls called sieve plates; cells have cytoplasm but no nuclei

• Plasmodesmata are strands of cytoplasm which extend from one sieve-tube member to the next through the sieve plates

• Each sieve-tube member has a companion cell which is nucleated; may control functions of both cell types and aid in transport function

– vascular tissue extends from roots through stems to leaves• in roots xylem and phloem are located in the vascular cylinder• In the stem it forms vascular bundles• In the leaves it forms leaf veins