Hypotheses about the origin of life. which is your favorite hypothesis and why

Charophytes or charophycean is the group of green algae that is closest related To the kingdom Plantae

Overview of Plants

Origin

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Green algae are named for their grass-green chloroplasts. These are similar to plants’ chloroplasts in ultrastructure, pigment composition and ability to store starch.

Within the groups of green algae, the members of the Phylum Charophyta (charophyceans) are the closest relatives to land plants. Land plants evolved from ancestors that were probably similar to modern complex charophycean algae (see also figure 29.1- the term streptophytes include charophyceans and true plants).

Similarities between complex charophyceans (e.g. Chara) and plants: • Molecular Genetics: Comparisons of the sequences of both nuclear and chloroplast genes point to

charophyceans as the closest living relatives of land plants • In addition, the following traits are only shared by charophyceans and land plants

ü Cell walls: rose-shaped complexes are used for cellulose synthesis; plasmodesmata (channel for communications between cells) are present

ü Formation of a phragmoplast (forms during plant cytokinesis to allow the formation of the new cell wall separating the two daughter cells)

ü Sexual reproduction and structure of flagellated sperm ü Peroxisome enzymes (that minimize loss of carbohydrates due to photorespiration)

New traits in plants (different from Charophyceans)

1. The presence of embryo: Embryophytes is a synonym for plants. A structure protects and feed the zygotes and embryos ü Multicellular and diploid ü Zygotes and embryos retained ü Depends on organic and mineral materials supplied by mother plant – placental transfer tissue

2. Distinct reproductive features: a. Alternation of generations (sporic life cycle). A multicellular

diploid sporophyte in addition to the multicellular haploid gametophyte in charophyceans. (Sporic life was present in other algae but not in charophyceans)

b. Dry air resistant reproductive cell (sporopollenin-walled spores)

c. Specialized structures to generate, protect, and disperse reproductive cells (gametangia and sporangia)

3. Tissues arise from apical meristems at growing tips.

Other important adaptations were appearing with the evolution of plants in the land (but not present in all plants, see figure 29.1

LE 29-4 Viridiplantae

Streptophyta

Plantae

Red algae Chlorophytes Charophyceans Embryophytes

Ancestral alga

Overview Plants

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Gametophytes: haploid (n) generation Role – to produce haploid gametes (by mitosis) Gametangia protects developing gametes from drying out and microbial attack. Two types of gametangia: Antheridia –gametangia producing sperm Archegonia –gametangia enclosing an egg Fertilization: Sperm and egg and fuse to form diploid (2n) zygote. Zygotes and embryos are retained in the gametophyte tissue, protected and fed (by placental

transfer tissue) and grow into multicellular diploid (2n) sporophytes. Sporophytes develop protective structures named sporangia, where sporopollenin-walled spores are produced. (Sporopollenin is a tough material that prevents cellular damage)

During evolution, plant sporophytes became larger and more complex See table 29.1 in your book

                          Bryophytes: The monophyletic liverwort, moss, and hornwort phyla are together known informally as the bryophytes. Bryophytes illustrate early-evolved features of land plants, such as a sporic life cycle involving embryos that develop within protective, nourishing gametophyte tissues Distinguishing bryophyte features:

ü Gametophytes are the dominant generation as opposed to dominant sporophyte generation in other plants

ü Sporophytes are dependent on gametophyte – small and short lived. In other plants, sporophytes are independent, large and long-lived

ü Nonvascular – lacking tissues for structural support and conduction found in other plants (bryophytes are shorter that most of other plants)

  See  the  life  cycle  of  the  moss  Sphagnum,    Figure  29.7  as  model  of  life  cycle  of   bryophytes    

Overview Plants

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Vascular Plants or Tracheophytes. The presence of vascular tissue gives name to the Vascular Plants or Tracheophytes. Go to figure 29.1 and note that a new trait appears for vascular plants: Lycophytes, pteridophytes, and other vascular plants generally possess stems, roots, and leaves having vascular tissues composed of phloem and xylem, cuticle, and stomata Vascular tissue: complex conducting tissue found in vascular plants. Compound by xylem and phloem

Xylem: Dead cells conducting water and minerals Phloem: Living cells conducting nutrients derived from photosynthesis (sugar, sap: sugar-rich water solution).

Lycophytes, pteridophytes are also known as seedless vascular plants, because they diverged form other plants before the appearance of the seed, but as other vascular plants, have specialized organs: Stems: Contain vascular tissue (phloem and xylem) and lignin and produce leaves and reproductive structures Roots: Specialized for uptake of water and minerals from the soil Leaves: Photosynthetic function, and conservation of water:

ü Waxy cuticle present on most surfaces of vascular plant sporophytes –prevents dessication ü Cutin found in cuticle – helps block pathogens ü Stomata – pores that open and close to allow gas exchange while minimizing water loss

Leaves are more developed in most pteridophytes (euphylls or megaphylls, see figure 29.1 and 29.21) Seedless vascular plants (lycophyte and pteridophyte) life cycle

ü Lycophyte and pteridophyte reproduction is limited by dry conditions, as is the case for bryophytes (needs water for the flagellated sperm to swim )

ü However, if fertilization occurs, lycophytes and pteridophytes can produce many more spores due to their larger sporophyte generation

o Vascular plant sporophytes are dependent upon maternal gametophytes for only a short time during early embryo development

o Stems of vascular plant sporophytes are able to produce branches, forming relatively large adult plants having many leaves

See the fern life cycle figure 29.12 and the video in Bb The ferns are pteridophytes, note the specific structures in the fern: rhizome, fronds, sori Compare seedless vascular plants vs. bryophytes. Make your own table.

Clades Primitive Features Advanced Features Bryophytes -Nonvascular

-Only moist environments -Small -Motile sperm -Needs water to reproduce

-Cuticle -Stomata -Rhizoid (…different than rhizomes) -Gametangia

Seedless Vasc. Plants -Motile sperm with flagellum -Needs water to reproduce -Only moist environments -Small Gametophyte

-Cuticle -Stomata -Roots and Leaves -Vascular system -Structural support -Dominant Sporophyte -Independent Gametophyte

Advantageous features for the adaptation to the land.

Overview Plants

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Seed plants or spermatophytes (meaning seed plants) are descendants from vascular plant ancestors that developed the following critical innovations for life in land: Reproductive innovations:

ü Seeds: allow plants to reproduce in diverse habitats ü Pollen: allows seed plants to disperse male gametophytes ü Ovules: provide protection and nutrition to female gametophytes and embryos ü Wood: strengthens plants, allowing them to grow tall and produce many branches, leaves, and

seeds To better understand the three first of these new innovations, let’s analyze what is new in the life cycle of seed plants:

1) Seed plants produce heterospores. In most seed plants, adult sporophytes develop two different kinds of sporangia: microsporangia & megasporangia (instead of only one sporangia). Microsporangia produce microspores by meiosis and megasporangia produce megaspores by meiosis. 2) Microspores undergo mitosis & develop into pollen grains, which are the young male gametopphytes 3) Macrospores undergo mitosis & produce female gametophytes (mega-gametophytes) containing eggs within archegonia. An ovule

consists of a megasporangium surrounded by one or two layers of tissue called integuments. The megasporangium produces spores that develop into mega-gametophytes. These mega-gametophytes remain within the tissues of the ovule and produce one or more egg cells 4) After fertilization, ovules develop into seeds. Mature seeds are ready for dispersal of the diploid generation and contain:

embryonic sporophyte (2n) stored food protective coat

Use the following table to recognize the three reproductive innovations: seeds, pollen and ovules. Two other derived traits related to reproduction are listed in the table (heterospory, already discussed, and reduced gametophytes). Remember that dominance of sporophyte domination and reduction of gametophyte is a trend in plant evolution (see figure )

Overview Plants

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Read in your book: 1) which are the ecological advantages of the seeds? Page 593. On your own: why do you think that pollen is a critical innovation in plants?

Seed plants include Gymnosperms (naked seeds) and Angiosperms (enclosed seeds, flowering plants). Gymnosperms include several groups; we are going to pay more attention to Conifers. See the figure 30.7 for the life cycle of the genus Pinus ( a conifer)

Overview Plants

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Compare the reproductive cycles of Gymnosperms and Angiosperms, what is common to them? What is different? See video in bb Reproductive cycle in Pinus, a Gymnosperm:

Reproductive cycle in angiosperms, (flowering plants):

Overview Plants

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Critical innovations in Angiosperms; Flower, endosperm, fruits A flower is a reproductive shoot, a stem branch that produces reproductive organs instead of leaves

Petals: attract animals for pollination. Sepal: protective layer of flower buds. Stamen: produce and disperse pollen.

anther: clusters of microsporangia (usually four) that produce pollen and then open to release it filament: sustain the stamen

Pistil: fused carpels (could be one, or more carpels fused) Carpel: Vase –shaped structure that produce, enclose and nurture female gametophytes. Contains veins of vascular tissue that deliver nutrients from the parent sporophyte to the development gametophyte.

stigma: receives and recognizes pollen of the appropriate species or genotype. Only appropriate pollen will be allowed to germinate style: long middle portion; encloses the elongated pollen tube which delivers nonflagellate sperm cells to ovules. ovary*: Encloses and protects ovules, also develop into fruits.

Ovaries are also considered a critical innovation in angiosperms and is not present in gymnosperms. Ovaries develops into fruits Within the ovule:

Diploid cell produces 4 megaspores by meiosis (3 die) and one is left. Surviving megaspore generate female gametophyte by mitosis Female gametophyte consists of 7 cells, one of which is the egg cell

Within the anther:

Diploid cells undergo meiosis producing 4 tiny, haploid spores (microspores) Pollen grains are immature male gametophytes. At the time of dispersal, the pollen grain is a two- or three-celled male gametophyte produced by mitotic division

Overview Plants

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During a later phase of development, a mature male gametophyte produces sperm cells      

Double fertilization: Pollen grains (immature male gametophytes) from the stamen are dispersed by the wind or transported by pollinators to the stigma of a flower. Pollen germinates and delivers two sperms to the female gametophytes by means of a long pollen tube, that is enclosed, fed and guided by the style. Once the sperms reach an ovule in the ovary double fertilization occurs. Double fertilization involves two events:

ü True fertilization: one sperm nucleus fuses with the egg to produce a diploid zygote that develops later into a young sporophyte

ü Triple fusion: the other sperm nucleus fuses with the two nucleus of the central cell, generating the first cell of the nutritive endosperm tissue

Double fertilization and therefore endosperm are present in angiosperms, but not in gymnosperms. Both are derived traits of angiosperms and the presence of endosperm is considered a critical innovation in angiosperms because provide a more efficient food storage to the seed.

Fertilized ovules develop into seeds, containing the young embryo (2n), the endosperm (result from triple fusion) and the coat (produced by sporophyte integuments). Ovaries and other parts develop into fruits. Ovary wall changes into a fruit wall (pericarp), and the ovules into seeds Fruit is a structure that encloses and helps disperse seeds. Dispersal helps reduce competition and allows colonization of new sites. Variation in mature fruits reflects seed dispersal adaptations. Mature seeds to be dispersed contain embryos that become dried and are protected by desiccation- resistant proteins and a tough seed coat. These adaptations enable seeds to withstand long periods of dormancy (metabolic slow down) Seed germination and seedlings Germination occurs only if seed encounters favorable conditions. Embryo absorbs water, becomes metabolically active, and grows out of seed coat (seedling)

Tube cell

Tube cell nucleus

Generative (sperm-producing) cell

Pollen coat and wall

Dominant independent sporophyte

Dependent gametophytes

(b) Sporophyte-dominant flowering plant (oak)

Flowers

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

(top left): © Biodisc/Visuals Unlimited

Ovule

242 µm

Antipodal cells

Two nuclei of central cell

Synergids

Sporangium

Megaspore wall

Female gametophyte (within megaspore wall)

Attachment to ovary

242 µm

Egg cell

Integuments

Micropyle opening

Overview Plants

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The Role of Coevolution in Angiosperm Diversification Coevolution: the process by which two or more species of organisms influence each other's evolutionary pathway. Most read: The Role of Coevolution in Angiosperm Diversification Co-evolutionary interactions between flowering plants and animals that serve as pollen- and seed- dispersal agents played a powerful role in the diversification of both angiosperms and animals

• Pollination Coevolution Influences the Diversification of Flowers and Animals. Pollinators transfer pollen from the anthers of one flower to the stigmas of other flowers of the same species. Pollinators foster genetic variability and plant potential for evolutionary change

o Constancy or fidelity – pollinators learn the flower characteristics and visit them preferentially

o Precise pollen transfer o Attract appropriate pollinator using attractive colors, odors, shapes, sizes Which advantages does animal pollination have over wind pollination? What is the pollination syndrome?

• Some flowers specialized for particular pollinators • Birds – odorless red flowers • Bees – blue, purple, yellow or white flowers with a sweet odor • If pollinator becomes extinct, plant may also face extinction

 

 

• Seed-Dispersal Coevolution Influences the Characteristics of Fruits and Animals As in the case of pollination, coevolution between plants and their animal seed-dispersal agents has influenced both plant fruit characteristics and those of seed-dispersing animals.

o Plant may have juicy, sweet fruit with small seed to readily pass through gut o Plants may signal fruit ripeness with color change

Overview Plants

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Plant Transport Vascular tissue: complex conducting tissue found in vascular plants. Compound by xylem and phloem

Xylem: Dead cells conducting water and minerals Phloem: Living cells conducting nutrients derived from photosynthesis (sugar, sap: sugar-rich water solution).

See the video and name the parts of the part of the plants and describe the process

Vascular tissue is present in the three plants’ organs: root, stem and leaves (in vascular plants).

Wood is listed as a critical innovation that appears in many seed plants. How is it possible the horizontal growth of the stem that produces the strong trunks that sustain the trees? While herbaceous plants produce mostly primary vascular tissues, woody plants produce primary and secondary vascular tissue. Woody plants begin life as herbaceous seedlings that possess only primary vascular systems. Secondary vascular tissue is formed from secondary or lateral meristems. Secondary xylem – wood Secondary phloem – inner bark (bark has both outer bark of mostly dead cork cells and inner bark (secondary phloem)

       

               

       

 

 

         

 

   

 

 

 

 

   

Overview Plants

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Secondary vascular tissues are produced by two types of secondary meristems (lateral meristems)

ü Vascular cambium ü Cork cambium

Vascular cambium

ü Produces secondary xylem to its interior and secondary phloem to its exterior

ü Secondary xylem conducts most of a woody plant’s water and minerals

ü Secondary xylem may transport water for several years

ü Usually only the current year’s production of secondary phloem is active in food transport

Cork cambium ü Produces cork ü Cork cells dead when mature and layered with

lignin and suberin