PLANTS

1. PLANTS

2. How Are Plants All Alike?

3. Plant Characteristics • Multicellular • Autotrophic (photosynthesis) • Chlorophylls a and b in thylakoid membranes • Surrounded by cell walls containing cellulose (polysaccharide) • Store reserve food as amylose (starch)

4. Plant Structure and Anatomy • Roots, stems, and leaves • Roots anchor the plant and draw water and minerals from the soil • Stems support the body and carry water and nutrients • Leaves are the main photosynthetic organ • Plant tissue • Three kinds of tissue in general • Dermal  Outer covering • Vascular  Fluid-conducting system • Ground  Support and photosynthesis

5. Plant Structure and Anatomy • Plant cells • Cells within the dermal tissue need to protect the plant from transpiration yet allowing gas exchange to occur • Ground tissue contains mainly of parenchyma cell, which are thin-walled and form the bulk of tissue in roots, stems, and leaves. These cells are very active in photosynthesis. Collenchyma and sclerenchyma cells support the plant • Vascular tissue has the xylem and phloem, which carries water and nutrients, respectively.

6. Roots • A growing seedling first sends a single primary root into the soil and as it grows bigger, secondary roots branch off the primary root • This enlarges the SA dramatically • Epidermis • Outer covering of the root • Has root hairs that make direct contact with the soil • The hairs are responsible for the large SA

7. Roots • Cortex • The layer of spongy cells beneath the epidermis • Parenchyma cells of the cortex move water from the epidermis to the vascular tissue • Vascular cylinder • The central region of xylem and phloem • Carries water and nutrients between roots and the rest of the plant

8. Roots

9. How Roots Work • Osmosis!! • Water moves out of damp soil into root hairs, which contain high concentrations of dissolved salts and sugars • The water passes through the root hairs and eventually into the vascular cylinder • What would happen if a plant was placed inside salt water? • Rapid water loss from roots is known as “root burn”

10. Roots • Active transport • Minerals going through cell membrane • This also brings in the water to the core • The Casparian strip • Endodermis—inner boundary of cortex that is very tight • Separates cortex from vascular cylinder • Known as the waxy layer that can control the entry into the core

11. Stems • Connects roots with the leaves: water and nutrients with photosynthesis • Epidermal tissues on the edge but different arrangement of ground and vascular tissue  For later. • Wood • As stem grows, new cells form between vascular cells, thus pushing outward and increasing the diameter • Vascular cambium: layer of dividing cells, usually the xylem • Phloem cells don’t grow as much and thus can get ‘cracked’  Cork cambiumproduces cork that form the bark

13. Stems • Cambium produces much more xylem in the summer than in colder weather • Historical information can be seen via annual tree rings • Nutrient transport and growth occurs within the thin layers of cells just under the bark  very delicate and easily damaged

14. Leaves • Main site of photosynthesis • Large SA with little mass  efficient in collecting solar energy • Attached to the stem by a petiole

16. Leaves • Epidermis • Covered with waxy cuticle • Stomata underneath for gas exchange • Guarded by two guard cells that open and close • Need to balance need for CO2 against need to conserve water • Mesophyll tissue • Packed with chloroplasts • Two types • Tall palisades on top • Spongy ones on bottom—lots of air space

17. Leaves • Leaf veins—vascular tissue • Xylem and osmosis • Xylem and phloem found as vascular bundles called ‘veins’

18. Leaves

19. Stomata Activity • Closed • Photosynthesis halts • Open • Photosynthesis can resume but too much transpiration could occur • Guard cells • When water flows in, increase in pressure causes a structural change that OPENS the stoma • When water flows out, decrease in pressure causes stoma to CLOSE

20. Stomata Activity • Several factors cause stomata to close: • Lack of water reshapes the guard cells • High temperatures stimulates cellular respiration, which can increase CO2 concentration within the air spaces • Other factors cause stomata to open: • Depletion of CO2 within the air spaces of the leaf, which occurs when photosynthesis begins • An increase in potassium ions (K+) into guard cells, which causes water to enter

21. Xylem • Water conducting system • Consists of two types of cells • Tracheids: long, thin cells that overlap and are tapered at the ends; function to support the plant as well as to transport the water • Vessel elements: generally wider, shorter, thinner walled, and less tapered; aligned end to end and differ from tracheids in that the ends are perforated to allow free flow through vessel tubes • Makes up most of wood and dead upon functional maturity

22. Phloem • Sugar conducting system via active transport • Consist of chains of sieve tube members/elements whose end walls contain sieve plates that facilitate the flow of fluid from one cell to the next • Alive at maturity, although they lack nuclei, ribosomes, and vacuoles • Connected to each sieve tube member is at least one companion cell that contain a full complement of cell organelles

23. Xylem and Phloem

24. Fluid Transport • Xylem transport • Water gets transported against gravity but doesn’t require any energy  From root to leaves • Fluid can be pushedup by root pressure via root pressure: Results from water flowing in to the roots from soil via osmosis; can push xylem sap upward only a few yards • The morning dew is due to root pressure  Guttation • Transpirationalpullcan carry fluid up the world’s tallest tress. Transpiration causes a negative pressure (tension) to develop and thus pulls up the sap • Cohesion of water due to strong attraction between water molecules makes it possible to pull a column of water from above • Transpirational pull-cohesion tension theory

25. Fluid Transport • Phloem transport • Phloem sap carries sugar from leaves into root and often to developing fruits as well • Translocation: Sugar gets distributed from various sources to sinks • The source is where sugar is being produced and the sink is where sugar gets stored or consumed • Movement of sugar into phloem highway creates a driving force because it establishes a concentration gradient  Causes water to come in and thus higher pressure occurs. • This pressure drives the movement of sugars and water through the phloem

26. Plant Growth • Tropisms: Derived from Greek and means ‘to turn’; plant responses to cues from their environment • Geotropism/Gravitropism • Response to gravity • Helps seedling grow toward sunlight • Different directions for root and stem • Thigmotropism • Response to touch • Phototropism • Response to light • First recognized by Darwin and his son (1880s)

27. Plant Hormones • Hormone is a substance produced in one part of an organism that affects activities in another part • Plant hormones help coordinate growth, development, and responses to environmental stimuli • Darwin’s experiment with phototropism: Figure 26-11 on p. 612 • Auxin • Indoleacetic acid (IAA) is a naturally occurring auxin; “To increase”; first plant hormone discovered • Stimulate cell growth and are produced by cells in the apical meristem, the rapidly growing region near the tip of a root or stem; preferential growth upward rather than lateral • Also stimulates stem elongation and growth by softening the cell wall • Produces phototropism due to unequal distribution • Mainly produced in the shoots and leaves

29. Plant Hormones • Cytokinins • Promotes cell division (cytokinesis, hence the name!) in lateral branches and leaf enlargement • Slows down the aging of leaves • Ratio between auxin and cytokinin concentration determines cell growth, rather than the level of either hormone by itself • However, can work antagonistically against auxins in relation to apical dominance • Produced in roots and travels upward in the plant • Gibberellin • Promote stem and leaf elongation • Work in concert with auxins to promote cell growth • Induce bolting, the rapid growth of floral stalk • Ex. Broccoli entering the reproductive stage  Sends up a tall shoot to ensure pollination and seed dispersal • Induction of growth in dormant seeds, buds, and flowers

30. Plant Hormones • Abscisic acid (ABA) • Inhibits growth! • Enables plants to withstand drought  Closes stomata during times of water stress • Promotes seed dormancy: Prevents seeds that have fallen on the ground in the fall from sprouting until the spring when conditions are more favorable • Ethylene gas • Small amount released when fruit tissues respond to auxin; Large amount released when plant is going through time of stress • Promotes fruit ripening • Aged flowers and leaves falling off  Facilitates apoptosis (programmed cell death) and promotes leaf abscission. Prior to death, cells break down many of their chemical components for the plant to salvage and reuse • Works in opposition to auxins

31. Controlling Plant Life Cycles • Annuals (marigolds, corn, peas), biennials (carrots, sugar beets), and perennials (trees and shrubs) • Whatever category a plant may fall into, timing is very important to a plant. • Must time their reproductive cycles so their reproductive cells will be ready at the same time as those of other members of their species • The environmental stimulus a plant uses to detect the time of year is the photoperiod, the relative lengths of day and night.

32. Controlling Plant Life Cycles • Circadian rhythm: The plant’s biological clock that is set to a 24-hour day • Long-day plants = short-night plants • Short-day plants = ? • Phytochrome is a pigment used by plants to detect day and night via changes in the length of light and dark periods each day • There are two forms: Pr (red-light absorbing) and Pfr (infrared light absorbing) • Pr  Pfr: when there is light present • Pfr  Pr: when it’s dark • This conversion enables the plant to keep track of time

33. Plant Reproduction • Asexual reproduction • Plants can clone themselves by vegetative propagation • A piece of the vegetative part (root, stem, or leaf) can produce an entirely new plant genetically identical to the parent plant • Naturally occurring example  Figure 26-14 • Agricultural use  Grafting to combine wanted characteristics of two different plants; done during dormancy

34. Plant Reproduction • The sexual reproduction in flowering plants is quite unusual • Alternation of generations life cycle • Diploid (2n) sporophyte stage and haploid (n) gametophyte stage • Two haploid gametes combine to form diploid zygote (2n), which then divides mitoticallyto produce the diploid multicellular stage called SPOROPHYTE (2n) • The sporophyte undergoes meiosisto produce a haploid spore. • Mitotic divisionleads to the production of haploid multicellular organisms called GAMETOPHYTES (n) • The gametophyte undergoes mitosis to produce gametes, which combine to form diploid zygotes and so on.

35. Alternation of Generations Gametophyte 2n Sporophyte 2n gametophyte 1n pollen 2n seed with plant embryo Ovary with 1n ovules (eggs) Sporophyte

36. Alternation of Generations

37. Alternation of Generations • For most plants, including ferns, conifers (cone producing plants), and angiosperms (flowering plants), the prominent generation is the sporophyte (2n) • For the moss (bryophyte), the prominent generation is the gametophyte (n) • The dominant sporophyte generation is considered more advanced evolutionarily than a dominant gametophyte generation

38. Plant Divisions

39. Plant Classification • Plants are divided into twogroups • Based on the presence or absence of an internal transport system for water and dissolved materials • Called Vascular System Vascular Bundles

40. Plant Classification • Bryophytes  Non-vascular plants • Ex) Mosses • Tracheophytes  Vascular plants • Seedless plants: Ferns (reproduction via spores) • Seed plants • Gymnosperms: Cone bearing • Cedars, sequoias, redwoods, pines, yews, and junipers • Angiosperms: Flowering plants • Roses, daisies, apples, and lemons • Monocots and dicots

41. Vascular System • Xylem tissue carries water and minerals upward from the roots • Phloem tissue carries sugars made by photosynthesis from the leaves to where they will be stored or used • Sap is the fluid carried inside the xylem or phloem

42. Multicellular Algae • Algae are photosynthetic aquatic organisms that are actually classified as protists • Although most are unicellular, some are multicellular (seaweed) and their reproductive cycles are quite similar to that of plants • Think of them as ‘honorary’ plants! • All algae contain chlorophyll a

43. Multicellular Algae • Brown algae • Contain carotenoids and xanthophylls • In the phylum Phaeophyta (dusky plants) • Giant kelps  Can be as long as 100 m • Common form is Fucus, which is found almost everywhere on the eastern coast of the US and is sometimes known as rockweed for the way in which it attaches itself to rocks • Most are salt-water organisms • Figure 24-2, p. 558

44. Multicellular Algae • Red algae • Get their color from a pigment called phycobilin and phycoerythrin • In the phylum Rhodophyta “red plants” • Most live in the ocean within the deep waters • Can absorb non-visible light via accessory pigments • Green algae • Most live in fresh water • In the phylum Chlorophyta “green plants” • Remarkably similar to green plants • Contain cellulose cell walls, chlorophyll a and b, and store food as starch

45. Nonvascular Plants • Do not have vascular tissue for support or conduction of materials • Called Bryophytes • Require a constantly moist environment Sporophyte stage Gametophyte Stage Moss Gametophytes & Sporophytes

46. Nonvascular Plants • Includes mosses (Bryophyta), liverworts (Hepatophyta), and hornworts (Antherophyta) Liverworts Hornworts

47. Bryophytes • Plants can’t grow as tall  Lack of lignin-fortified tissue • No true roots, stems, or leaves • Cells must be in direct contact with moisture and thus live close to the ground • Materials move by diffusion cell-to-cell • Sperm must swim to egg through water droplets  Contains flagella • Exhibit alternation of generations • Gametophyte generation is dominant • Partial adaptation to life away from water • Waxy covering

48. Sporophytes Gametophytes

49. Bryophytes • To a limited extent, bryophytes are able to gather water from moist soil as they are anchored by rhizoids, which are thin filaments that absorb water and nutrients from the soil. • Mosses • When a moss spore lands on wet soil, it germinates and grows into a tangle of protonema • As the protonema gets larger, its filaments become more organized and starts growing upward • These moss plants are the gametophyte stage of the moss life cycle

50. Moss • Gamete formation and fertilization • Gametes produced from gametophytes • Must be in water for sperm to swim • Diploid zygote grows into sporophyte, which becomes dependent on the gametophyte • Spore formation • From capsule of sporophyte Meiosis