Plant Support & Tissue Systems

Dermal Tissue

• Epidermis (non-woody plants)
  • Covered by waxy cuticle to prevent water loss.
• Periderm: Replaces the epidermis in woody plants.
• Trichomes: Epidermal outgrowths for various functions.

Three Tissue Systems

• Dermal Tissue System
  • Epidermis / Periderm
• Vascular Tissue System
  • Xylem
  • Phloem
• Ground Tissue System
  • Parenchyma
  • Collenchyma
  • Sclerenchyma
• Tissue systems are continuous throughout the plant body and connect all the plant organs.

Ground Tissue

• Parenchyma
  • Thin-walled, living cells.
  • Metabolic functions include:
    • Synthesis using chloroplasts.
    • Storage with plastids and vacuoles.
  • Can divide and differentiate for repair and replacement.
• Collenchyma
  • Unevenly thickened primary cell walls.
  • Flexible, living cells.
  • Support young parts of the plant shoot.
  • Do so without restraining growth.
• Sclerenchyma
  • Thick secondary walls containing lignin.
  • Dead at maturity.
  • Support tissue in plants.
  • Provides some protection.

Vascular Tissue

• Xylem
  • Tracheids and vessel elements with secondary cell walls containing lignin.
  • Dead at maturity.
  • Support and transport.
  • Conducts water & minerals upward in the plant.
  • Vessel elements have partially perforated end walls.
• Phloem
  • Sugar transporting tissue.
  • Sieve-tube elements.
    • Alive at maturity (primary walls only).
    • Lack most organelles.
    • Sieve plates porous for fluid flow between cells.
  • Companion cells:
    • Nucleus and ribosomes serve both cells.
    • Connected to sieve-tube elements via plasmodesmata.

Meristems

• Meristems: Perpetual embryonic tissue that allows for growth throughout the plant life.
• Apical meristems at tips of roots and shoots:
  • Elongate, causing growth in length (primary growth).
• Lateral meristems add thickness to woody plants (secondary growth):
  • Vascular cambium: Located between xylem and phloem, one cell layer thick.
  • Cork cambium: Produces cork.

Primary and Secondary Growth

• Primary growth:
  • Elongation of shoots and roots via apical meristems.
• Secondary growth:
  • Addition of thickness to woody plants via lateral meristems (vascular cambium and cork cambium).
  • Vascular cambium produces secondary xylem (wood) and secondary phloem (inner bark).
  • Cork cambium produces cork (outer bark).
• Growth rings:
  • Result from variations in secondary xylem growth.
  • Early wood has larger cells, late wood has smaller cells.

Stem Organization

• Eudicot Stem:
  • Vascular bundles in a ring with pith inside and cortex outside.
  • Vascular cambium present.
• Monocot Stem:
  • Vascular bundles scattered in ground tissue.
  • No vascular cambium.

Plant Diversity

• Diversity: Roughly 290,000 living species
• How did plants change the world?

Kingdom Plantae

• Eukaryotic and multicellular.
• Photosynthetic autotrophs.
• Cellulose cell walls.
• Starch: Primary carbohydrate food reserve.

Land Plants

• Plants are defined as embryophytes, plants with embryos.
• Monophyletic group.
• Algal ancestor - similar to the modern Charophytes.
• Accumulation of traits that facilitated survival on land
  • e.g. sporopollenin.

Move to Land: Opportunities and Challenges

• Opportunities:
  • Space.
  • Unfiltered sunlight.
  • Atmosphere with plentiful $CO_2$.
  • Nutrient-rich soil.
  • Few herbivores or pathogens.
• Challenges:
  • Scarcity of water.
  • Lack of structural support.

Derived Traits of Plants

• Five key traits appear in nearly all land plants but are absent in charophytes:
  • Apical meristems
  • Alternation of generations
  • Multicellular, dependent embryos
  • Multicellular gametangia
  • Walled spores produced in sporangia

Apical Meristems

• Apical Meristems
  • Continual growth in these meristems
  • Elongation of shoots and roots

Alternation of Generations

• All land plants show alternation of generations in which two multicellular body forms alternate
  • Gametophyte (n)
  • Sporophyte (2n)

Dependent Embryos

• The diploid embryo is retained within the tissue of the female gametophyte
  • Nutrients
  • Protection against dehydration
• Land plants are called embryophytes because of the dependency of the embryo on the parent

Multicellular Gametangia

• Protected from dehydration
• Female gametangium produces egg cells
• Male gametangium produces sperm cells
• The gametophyte→ gamatangia→ gametes

Walled Spores Produced in Sporangia

• The sporophyte  sporangia  spores
• Haploid spores produced by meiosis
• Spore walls contain sporopollenin
  • resistant to dehydration
  • dispersal without water
• A spore is a haploid cell that can develop into a new organism (gametophyte) without fusing with another cell

Additional Derived Traits

• The epidermis is covered by a waterproof cuticle.
  • Prevents excess water loss
• Mycorrhizae - Symbiotic root associations with fungi.
  • More effective absorption of water and minerals.
  • May have allowed plants to colonize land before the evolution of roots.

Evolution of Land Plants

• Ancestral species gave rise to a vast diversity of modern plants
• Origin of land plants (about 470 mya)
• Origin of vascular plants (about 425 mya)
• Origin of extant seed plants (about 305 mya)

Bryophytes

• Hepatophyta: liverworts
• Anthocerophyta: hornworts
• Bryophyta: mosses
• Bryophytes are represented by three phyla of small herbaceous plants
• The gametophyte generation dominant
• Bryophyta have cuticle and stomata, Hepatophyta do not

Bryophytes Features

• Bound to aquatic environment:
  • No vascular tissues
  • Usually small (2-5cm)
  • Lack roots and true leaves
    • Rhizoids
    • Leaf-like structures
  • Flagellated sperm
    • require water for fertilization
• Terrestrial Features:
  • Spores with resistant coats (sporopollenin)
    • Aerial dispersal of spores
  • Mosses & hornworts have stomata
  • Gametes in protective gametangia
  • Embryophytic condition

The Ecological and Economic Importance of Mosses

• Inhabit diverse and sometimes extreme environments
  • They are especially common in moist forests and wetlands
• Some mosses might help retain nitrogen in the soil
• Sphagnum, or “peat moss,” forms extensive deposits of partially decayed organic material
  • Important organic carbon reservoir
  • Overharvesting of Sphagnum and/or a drop in water level in peatlands could release stored $CO_2$ to the atmosphere

Gametophyte-sporophyte relationships in different groups

• In nonvascular plants Gametophyte is dominant, and sporophyte is Reduced and dependent on gametophyte for nutrition
• In seedless vascular plants Sporophyte Dominant, and Gametophyte is Reduced, independent (photosynthetic and free-living)

Seedless Vascular Plants

• Dominant sporophytes and small, independent gametophyte

Seedless Vascular Plants Features

• Terrestrial Features:
  • Vascular tissues and lignin in walls
    • support for tall growth
    • transport systems for water & food
  • True roots
    • Roots anchor vascular plants
    • absorb water and nutrients from the soil
  • True leaves
    • increase surface area to capture sunlight
    • megaphylls in ferns, microphylls in Lycophyta
• Linked to moist environments:
  • Water required for fertilization
    • flagellated sperm

Sporophylls and Spore Variations

• Sporophylls are modified leaves with sporangia
• Sori are clusters of sporangia on the undersides of sporophylls
• Strobili are cone-like structures formed from groups of sporophylls
• Most seedless vascular plants (such as ferns) are homosporous
• All seed plants are heterosporous
  • Heterosporous species produce megaspores, which give rise to female gametophytes, and microspores, which give rise to male gametophytes

Phylum Lycophyta

• Growth forms: Prostrate and epiphytic
• All possess microphylls

Phylum Monilophyta

• Ferns:
  • Large divided leaves - fronds
  • Sporophylls
    • carry sporangia on underside of the fronds –sori
  • Rhizomes
    • Stems give rise to leaves and roots
  • Fiddleheads - immature ferns
• Horsetail
• Whisk fern

Evolution of Land Plants

• Origin of land plants (about 470 mya)
• Origin of vascular plants (about 425 mya)
• Origin of extant seed plants (about 305 mya)

Seeds

• Seeds and pollen grains are key adaptations for life on land
• Evolutionary Advantage of Seeds over Spores:
  • multicellular - much more complex and resistant :
    • have a supply of stored food
    • may remain dormant for days or years
    • may be transported long distances by wind or animals
• A seed is a embryo, along with its food supply, packaged in a protective coat
  • Seed coat (derived from integument)
  • Embryo (2n) (new sporophyte)
  • Food supply (female gametophyte tissue) (n)

Seed Plant Characteristics

• In addition to seeds, the following are common to all seed plants:
  • Reduced gametophytes
  • Heterospory – megaspores and microspores
  • Megasporangium (forming the megaspore) is retained within the parent sporophyte.
    • The ovule is the megasporangium, megaspore, and protective integument(s) together.
    • The female gametophyte develops inside the ovule and produces gametes (eggs).
  • Microsporangia form microspores that develop into pollen grains.
    • Pollen grains contain the male gametophytes.
    • Sperm develops inside pollen grain
    • water not needed for reproduction

Gametophyte-sporophyte relationships in Seed plants

• In Seed plants Sporophyte is Dominant, and Gametophyte is Reduced (usually microscopic), dependent on surrounding sporophyte tissue for nutrition

Evolution of Land Plants (Seed Plants)

• The first seed plants appear in the fossil record with fern-like leaves, woody stems, and produced seed-like structures

Gymnosperms

• The ovules and seeds of gymnosperms are not enclosed by ovaries - naked seed plants
  • develop on the surfaces of modified leaves (cones)
  • male (pollen) and female (ovulate) cones
  • Microscopic gametophyte (inside the cones), large long-living sporophyte (the tree itself)
• Adaptation to leaves for drier conditions:
  • needle-shaped leaves
  • thick cuticles
  • small surface areas

Gymnosperms: Four Phyla

• Phylum Cycadophyta (cycads)
  • Thrived during Mesozoic, but relatively few species exist today
  • Unlike most seed plants, cycads have flagellated sperm
  • Individuals have large cones and palmlike leaves
• Phylum Ginkgophyta
  • Consists of a single living species, Ginkgo biloba
  • Like cycads, has flagellated sperm
  • It has a high tolerance to air pollution and is a popular ornamental tree
• Phylum Gnetophyta
  • This phylum comprises three genera:
  • Species vary in appearance, and some are tropical whereas others live in deserts
• Phylum Coniferophyta (conifers)
  • The largest of the gymnosperm phyla
  • Most conifers are evergreens and can carry out photosynthesis year round
  • Some of the largest and oldest living organisms

Characteristics of Angiosperms

• All angiosperms are classified in a single phylum, Anthophyta
• They are the most widespread and diverse of all plants
• Angiosperms have two key adaptations
  • Flowers
  • Fruits

Angiosperms: Flowers & Fruit

• Flowers are complex reproductive structures
  • evolved from leaves and shoots
  • Sites of pollination and fertilization
  • Ovule develops into the seed
  • Ovary develops into the fruit (unique to Angiosperms)
• Fruit can be fleshy or dry.
• Function to:
  • Protect seed
  • Disperse seed

Angiosperms Diversity

• The two main groups of angiosperms are monocots (one cotyledon) and eudicots (“true” dicots)
• The clade eudicot includes most groups formerly assigned to the dicot (two cotyledons) group