Plants

Key Terms:

  • Radicle: the first part of a seedling (a growing plant embryo) to emerge from the seed during the process of germination.

Land plants have evolved to occupy a specific niche:

  • Sessile
    • Must avoid being eaten
    • Find a mate and reproduce
    • Disperse their offspring
  • Autotrophic
    • Collect raw materials for photosynthesis
    • Expose themselves to sunlight
  • Terrestrial
    • Avoid drying out
    • Deal with gravity
  • Multicellular
    • Find ways to transport substances more quickly and over larger distances

Plant Cells

General Characteristics:

  • Cell wall – provide support
  • Large central vacuole - provides
  • Chloroplasts – photosynthesis

Three tissue types:

  • Vascular tissue
    • Xylem and Phloem
  • Dermal tissue
    • Covers all parts of the plant
  • Ground tissue
    • Everything else

Ground tissue: 3 subtypes

  • Parenchyma - thin cell walls
  • Collenchyma - medium cell walls - firm yet flexible
  • Sclerenchyma - thick, hard cell walls (e.g. spines)

Vascular tissue:

  • Carbohydrates made in the leaves must be carried to all other living cells
    • Phloem
  • Water and dissolved nutrients must be taken up by the roots and transported to the rest of the plant
    • Xylem

Roots - Anchor

  • Root systems are specialized to anchor a plant into the type of soil it grows in.

Roots - gather/store water and minerals

  • Vascular tissue
    • Concentrated in the centre of the root, and surrounded by a waterproof endodermis.
    • This regulates the movement of mineral ions and water into the vascular tissue
    • Plants drive minerals into the vascular tissue by active transport.
    • Water follows, due to osmosis.
    • This increases the pressure in the xylem, which in turn pushes the water (and minerals) up into the stem
  • Root hairs
    • Tiny extensions of individual epidermal cells on plant roots
    • Increase surface area available for absorption of water and nutrients
  • Cortex
    • Outer layer of roots specialized in some plants to store large amounts of carbohydrates

Stems

Stems Functions

  • Supports the plant
  • Spaces the leaves to maximize the amount of light and air they get
  • Contain xylem and phloem
  • Stores food

Types of stems - herbaceous

  • Annual plants
  • Thin, soft green, short-lived, and contain little or no wood
  • Can photosynthesize
  • Usually do not grow taller than 1 meter (exception: palm tree)
  • Contain Collenchyma Cells
      • allows plants to bend without being broken

Types of stems - woody

  • Perennial plants
  • Increase in diameter each year as more vascular tissue is created
  • Layers in Woody Stem:
    • Heartwood – dead xylem that no longer functions
    • Sapwood – xylem that still conducts fluid
    • Bark – contains living phloem and periderm
      • Periderm:
        • Cork Cambium – stem growth

(width)

        • Cork – Outer layer of bark

Leaves

Leaf Function

→ the major site of photosynthesis

  • Problem:
    • Large surface area and exposure to the air provides two ingredients for photosynthesis.
    • BUT - these characteristics increase the rate of water loss.
    • Leaves are specialized for the niche of the plant they belong to.

Layers of the leaf

  • Epidermis
    • The outer covering of the leaf
  • Cuticle
    • waxy substance secreted by epidermal cells
    • helps reduce water loss by evaporation
  • Mesophyll
    • Photosynthetic region between the upper and lower surface of the leaf
    • Contains parenchyma cell
    • Specialized cells that have lots of chlorophyll
  • Palisade Mesophyll
    • Under the upper epidermis
    • Primary site for photosynthesis
  • Spongy Mesophyll
    • Above the lower epidermis
    • Randomly arrange with large spaces for rapid carbon dioxide diffusion

Guard Cells

  • Contain chlorophyll
  • During photosynthesis, become swollen with water
    • Forces stomata open
  • Therefore on sunny days
    • Stomata are open providing lots of CO2
  • Dark night
    • No photosynthesis – stomata close
    • Prevents water from escaping

Transpiration Pull

  • Around 98% of the water that reaches the leaves is lost to evaporation through the stomata (transpiration)
  • This creates a negative pressure that acts to pull water up to replace the lost water
  • This is the second mechanism which plants use to transport water up to the leaves.

Sugar Transport in Phloem

  • Photosynthesis in the leaves produces sugars
  • Sugars are actively transported into the phloem.
  • Water follows by osmosis.
  • Pressure in phloem increases, so sap moves towards where sugar is being removed from phloem.

Sexual Reproduction in Angiosperms

  • Most diverse and widespread plants on Earth
  • Reproductive organ - FLOWER

The flower - 4 sets of modified leaves

  • Flowers have 4 organs:
    • Sepals
      • Protect the flower bud and can look like small leaves of flower petals
    • Petals
      • Colorful structures that attract pollinating insects
    • Stamens
      • Male reproductive organ
      • Composed of a filament and anther
      • Filament: supports the anther
      • Anther: produces pollen grains, which contain sperm cells, through mitosis and meiosis
    • Pistils
      • Female reproductive organ
      • Composed of a stigma, a style and a ovary
      • Pollination occurs at the stigma. The style connect the stigma to the ovary, which contains one or more ovules

During sexual reproduction

  • Anther releases pollen that goes down style and enters the ovary
  • After fertilization, seeds, each containing an embryo and a food supply, form within a fruit that develops from the ovary wall
  • Fruits are dispersed by wind, or by animals, and the seeds are released and can grow into a new plant

Variations among flowers - structural differences

  • Complete vs. incomplete flowers
    • A flower is considered incomplete if it is missing one or more flower organs (sepals, petals, stamens, and pistils)
  • Perfect vs. imperfect flowers
    • A flower that does not contain both the male and female reproductive organs (pistil and stamen) and is considered imperfect

Variations among flowers - pollination mechanisms

  • Pollination
    • Transfer of pollen from the male anther to the family stigma
  • Self-pollination and cross-pollination
    • A plant can pollinate itself or another flower of the same plant
    • This leads to inbreeding and loss of genetic variation
  • Animal pollination
    • Flowers are brightly coloured, have strong scents and produce nectar to attract pollinators
    • As animals move from one plant to another, they carry pollen with them
  • Wind pollination
    • Usually lack colourful or fragrant flowers
    • Produce large quantities of light pollen grains that can be carried by wind

Angiosperm classification

  • Divided into two main groups based on a structure called a seed lead or cotyledon
  • The cotyledon is a structure in the seed that helps to nourish the plant as it first starts to grow (before leaves are available for photosynthesis)

Monocot and Dicot

  • Monocots
    • Produce seeds which only have one cotyledon (seed leaf)
    • Examples: corn, water lilies, onions, orchids, grasses
  • Dicots
    • Produce seeds which have two cotyledons
    • Examples: beans, maples, oaks, peas, beans, potatoes
  • There are many other characteristics that can be used to distinguish monocots and dicots
  • The arrangement of vascular tissue in roots and stem, the number of petals and the arrangement of veins the leaves all differ

Plant Growth

  • Plants grow at specialized meristematic tissues
  • Meristem
    • Specific regions in plants that undergo mitosis
    • Cells produced by meristematic tissue eventually differentiate to become other types of tissues
  • Apical meristems
    • Meristems at the root tips and shoot tips
  • Lateral meristems
    • Cylindrical regions in roots and stems
    • Responsible for increases in diameter

Types of Lateral Meristem

  • Vascular cambium
    • Produces cells that will become new xylem and phloem tissue
  • Cork Cambium
    • Produces cork and bark

Growth of Lateral Meristem

  • Primary growth
    • All growth in length of roots and stems throughout a plant’s entire life
    • All growth in the diameter of roots and stems which occurs in the first year of a plants life
  • Secondary growth
    • Plant growth originating at the lateral meristems which results in the increase in the diameter of roots and stems after the first year of a plants life

Plant growth and development

  • Seeds germinate when conditions are ideal - they absorb water, and the radicle penetrates the seed coat
  • Germination and growth is influenced by the presence of plant hormones, as well as environmental factors

Plant Hormones:

  • Hormones act as chemical signals between cells and tissues in different parts of the plant
  • Can stimulate or inhibit plant growth

Stimulatory Hormones

  • Auxins
    • Stimulate cell division and elongation in stems and roots
    • Regulate cell expansion in plant responses to light and gravity
    • Prevents premature fell of leaves, flowers, fruits
    • Promotes flowering
    • Helps in cell division and xylem differentiation
  • Cytokinins
    • Stimulate lateral shoot growth
    • Stimulates the formation of chloroplasts in leaves
  • Gibberellins
    • Stimulate cell elongation in the stem
    • Stimulate seed germination

Ethylene

  • Both stimulatory and inhibitory
  • Gaseous
    • Promotes ripening of fruits
    • Promotes growth of seeds and buds
    • Promotes leaf aging and release

Plant response to environmental stimuli

  • Tropic response
    • The growth response of a plant to an external stimulus
  • Positive Tropism
    • The plants growth is towards the stimuli
  • Negative tropism
    • The plants growth is away from the stimuli

Tropic Responses

  • Phototropism
    • Growth response to light caused by an unequal distribution of auxin
    • More auxin is present on the side of the plant further away from the light
    • The auxins cause these cells to elongate
      • The stem curves towards the direction of light
  • Gravitropism
    • Growth response to gravity
    • Stems exhibit negative gravitropism
      • Plant grows upwards, pushing against gravity
  • Thigmotropism
    • Growth response to mechanical stimuli
      • I.e. contact with an object
  • Hydrotropism
    • Growth of roots towards water

Other Factors that effect plant growth

  • Plant growth is also influence by light energy from the sun, carbon, air, and water from teh soil
  • Ex. photoperidism: plant responses to changes in the length of day
    • At light latitude, length of day changes a lot - and so does temperature
    • Therefore, plants go into dormancy in response to lengthening days