Plant Biology Notes

Kingdom Plantae

1.1 Structure and Function of Various Plant Parts

• Each plant organ has specific functions contributing to the plant's overall success.
• The main plant organs are:
  • Root
  • Stem
  • Leaf (vegetative organs)
  • Flower (reproductive organ)

1) Root

• Main functions:
  • Storage (e.g., carrot)
  • Anchoring the plant
  • Water and nutrient uptake
• Two basic types of roots:
  • Tap
  • Fibrous

a) Tap

• Prominent main root directed vertically downward.
• Numerous smaller lateral roots.
• Examples: carrot, beet, radish, dandelion.

b) Fibrous

• Many long, slender roots of about equal size.
• Example: grasses.
• Some roots form symbiotic relationships with bacteria.
  • Bacteria fix atmospheric nitrogen.
  • Plants receive a built-in source of fixed nitrogen.
  • Bacteria receive sugars from the plant.
• Most effective symbiosis occurs in the legume family (peas, beans, etc.).
• Legumes have swellings along the roots called nodules.
  • Nodules are 'infected' by bacteria from the Rhizobium genus.

2) Stem

• Major functions:
  • Mechanical support for other plant parts (leaves, flowers).
  • Conducts water and nutrients to other organs.
• In some cases, stems are involved in photosynthesis and nutrient storage.
• Water and nutrients are conducted through:
  • Vascular tissues
  • Simple diffusion (in plants lacking vascular tissues, such as mosses)

Vascular Tissues

• Xylem: conducts water and inorganic nutrients (minerals) up the stem.

• Phloem: conducts water and photosynthetic products (organic materials) up and down the plant.

• Many plants have modified stems serving other purposes.

Stem Modifications

• Stolons (runners):
  • Horizontal stems (e.g., strawberry plant).
  • 'Run' the plant further along the ground.
  • Used in asexual reproduction.
• Bulbs:
  • Modified stems.
  • Underground, vertical bases made mostly of enlarged leaves.
  • Used for storage of sugars and asexual reproduction.
• Tubers:
  • Enlarged ends of rhizomes.
  • Serve as food storage.
  • Can also be used in asexual reproduction.
  • Example: potato.
• Rhizomes:
  • Horizontal stems that grow just below or along the surface.
  • Example: edible part of the ginger plant.

3) Leaf

• Organ where most photosynthesis occurs.

• Leaf shape is characteristic of the plant species.

• General structures:

  • Lamina (blade): flat, broad portion.
  • Stalk

• Leaves are adapted to perform photosynthesis.

  • Broad surface maximizes photosynthesis.
    • Allows for maximum absorption of light energy.
    • Efficient diffusion of carbon dioxide and oxygen.
  • Broad leaves facing upwards are ideal solar collectors.

• Stomata (singular: stoma):

  • Function in gas exchange (carbon dioxide and oxygen).
  • Pore-like structures surrounded by two guard cells.
  • Guard cells regulate the opening and closing of stomata.
  • Stomata are located at the bottom of the leaf.

• Internal arrangement of tissues (in cross-sections):

  • Epidermis: outermost layer of cells extending over the leaf surface.
  • Mesophyll: interior of the leaf between the upper and lower epidermis.
    • Palisade mesophyll: upper region of the mesophyll.
    • Spongy mesophyll: lower region of the mesophyll.
  • Vascular tissue: extends through the spongy mesophyll.
    • Carries food to all parts of the plant.
    • Gives support to the leaf.
    • Brings water to the sites of photosynthesis.

Leaf Modifications

• Tendrils:
  • Forms a coil that brings the plant closer to support.
  • Typically modified leaves, but some are modified stems (e.g., grapevines).
• Spines:
  • The spines of cacti are actually leaves.
  • Photosynthesis is carried out mainly by fleshy green stems.
• Bracts:
  • Modified leaves that surround a group of flowers (e.g., red parts of the poinsettia).
  • Brightly colored to attract pollinators.
• Reproductive leaves:
  • Some plants produce plantlets that fall off and take root in the soil.

4) Flower

• Composed of male and female reproductive structures.
• Surrounded by attractive or protective leaf-like structures (sepals and petals), collectively known as the perianth.
• Sepals:
  • Develop first.
  • Usually green and leaf-like.
  • Provide protection for internal structures.
  • Small amount of food through photosynthesis.
• Petals:
  • Leaf-like, often brightly colored.
  • Designed for protection.
  • Attractants for animals (e.g., insects) necessary for gamete transfer or pollination.
• Stamens:
  • Inside the petals.
  • Each stamen consists of an anther suspended on a filament.
  • Anther: site of pollen grain production (multicellular/multinuclear haploid structures that form male gametes).
• Carpel:
  • Most central structure within the flower.
  • Usually composed of:
    • Stigma: sticky surface for receiving pollen grains.
    • Style: slender stalk through which pollen tubes grow.
    • Ovary: containing ovules.
      • Ovules contain the egg inside the embryo sac.
  • During fertilization, the pollen tube transfers the male gamete to the egg in the embryo sac.

1.2 Plant Diversity

• Plants are the green, photosynthetic kingdom.
• Non-motile.
• Some are terrestrial, others aquatic.
• Divided into several groups:
  • Bryophyta (Mosses)
  • Pterophyta (Ferns)
  • Seed plants

A) Bryophyta or Mosses

• Live in damp, shady, terrestrial environments.
• Short (biggest species about 20 cm).
• Nonvascular (avascular) plants.
  • Lack vascular tissue to transport nutrients.
  • Water and nutrients transported via diffusion.
• Live in clumps.
  • Provides structural support.
  • Allows water retention.
• Require water for fertilization (sperm transfer).

B) Pterophyta or Ferns (seedless vascular plants)

• Terrestrial plants.
• Located in areas of damp soil, high humidity, and low light.
• Advanced over mosses through the evolution of vascular tissue.
  • Allows rapid, long-distance transport of materials.
  • Allowed plants to increase in size.

C) Seed plants

• Most diverse and successful group of terrestrial plants.
• Dominates virtually all terrestrial habitats.
• Includes all trees, shrubs, grasses, annual and perennial herbs.
• Two subgroups:
  • Gymnosperms (exposed seed plants)
  • Angiosperms (enclosed seed plants)

1) Gymnosperms

• Bear naked ovules that develop into seeds at the base of scales in cones.
• Includes some of the largest and oldest organisms.
• Typically divided into four phyla:
  • Phylum Conipherophyta (conifers/cone-bearing plants)
  • Phylum Ginkogophyta (Ginkgo biloba, characteristic leaves)
  • Phylum Cycadophyta (cycads, more significant in the past)
  • Phylum Gnetophyta (odd plants sharing gymnosperm characteristics)
• Produce seeds but do not flower.
• Most members have both male and female parts on the same plant (different locations).
• Always woody trees or shrubs.

2) Angiosperms

• Flowering plants.
• Currently the most successful (abundant) and dominant type of plant.
• Seed plants with reproductive structures called flowers and fruits.
• Classified in a single phylum, Anthophyta (from Greek anthos, flower).
• Ovules are contained in the ovary.
• Ovaries mature into a fruit.