Plant anatomy and morphology FLOWERS LECTURE pt 2/2 lecture 3

AGRI 103: AGRONOMY

PLANT ANATOMY 2

Plant Organs: Flowers

  • The process of sexual reproduction in plants begins in the flowers.

  • Flower forms can vary significantly among different plant families.

  • In the context of Saskatchewan Polytechnic, understanding flowers is essential for agronomy studies.

Parts of a Flower

A general flower consists of four key parts:

  1. Stamens (male component):

    • Filament: The stalk that supports the anther.

    • Anthers: Produce pollen which contains sperm necessary for sexual reproduction.

    • Most flowers have multiple stamens to enhance reproduction potential.

  2. Pistils (female portion):

    • Comprises three parts:

      • Stigma: The sticky surface that receives pollen.

      • Style: The tube through which the pollen tube travels.

      • Ovary: Contains the ovules which hold the eggs for fertilization.

  3. Petals:

    • Showy, modified leaves that serve to attract pollinators and may protect immature stamens and pistils.

    • Often wither and die after fertilization, marking the end of the flower's reproductive phase.

  4. Sepals:

    • Green, leaf-like structures located at the base of the petals, providing protection for the developing flower.

Pollination and Fertilization

  • Pollination: The process begins when pollen attaches to the stigma.

  • A pollen tube is then produced which allows sperm to travel through the stile to reach the ovary.

  • The ovary contains ovules, which hold the eggs (female gametes).

  • Fertilization occurs when male sperm unites with female ovules, resulting in fertilized eggs that develop into seeds within the ovary.

Types of Pollination

  1. Self-pollination:

    • Defined as the transfer of pollen from the anther to the stigma of the same flower.

  2. Cross-pollination:

    • Involves the transfer of pollen from the anther of one plant to the stigma of another plant.

Classification of Flowers

Flowers can be categorized based on:

  1. Presence of Flower Parts:

    • Complete Flowers: Include all four parts (stamens, pistils, petals, sepals).

      • Example: Flowers of legumes (e.g., pea flowers).

    • Incomplete Flowers: Lack one or more of the four parts.

      • Example: Grass flowers which do not have sepals or petals.

Grass Flowers

  • Grass flowers are referred to as florets; they lack the traditional sepals and petals, having instead leaf-like coverings called:

    • Lemmas: Structures surrounding the stamen and some pistils.

    • Paleas: Another layer beneath the lemma.

    • Glumes: Additional coverings located on spikelets.

    • Awns: Long appendages that may be present on lemmas in certain grass species.

Perfect vs. Imperfect Flowers

  1. Perfect Flowers:

    • Have both stamens and pistils present in the same flower.

  2. Imperfect Flowers:

    • Lack either the stamen or the pistil.

    • Divided into two categories:

      • Monoecious Plants: Have separate male and female flowers on the same plant.

      • Dioecious Plants: Have distinct male and female plants, requiring both for fertilization.

Examples of Monoecious and Dioecious Plants

  • Monoecious Example: Corn (Zea mays)

    • Corn Tassel: Male, staminate flowers found at the top of the plant.

    • Corn Ears: Female, pistillate flowers located along the sides of the plant with the associated corn silks being elongated styles with sticky stigma.

  • Dioecious Example: Hemp (Cannabis sativa)

    • Has male and female plants distinctly separated.

    • Hops (Humulus lupulus): Also dioecious in nature.

Inflorescences

  • Inflorescences refer to the different arrangements of flowers clustered together on branches.

  • Individual flowers within inflorescences are called florets and can be borne on a stalk called a pedicel.

  • Types of inflorescences include:

    1. Spike: Florets borne directly on the main stalk.

      • Example: Wheat and barley are examples of compound spikes with spikelets.

    2. Raceme: Florets are borne on pedicels.

      • Example: Most legumes form racemes, such as canola.

    3. Panicle: A compound raceme, where branches arise from the main stem.

      • Example: Oats show this structure.

Fruits

  • Fruits are defined as the ripened or mature ovary containing the seeds after fertilization.

  • Fruits can be classified as:

    1. Fleshy Fruits

    2. Dry Fruits:

    • Have hard seeds contained within the fruit wall (termed "grains").

    • Dehiscent Fruits: Ovaries that split open at maturity to release seeds; examples include legumes and canola.

    • Indehiscent Fruits: Ovaries that remain closed when mature, examples include grasses and sunflowers.

      • Special case: The seed of grasses like wheat and corn is classified as a caryopsis, which is a type of fruit where the ovary wall (pericarp) is fused to the seed.

Seeds

  • Seeds are mature, fertilized ovules located within the ovary of the flower.

  • Each seed contains:

    • An embryo (immature plant) that carries the genetic material essential for propagating the species' future generations.

    • Food storage reserves necessary for the developing embryo.

    • A seed coat that protects the embryo and food reserves.

Monocots vs. Dicots

  1. Monocots:

    • Have a single seed leaf (cotyledon) within the seed.

    • Store their main food reserves in the endosperm.

  2. Dicots:

    • Have two seed leaves (cotyledons) in the seed.

    • Store all their nutrients within these two cotyledons.

  • Visual Aids:

    • Cross-section of a corn seed shows the endosperm and single cotyledon.

    • Cross-section of a bean seed illustrates the two cotyledons full of nutrients.