7.0 | Sexual Propagation & Seed Biology

Plant Anatomy of Flowers, Fruits, and Seeds

Flowering Plants: Angiosperms

[IMAGE TO INSERT: Flowering plant diagram showing colorful flowers]
Mature flower diagram showing labeled parts. Image: Mariana Ruiz Villarreal, Public Domain via Wikimedia Commons

Angiosperms are plants that produce flowers and seeds enclosed within a carpel. This large and diverse group includes herbaceous plants, shrubs, grasses, and most trees. With approximately 300,000 species, flowering plants represent the largest and most diverse group within the kingdom Plantae.

Flowers are exclusive to angiosperms and serve as organs for sexual reproduction. They produce gametes (mature male or female sex cells capable of uniting in sexual reproduction) and play a crucial role in pollination.

Flower Types

Flowers are classified into two main types:

Complete Flowers (Perfect Flowers): These flowers contain all four main parts – stamen, pistil, petals, and sepals. They possess both male and female reproductive structures.

Incomplete Flowers (Imperfect Flowers): These flowers are missing one or more parts, specifically either the male or female reproductive structures:
– Female flowers: Only produce fruit; contain pistil, stigma, style, and ovary
– Male flowers: Only produce pollen; contain stamen, anther, and filament

Basic Flower Parts

Flowers consist of several distinct parts, each with specific functions:

Peduncle: The flower stalk that supports the flower structure.

Receptacle: The part of the flower stalk that bears the floral organs.

Sepal: Leaf-like structures at the flower base that protect young buds. Collectively, sepals are known as the calyx.

Petal: Located in and above the sepals, petals attract pollinators with their colors and patterns.

Male Reproductive Parts (Stamen)

The stamen is the male reproductive part of the flower and is responsible for producing pollen grains containing male sex cells.

Filament: The stalk of the stamen that holds and supports the anther.

Anther: The pollen-bearing structure located at the top of the filament.

Pollen: Grains containing the male sex cells necessary for fertilization.

Female Reproductive Parts (Pistil)

The pistil is the female reproductive part of the flower, responsible for producing seeds.

Stigma: The sticky top surface of the pistil that is receptive to pollen grains.

Style: The stalk of the pistil through which the pollen tube grows.

Ovary: The base of the pistil that matures to become fruit after fertilization.

Ovule: Located within the ovary, ovules carry the female sex cells and develop into seeds after fertilization.

Fruits

Fruits are the developed ovaries of flowers, sometimes including other flower parts, that form after pollination and fertilization. Angiosperms are also known as fruiting plants because flowers are essential for fruit production – no flowers means no fruits. Fruits serve the vital functions of protecting and disseminating seeds.

Fruits may be classified based on their structure and texture:

Simple Fruits: Formed from one ovary
Aggregate Fruits: Formed from a single flower with many ovaries
Multiple Fruits: Developed from a fusion of separate flowers on a single structure

Common Fruit Types

Berry: The most common domesticated fruit type, consisting of many seeds surrounded by soft tissue. Examples include grapes, tomatoes, cucumbers, eggplant, bananas, and chili peppers.

Drupe (Stone Fruit): A single seed surrounded by a hard pit (endocarp). Examples include coffee, mango, olive, coconut, pistachio, cashew, almonds, cherries, peach, plum, apricot, and nectarine.

Grain: A single seed surrounded by a paper-thin dry fruit. Grains are the fruits of grasses and include cereals like wheat, rice, corn, and oats.

Legume: Many seeds surrounded by a pod that opens along two seams when dry. Examples include peas, beans, chickpeas, lentils, soybeans, and peanuts. Legumes are beneficial for soil health.

Pome: Many seeds surrounded by flower petals that have fused together to form a meaty layer. These are sometimes called accessory fruits. Examples include apples and pears.

Nut: A single seed surrounded by a rock-hard layer that is dry at maturity. True nuts have a very hard pericarp. Examples include acorns, hazelnuts, and chestnuts.

Seeds

Seeds are essential structures for plant reproduction and propagation. They serve as a dispersal unit, allowing plants to spread and colonize new areas away from the parent plant.

Seeds contain three main parts:

Seed Coat: The protective outer covering that shields the embryo from damage and prevents premature germination. The seed coat is mostly impermeable and ranges from one to several cells thick. It includes the hilum (scar of attachment to the ovary) and the micropyle (a tiny hole for gas and water diffusion).

Cotyledon (Seed Leaf): Provides a temporary food supply for the developing embryo. Cotyledons store nutrients necessary for germination and early growth.

Embryo: An undeveloped plant contained within the seed, consisting of the embryonic shoot (epicotyl and plumule) and embryonic root (hypocotyl and radicle).

Monocots and Dicots

Seeds are classified into two major groups based on the number of cotyledons:

Monocots (Monocotyledons):
– Embryos with a single cotyledon
– Flower parts in multiples of three
– Have adventitious root systems
– Store nutrients in the endosperm
– The cotyledon is called the scutellum
– Include the coleoptile (protective sheath for shoot) and coleorhiza (protective sheath for root)
– Examples: grasses, corn, wheat, rice, lilies, orchids

Dicots (Dicotyledons):
– Embryos with two cotyledons
– Flower parts in multiples of four or five
– Have roots which form from the radicle (taproot system)
– Store nutrients primarily in the cotyledons
– Include the epicotyl (embryonic shoot), hypocotyl (embryonic stem), and radicle (embryonic root)
– Examples: beans, peas, sunflowers, roses, oak trees