Plant Reproduction: Flower Structures

• Plant Reproduction

Structure of an Idealized Flower

• Stamen is the male part which comprises the anther and filament
• Carpel is the female part made up of the stigma, style, and ovary
• Petals are the colorful parts used to attract pollinators
• Sepal protects the flower bud
• Receptacle attaches the flower to the stem

Types of Flowers Based on Whorls

• Whorls refer to the arrangements of flower parts in a circular pattern
• Complete flowers have all four parts: sepals, petals, pistils, and stamens
• Incomplete flowers are missing one or more of the four integral parts (sepals, petals, pistils, and stamens.)

Types of Flowers Based on Reproductive Whorls

• Flowers can also be categorized by the presence and arrangement of reproductive structures

• Perfect/Bisexual flowers have both male and female reproductive structures (androecium and gynoecium) in the same flower

• Gumamela (Hibiscus) is an example of a perfect/bisexual flower

• Imperfect/Unisexual flowers lack either male or female reproductive structures

• Staminate flowers are male, only containing stamens

• Carpellate/pistillate flowers are female, only containing carpels or pistils

• Squash, cucumbers, corn, and grasses exemplify imperfect/unisexual flowers

Gametophyte Development

• Gametophyte development refers to the formation and development of the gametophyte stage in the life cycle of plants and some algae
• Gametophytes produce gametes for sexual reproduction

Gametophyte Development Details

• Male gametophytes (pollen grains) develop within the microsporangia of anthers

• Each microsporangium has diploid microsporocytes

• Microsporocytes divide by meiosis, producing four haploid microspores that develop into pollen grains

• The generative cell divides to form two sperm cells making the pollen grain a mature male gametophyte

• The nucleus of the tube cell is also a key component of the mature pollen grain

• Female gametophytes (embryo sacs) develop within the ovule inside the ovary

• The ovule's megasporangium contains a diploid megasporocyte

• The megasporocyte divides by meiosis into four haploid cells, but only one survives as the megaspore

• The megaspore undergoes three mitotic divisions to form the multicellular female gametophyte, or embryo sac

Double Fertilization

• Pollen grain germinates and the pollen tube grows down the style towards the ovary
• The pollen tube discharges two sperm into the female gametophyte inside the ovule
• One sperm fertilizes the egg, forming a zygote
• The other sperm combines with two polar nuclei, forming a triploid cell that develops into the nutritive tissue called endosperm

Ovule vs Egg Cell

• Ovule is a structure that develops into a seed while the egg cell is the female gamete
• Ovule is located inside the ovary , the egg cell is inside the embryo sac within the ovule
• Ovule supports fertilization and turns into a seed whereas the egg cell fuses with sperm to form zygote
• Ovule is multicellular, composed of integuments and the embryo sac while the egg cell is single and haploid
• After fertilization, the ovule becomes a seed and the egg cell becomes an embryo

From Ovule to Seed

• After double fertilization, the ovule develops into a seed
• The ovary develops into a fruit enclosing the seeds
• As the embryo develops from the zygote, the seed stockpiles proteins, oils, and starch depending on the species
• Nutrients are stored in the endosperm

Embryo & Seed Development in Cotton

• The zygote goes through mitosis to form the proembryo and suspensor, which supports the embryo in the endosperm
• The cotton seed has a seed coat and contains the mature ovule within which is the mature embryo
• In the seed, the radicle (embryonic root) is at one end, and the shoot tip at the other end
• Cotyledons form, which are seed leaves that store food. After germination, cotyledons expand and become photosynthetic

From Ovary to Fruit

• While seeds develop from ovules the flower ovary develops into a fruit
• The fruit when mature, aids in dispersal by wind or animals
• Fertilization triggers hormonal changes leading the ovary to transform into a fruit
• If the flower hasn't been pollinated, the fruit usually doesn't develop, and the flower withers and falls away
• During fruit development, the ovary wall becomes the pericarp, the thickened wall of the fruit

Developmental Origin of Fruits

• Simple fruits develop from a single carpel (or several fused carpels) of one flower ex. pea, lemon, peanut
• Aggregate fruits develop from many separate carpels of one flower ex raspberry, blackberry, strawberry
• Multiple fruits develop from many carpels of many flowers forming an inflorescence ex. pineapple, fig
• Accessory fruits develop largely from tissues other than the ovary ex. apple