Flower Structure, Pollination, and Fertilisation
General Structure and Functions of a Flower
A flower is composed of a flower stalk, known as the pedicel, and the apex of this stalk, referred to as the receptacle. Attached to the receptacle are four primary groups of floral structures: the calyx on the outermost layer, the corolla (the colored parts), the male reproductive parts known as the androecium, and the female reproductive parts known as the gynoecium or pistil.
The calyx consists of individual units called sepals, which are typically green in color. When sepals are free from one another, they form a polysepalous calyx; when they are fused together, they form a gamosepalous calyx. The primary function of the calyx is to protect the internal parts of the flower, particularly during the bud development stage. In certain flowers, an additional ring of sepal-like structures called the epicalyx may be found just beneath the calyx.
The corolla is made of petals, which are often brightly colored, large, and conspicuous in insect-pollinated flowers to attract pollinators. If the petals are fused, the corolla is described as gamopetalous, and if they are free, the term polypetalous is used.
The androecium is the male part of the flower and consists of one or more stamens. Each stamen is made of a long stalk-like structure called the filament, which supports the anther at its tip. The anther contains four pollen sacs, which house the pollen grains containing the male gametes. Stamens may be fused into a staminal tube, as seen in the Hibiscus flower, or they may remain free, as seen in the Crotalaria flower.
Classification and Terminology of Flower Parts
Flowers are classified based on the presence or absence of specific parts. A complete flower is one where all floral parts—calyx, corolla, androecium, and gynoecium—are present. If one or more of these parts are missing, the flower is considered incomplete.
Flowers can also be classified by their reproductive organs. A unisexual flower contains only one of the reproductive parts, either male (staminate flower) or female (pistillate flower). A hermaphrodite or bisexual flower, such as Hibiscus, contains both male and female reproductive organs.
Plants are described as monoecious when both male and female flowers are borne on the same individual plant, such as maize and oil palm. Conversely, dioecious plants bear either male or female flowers on separate plants, meaning there are distinct male and female plants, as seen in the paw paw.
Floral parts are categorized into non-essential parts (calyx and corolla) and essential parts (androecium and gynoecium), which are directly involved in reproduction. Flowers are also described by their symmetry. Regular or actinomorphic flowers, such as morning glory, are radially symmetrical and can be divided into two similar halves by any vertical section passing through the center. Irregular or zygomorphic flowers, such as Crotalaria, are bilaterally symmetrical and can only be divided into two equal halves in one specific plane.
A flower with a stalk is called a pedicillate flower. Flowers that occur singly are termed solitary, while those that grow in clusters form an inflorescence.
Types of Ovaries and Gynoecium Structures
The gynoecium or pistil forms the female reproductive organ and consists of carpels, each made of an ovary, a style, and a stigma. The ovary contains the ovules. Ovaries are classified based on their position on the receptacle. If the ovary is above the other floral parts, it is a superior or hypogynous ovary, as seen in Hibiscus. If the receptacle surrounds the carpel with floral parts arising around the ovary, it is perigynous, as in the rose flower. If the other floral parts occur above the ovary, it is an inferior or epigynous ovary, as seen in apple flowers.
Gynoecium types are also determined by the number and fusion of carpels. A monocarpous gynoecium has a single carpel, such as in beans. A polycarpous gynoecium has two or more carpels. If the carpels in a polycarpous gynoecium are free, it is termed apocarpous (e.g., rose and Bryophyllum). If the carpels are fused together, the gynoecium is termed syncarpous (e.g., Hibiscus).
Pollination and Its Agents
Pollination is defined as the transfer of pollen grains from the anther to the stigma. Self-pollination occurs when pollen is transferred from the anther to the stigma of the same flower or another flower on the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different plant but of the same species.
Pollination often requires agents to facilitate the transfer of pollen grains. The primary agents are insects and wind. Insect pollinators include bees, butterflies, and mosquitoes, which visit flowers to obtain nectar. Wind pollination commonly occurs in the grass family, including plants like maize, wheat, and oats. For pollination to be successful, flowers are structured to enable the specific agent to carry out the transfer without hindrance.
Adaptations to Insect and Wind Pollination
Insect-pollinated flowers, known as entomophilous flowers (e.g., Hibiscus), have the following adaptations: they are large and conspicuous with brightly colored petals, inflorescences, or bracts; they are scented and produce nectar; anthers are small and firmly attached to filaments so insects rub against them; pollen grains are relatively large, heavy, and rough or sticky to adhere to insect bodies; and stigmas are small, sticky, and positioned inside the flower.
Wind-pollinated flowers, known as anemophilous flowers (e.g., Zea mays), exhibit different adaptations: flowers are small and inconspicuous with no scent or nectar; anthers are large and loosely attached to flexible filaments to release pollen easily in the wind; pollen grains are usually small, smooth, and light for airy dispersal; and stigmas are long, feathery, and hang outside the flowers to act as nets for trapping airborne pollen. The structure of wind-pollinated flowers is generally simple without a particular shape.
Mechanisms Preventing Self-Pollination
Most plant species have developed mechanisms to discourage self-pollination and self-fertilization to enhance cross-pollination, which provides variations for better adaptability. Even in hermaphrodite and monoecious plants, natural provisions exist to prevent selfing. Bright colors, scents, and nectar attract insects to ensure pollen is moved between different flowers.
Specific temporal mechanisms include protandry and protogyny. In protandrous flowers, the stamens ripen before the stigma is mature, allowing anthers to shed pollen before it can be received by the same flower's stigma. This difference in ripening times effectively forces cross-pollination.
The Process of Germination and Double Fertilization
Fertilization is the fusion of male and female nuclei within the embryo sac. After pollination, the pollen grain adheres to the stigma, which secretes a sticky substance that stimulates the pollen grain to germinate. The grain absorbs nutrients and develops an outgrowth called the pollen tube. This tube grows rapidly through the style toward the embryo sac, obtaining nourishment from surrounding tissues.
During germination, the tube nucleus stays at the tip of the growing pollen tube. Behind it, the generative nucleus divides by mitosis into two male gamete nuclei. The pollen tube enters the ovule through an opening called the micropyle, penetrates the embryo sac wall, and bursts. The tube nucleus then disintegrates.
Double fertilization, unique to flowering plants, then occurs. One male nucleus fuses with the egg cell nucleus to form a diploid () zygote that develops into an embryo. The other male nucleus fuses with the polar nuclei to form a triploid () nucleus. This triploid nucleus subsequently develops into the endosperm, which serves as a food reserve. After fertilization, the ovary, fertilized ovule, and the entire flower undergo changes; for instance, the protective calyx usually dries up as the seed and fruit develop.
Questions & Discussion
Study Question 3: Given a flower, explain how you can show its symmetry. (Answer: This involves performing vertical sections through the center of the flower to determine if it can be divided into similar halves in multiple planes (actinomorphic) or only one specific plane (zygomorphic)).
Study Question 4 (a): Name the type of cell division that produces the gametes. (Answer: Meiosis).
Study Question 4 (b): From your knowledge of the cell division you have named in (a) above, state the chromosome constitution of the gametes. (Answer: Gametes are haploid (), meaning they contain half the number of chromosomes of the parent cell).