The angiosperm flower is the primary reproductive structure that provides the platform for the eventual production of seed and the continuation of the species through time. As such, there are both male and female components of the flower which may occur together on the same structure or occur separately. The generalized flower is made-up of essentially four whorls of specialized parts. The outer whorl is composed of leaf-like structures collectively called the calyx. Each individual leaf-like structure is a sepal. The general function of the sepals is to protect the flower bud as it matures. The next whorl inward is represented by structures called petals. Collectively, each group of petals is called the corolla. These structures are involved in attracting and even guiding a pollinator to the center of the flower. The corolla is often modified into different forms or shapes to accommodate a specialized pollinator. Sometimes it is very difficult to distinguish sepal from petal due to the fact that they all appear the same. Identical sepals and petals are called the perianth collectively.
Moving inward still further into the flower the next whorl is made of parts representing the male component of the flower. Each component part is called a stamen. It has two basic parts‚ the anther bearing the pollen and a filament or stalks which attaches the stamen to the base of the flower. Finally, the innermost whorl is the pistil or carpel functioning as the female part of the flower. The pistil has three basic structures. The stigma is the receptive part of the flower where pollen must first land (pollination). Once the pollen is on the stigma it germinates, sending out a pollen tube containing two cell nuclei which move down the pollen tube as it grows through the second part of the female flower structure termed the style. The basal part of the pistil is the ovary which has within it one or more ovules. It is within the ovules that the pollen tube must grow and deliver the sperm nucleus to the embryo sac. This event allows for the joining of sperm and egg nuclei (fertilization). Once this has occurred then nutrients are created to nurture the development of seed structures. It is within the seed that seed leaves or cotyledons develop. As the seed germinates the cotyledons are the first leaf-like structures to appear. Some seeds produce a single cotyledon and others produce two cotyledons. It is on this basis that the classification of Angiosperms (Flowering Plants) is divided into Monocotyledons (Monocots) and Dicotyledons (Dicots), respectively.
Flower Parts A flower is a highly differentiated and specialized branch of the stem bearing modified leaves or flower parts. It is the site of sexual reproduction in these plants and is their most distinctive structure. The great variety of forms acts as a guide in separating flowering plants into the major groups. The terms for the individual structures in the flower are as follows
Accessory organs are not directly involved in pollination. They include:
- Perianth The outer floral parts, composed of the calyx and the corolla.
- Tepal used when calyx and corolla are very similar and not easily distinguished as in Lilium catesbaei (Pine lily).
- Calyx The ring of sepals making up the outermost, leaf like part of the flower. Sepals are commonly green, but may be almost any color and serve primarily as protection for the other floral parts.
- Corolla The inner set of leaf like parts lying just within the calyx and composed of petals. Petals are generally white or brightly colored to attract pollinating insects to their nectar. They also serve as protection for the innermost organs.
- Receptacle or torus The apex of the pedicel upon which the organs of a flower are developed.
- Floral Bracts Modified leaves which can simulate petals and add the conspicuous part to otherwise inconspicuous flowers. Examples are the red bracts surrounding the small Euphorbia pulcherrima (poinsettia) flowers; the purple, red, or white leaves enclosing the small white flowers of Bougainvillea spp.; or the white leaves of Cornus florida (flowering dogwood). Many plants have floral bracts which are not colorful such as Fittonia vershaffeltii (silver nerve plant).
- Reproductive Organs These are directly involved in pollination and fertilization, hence their presence usually determines the survival of the species.
- Stamens Male reproductive organs attached to the receptacle in some species inside the corolla or in other species to the corolla itself.
- Filament Thin stalk which attaches the anther to the rest of the flower. Its attachment is called basal if it is at the lower end of the anther as in Tulipa spp. (tulip) and versatile if it is lateral, near the center of the anther as in Crinum spp.
- Anther Lobed, oblong, bag-like appendage at the top of the filament which produces the pollen grains which develop the male germ cells. Anthers are usually yellow and when young have from one to four cavities (cells) in which pollen grains arise. When mature, the anther usually contains two cavities from which the pollen grains are released by the formation of apical pores or longitudinal slits in the cavity wall.
- Pollen Grains Usually appear as tiny specks barely visible to the unaided eye, but are produced in such quantity that they often form a layer of powder. Each grain is usually two-celled, spherical, ovoid or disk-like in appearance, whose surface is marked with ridges, spines and germ spores. Pollen grains, collectively known as pollen, are so characteristic of the different species that they are used for identification purposes.
- In orchids and milkweeds, pollen is in masses known as pollinia.
- Stamens are called opposite when they are opposite to the petals as in Vitis rotundifolia (muscadine grape) or Rhamnus caroliniana (Carolina buckthorn) and alternate when they alternate with the petals as in Viola odorata (garden violet). The stamens are usually free or separate from each other, but in a few families they are united either by their filaments Hibiscus rosasinensis (hibiscus) and Lycopersicon esculentum (tomato) or by their anthers as in Helianthus annuus (sunflower). Pistils The female reproductive organs usually occur in the very center of the flower and are often surrounded by the stamens, petals and sepals. Flowers may have just one simple pistil as in Lathyrus odoratus (sweet pea) or two, three, four, five, or more separate pistils as in Consolida spp. (larkspur). A carpel refers to either a simple pistil or one of the segments of a compound pistil. United carpels are found in Dietes iridioides (African iris) and Antirrhinum majus (snapdragon). Pistils are usually flask- or bottle-shaped.
- They are composed of three parts
- Style The elongated stalk or neck connecting the ovary with the stigma.
- Ovary Enlarged, bulbous, basal part of the pistil, which bears the ovules (the egg-containing units which, after fertilization, become the seeds) attached either to its central axis or to its inner wall. The tissue to which the ovules are attached is called the placenta. Each ovule usually contains one egg, the female gamete or sex cell. The ovule normally develops into a seed when fertilized. Generally there are two or more ovules per carpel. In some plants, only one may mature into a seed. The ovary normally develops into a fruit containing seeds. An example is the pod of Phaseolus lunatus (lima bean).
- Stigma The tip of the pistil especially adapted to receive the pollen grains. The stigma may be expanded into a bulb or disk or divided into two or more slender parts. The stigma is often located atop a style.
- A pistil is said to be compound when several or many carpels become united. Carpels are united so that a compound ovary often contains as many cavities as there are carpels. In some flowers, a compound ovary becomes “one-celled” by the disappearance of the partitions between the different carpels as in Primula spp. (primrose). Union of carpels may be so complete that it includes the styles and stigmas as well as the ovaries.
Different classification or different classes of flowers are based on the presence or absence of their parts
- Complete flowers are made up of calyx, corolla, stamens and a pistil or pistils (the four “regular parts”).
- Incomplete flowers lack one or more of the four regular parts of a complete flower as in all of the Fagaceae (oak family), Betulaceae (birch family) and Juglandaceae (walnut family).
- Perfect flowers have both stamens and pistils, but not necessarily sepals or petals.
- Imperfect flowers lack either stamens or pistils and may or may not have sepals or petals.
- Naked flowers are without petals (apetalous) or sepals (asepalous) as in Zantedeschia spp. (calla lily).
- Apetalous flowers lack petals as in Elaeagnus pungens (silverthorn), Hydrangea spp. and Cornus florida (flowering dogwood).
- Staminate (male) flowers have a stamen or stamens, but no functional pistils.
- Pistillate (female) flowers have a pistil or pistils, but no functional stamens.
Three terms are applied to plants based on their flowering characteristics
- Monoecious plants bear both staminate and pistillate flowers on the same plant as in Quercus spp. (oak) and Zea mays (corn).
- Dioecious plants bear staminate flowers on one plant and pistillate flowers on a different plant, hence the terms male and female plants. Ilex species (holly) and all cycads and many conifers are examples.
- Polygamous plants bear staminate, pistillate and hermaphroditic (bisexual – both sexes present and functional in the same flower) flowers on the same plant. An example is Acer rubrum (red maple).