There are already about 250,000 species of flowering plants that have been discovered and named. The basis for their diversity comes from their incredible reproductive success in a wide variety of habitats.
The success of this group is also reflected by the diversity of their flowers that show astonishing displays of different forms, sizes, shapes, and colors—all of these to lure pollinators and effect sexual reproduction.
Flowers are considered as an organ system because they are made up of two or more sets, or whorls, of leaflike structures. A typical flower is composed of four whorls, which are the sepals, petals, stamens, and a pistil with one or more carpels. Much of the variation among flowers is based on variation of these basic parts.
Complete and Incomplete Flowers
A flower that has all four whorls of floral parts is said to be a complete flower (such as the hibiscus and the lily). An incomplete flower lacks any one or more of these parts (such as those of elms, willows, oaks, and plantains).
With or without sepals and petals, a flower that has both stamen and pistil is called a perfect flower. Thus, all compete flowers are prefect, but not all perfect flowers are complete. In contrast, flowers that have only stamens or only pistils are called imperfect flowers.
Unisexual and Bisexual Flowers
Unisexual flowers are either staminate (bearing stamens only) or pistillate (bearing pistils only) and are said to be imperfect. Bisexual flowers are perfect because they have both stamens and pistil.
When staminate and pistillate flowers occur on the same individual, the plant is called monoecious (examples include corn and the walnut tree). When staminate and pistillate flowers are borne on separate individual flowers, the plant is said to be dioecious (examples include asparagus and willow).
Superior or Inferior Ovaries
An inferior flower has an ovary below where the sepals, petals, and stamens are attached (as do daffodils and sabatia). Some flowers show an intermediate type, where the receptacle partly surrounds the ovary; the petals and stamens branch from the receptacle about halfway up the ovary (as in cherry, peach, and almond flowers).
Hypogynous, Epigynous, and Perigynous Flowers
Flowers inwhich the sepals, petals, and stamens appear to be attached to the upper part of the ovary due to the fusion of the hypanthium are called epigynous, and the ovaries of such flowers are said to be inferior (as in cornus and narcissus).
Flowers in which the hypanthium forms a cuplike or tubular structure that partly surrounds the ovary are called perigynous. In such flowers, the sepals, petals, and stamens are attached to the rim of the hypanthium, and the ovaries of such flowers are superior.
Fused and Distinct Floral Parts
The parts of a flower may be free or united. Fusion of like parts (such as petals united to petals) is called connation. When like parts are not fused, they are said to be distinct (one petal is distinct from another petal).
Fusion of unlike parts (stamens united to petals) is called adnation, and the contrasting condition is called free (stamens are free from petals). Fused structures may be united from the moment of origin on ward, or they may initially be separate and grow together as one later in development.
Regular and Irregular Flowers
In many flowers, the petals of similar shape radiate from the center of the flower and are equidistant from one another. Such flowers are said to have regular or radial symmetry.
In these cases, even though there may be an uneven number of sepals and petals, any line drawn through the center of the flower will divide it into two similar halves. The halves are either exact duplicates or mirror images of each other. Flowers with radial symmetry are also called actinomorphic flowers (examples: stonecrop, morning glory).
Flowers with irregular or bilateral symmetry have parts arranged in such a way that only one line can divide the flower into equal halves that are more or less mirror images of each other. Flowers with bilateral symmetry are also called zygomorphic flowers (examples: mint, pea, snapdragon). A few flowers have no plane of symmetry and are referred to as asymmetrical.
Corolla is the collective term for all the petals of a single flower. This is usually the showy part of the flower. In fused corollas, any extension of the petal beyond its fused part is called the limb.
The tubelike structure where the petals are united at the bottom of the fused corollas is called the tube. The opening at the top of the tube in fused corollas is called the throat.
In the following different types of corolla shapes, numbers 1 to 6 are actinomorphic, while numbers 7 to 11 are zygomorphic.
- Rotate: wheel-shaped with a short tube and large limb (example: bluets).
- Campanulate: bell-shaped with an extended, flaring tube (example: bellflower).
- Funnelform: funnel-shaped with a continuously expanding tube and little flaring (example: bindweeds).
- Tubular: an elongated tube with minimal limb (example: trumpet vine).
- Salverform: an elongated tube with a conspicuous limb, trumpet-shaped (examples:Russian olive, morning glory).
- Urceolate: an inflated tubewith a terminal constriction, urn-shaped (example: highbush blueberry).
- Bilabiate: two-lipped, usually because of the presence of a landing platform formed by basal lobes (examples: snapdragon, salvia).
- Ligulate: petals connate at the margins to form a strap-shaped corolla (example: asters).
- Galeate: helmet-shaped(example: pedicularis).
- Spurred: with an extension or spur that often produces nectaries (examples: impatiens, utricularia).
- Papilionaceous: like a butterfly with a central standard petal and lateral wing petal (example: lupines).
Flowers of Monocots and Dicots
Floral variation provides part of the basis for dividing the flowering plants into two major groups: the dicotyledons and the monocotyledons.
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The informal name “dicot” is given to plants having two cotyledons (seed leaves) in each seed; “monocot” refers to plants that have one cotyledon in the seed. In monocots, the flower parts occur in threes or multiples of three; for example, three sepals, three petals, six stamens, and a pistil with three carpels.
In dicots, flower parts usually occur in fours or fives or multiples of four or five. Although dicots and monocots may have other numbers of floral parts, many other features are unique to each group.
Dicots include about 80 percent of all angiosperm species, including many herbaceous plants and all woody, flower-bearing trees and shrubs. Monocots are primarily herbaceous, but they also include some trees, such as palms and Joshua trees.
Types of Inflorescence
Flowers may be solitary, or they may be grouped together in an inflorescence, a cluster of flowers. An inflorescence has one main stalk, or peduncle. It may also bear numerous smaller stalks called pedicels, each with a flower at its tip.
The arrangement of pedicels on a peduncle characterizes different kinds of inflorescences. Some of the common types of inflorescences are as follows:
- Spike: The flowers,which are with a very short or with no pedicel, are attached along the elongate and unbranched peduncle of the inflorescence (examples: plantain, spearmint, tamarisk).
- Raceme: The flowers are with pedicels of about the same length, which are attached along the elongate and unbranched peduncle of the inflorescence (examples: lily of the valley, snap-dragon, mustard, currant). The oldest flowers are at the base of the inflorescence and the youngest at the apex.
- Panicle: The flowers are with pedicels, which are attached along the branches arising from the peduncle of the inflorescence (examples: oats, rice, fescue).
- Corymb: The flowers arewith pedicels of unequal length,which are attached along an unbranched, elongate peduncle, forming a flat-topped inflorescence (examples: hawthorne, apple, dogwood).
- Umbel: The flowers are with pedicles, which are all attached at about the same point at the end of the peduncle—this is specifically called a simple umbel (examples: onion, geranium,milkweed). A compound umbel is formed when the peduncle produces branches that end at approximately the same level, forming a flat top, and the ends of these branches arise from a common point (examples: carrot, dill, parsley).
- Head: The flowers do not have pedicels, and they all cluster tightly on the expanded tip of the peduncle (examples: sunflower, daisy, marigold). This type of inflorescence is also referred to as capitulum.
- Cyme:The flowerswith pedicels are located at the ends of the peduncle and lateral branches as well as along the length of the lateral branches. The youngest flowers in any cluster occur farthest from the tip of the peduncle (example: chickweed).
- Catkin: The flowers have no pedicels, are unisexual (either staminate or pistillate), and are attached along the length of the peduncle (examples: hazelnut, willow, birch, walnut). The flowers are usually very small and fall as a group. This type of inflorescence is also referred to as ament.
- Spadix: The flowers have no pedicels and are attached along the length of the thickened or fleshy peduncle, which is enveloped by a conspicuously colored bract called a spathe (example: philodendron, anthurium).
Some types of inflorescences characterize different groups of plants. For example, nearly all members of the carrot family (Apiaceae) have compound umbels. All members of the sunflower family (Asteraceae) have heads, including chrysanthemums, zinnias, marigolds, and dandelions. All members of the arum family (Araceae) have a spadix inflorescence.