FLOWERS
Introduction
The term angiosperm is derived from two Greek words: angeion, meaning “vessel,” and sperma, meaning “seed.” The “vessel” is the carpel, which is like an inrolled leaf with seeds along its margins.
EXAMPLES OF FLOWERING PLANTS
Rafflesia flower
The flowers themselves range in size from the minute flowers of duckweed. Wolfia columbiana, whose entire plant body is only 0.5 to 0.7 millimeter (0.02 to 0.03 inch) wide, with flowers little more than 0.1 millimeters long to the enormous Rafflesia flowers (Fig. 8.2 a) of Indonesia that are up to 1 meter (3 feet, 3 inches) in diameter and may weigh 9 kilogram (20 pounds) each.
Amorphophallus titanium
A unique species known as the corpse flower (Amorphophallus titanium) produces an interesting bloom (also called an inflorescene, discussed later in this chapter) that deserves mention for its large size and offensive odor. The bloom itself (Figure 8.2b), which can be over 2 meters (6 feet, 6 inches) tall, protects the smaller flowers within and is derived from an underground corn that can remain hidden for many years. Carrion beetles are attracted to an odor of rooting flesh emitted by the bloom and presumably act as pollinators when they travel from one flower to the next.
Catkins (Alnus)
The rarity and impressive display of corpse flower blooms (less than 100 in recorded history) inspire curious spectators to visit and enjoy botanical gardens around the world. Flowers may
be any color or combination of colors of the rainbow, as well as black or white, they may have virtually any texture, from filmy and transparent so thick and leathery from spongy to sticky, hairy, prickly, or even dewy wet to the touch. Flowers of many trees, shrubs, and garden weeds are quite inconspicuous and lack odor (Fig. 8.3), but other flower are strikingly beautiful, particularly when examined with a dissecting microscope.
DICOTS AND MONOCOTS
The flowering plants (angiosperms) that make up the division Magnoliophyta may be grouped into two major plant groups: Dicotyledoneae (or Magnoliopsida) and Monocotyledonae (or Liliopsida).
a) Dicots-
• Monocot plants have one cotyledon. They also have long narrow leaves with parallel veins. Cutting a cross section from the stem of a monocot shows the vascular bundles scattered around in the plant tissue. The young plant stores food in the form of starches and other nutrients in a structure called the endosperm.
• Another key characteristic for identifying monocots is by the number of flowers or flower parts which are arranged in groups of three. Also, the pollen grains of monocot plants have a single pore or furrow making them monosulcate (from the Greek word mono meaning ‘single’ or ‘one‘and the Latin word sulcatus meaning ‘furrow’) and new roots originate from the stem of the plant. Some examples of monocots are lilies, orchids, corn, rice, wheat, barley, pineapple, sugar cane, bananas, palms, and grasses.
b) Monocots-
• As opposed to monocots, dicots (also called eudicots) have two cotyledons during germination which supply the young plant with food and nutrients. The leaves of dicot plants come in a variety of shapes and sizes and the veins form branching patterns. Microscopic examination of dicot seeds shows a structure called the hilum which is a scar on the seed coat where the ovary was attached. This feature is not seen in monocots. Also, different from monocots is the roots of dicot plants originate from the radicle.
• Another way dicots are distinct from monocots is their flowers and flower parts are arranged in multiples of four or five. In addition, the cross section of a dicot stem shows the vascular bundles arranged in a circular pattern. Unlike monocots, the pollen grains of dicot plants have three pores and are called trisulcate. Dicot plants can also have bark and secondary growth increases the diameter (girth) of the plant. Examples of dicots include potatoes, tomatoes, apples, pears, peaches, cauliflower, peppers, broccoli, and cabbage.
The Four Main Flower Parts
Sepals: the protective, leaf-like outer parts of a flower. Petals: an often colorful structures that attract pollinating animals to the flower.
Stamens: the male parts of a flower. Each stamen consists of a stalk called a filament and a pollen-producing tip called an anther. The stamens of many flowers are designed to shed pollen onto a pollinating animal such as a bee or hummingbird.
Carpels / Pistil: the female parts of a flower. A flower's carpels are often fused into a single structure known as a pistil. At the tip of the carpel / pistil is a pollen collecting area known as the stigma. This is where a pollen grain must be deposited in order for pollination to take place.
Whorl: A flower’s sepals, petals, stamens and carpels grow in rings known as whorls around the receptacle.
The Male Parts of a Flower (The Androecium)
Surrounded by a whorl of sepals and a whorl of petals (which together known as the perianth) are the reproductive parts of a flower.
The male parts of a flower, which collectively are called the androecium, are arranged in a whorl around the flower's female parts (the Gynoecium).
Stamen: The stamen is the male part of the flower. It consists of a stalk called a filament and a head called an anther. The number of stamens in a flower varies from species to species.
Anther: The anther is the bulbous head at the tip of the filament. It contains microsporangia, or pollen sacs, in which pollen is produced.
Pollen is the microscopic, dust-like powder in which a plant’s male reproductive cells are dispersed.
The anthers are usually positioned in such a way that a pollinator (an animal such as a bee or a hummingbird) will brush against them while visiting the flower. This results in pollen being transferred to the pollinator's body, where it will be transported to the female parts of another flower (see the Life Cycle of a Plant).
Filament: The filament is the stalk that bears the anther.
The Female Parts of a Flower (The Gynoecium)
At the center of the flower, surrounded by the whorl of stamens, is the gynoecium – the flower’s female parts.
Carpel: The carpel is the individual female reproductive part of a flower. It consists of an ovary, a style and a stigma. Depending on species, a flower may have either a single carpel, or several. In many species the carpels are fused (joined), forming a structure known as a pistil.
Stigma: The stigma is a located at the tip of the carpel / pistil. It offers a receptive, often sticky or feathery, surface that captures pollen. Upon landing on the stigma, a pollen grain (of the same species) will begin to germinate (see the Life Cycle of a Plant).
Style: The style is the stalk between the stigma and the ovary. In some species (such as tulips and poppies) it is very short and / or indiscernible from the stigma and ovary.
When a grain of pollen germinates, it extends a structure called a pollen tube through the style to the ovary. The male reproductive cells from the pollen will then travel through the pollen tube to fertilize the egg.
Ovary: Located at the base of the carpel or pistil is the ovary. Chambers within the ovary contain one or more ovules. These egg-containing structures, once fertilized, will become seeds. If the ovary is positioned above the calyx and corolla, it is said to be superior; if it is below the calyx and corolla it is said to be inferior.
Calyx: The entire whorl of sepals is known as the calyx. Its function is to protect the inner parts of the flower during the bud stage.
Corolla: The corolla is the collective term for a flower’s whorl of petals.
