Flowers arc modified branches bearing four sets of floral organs. The tloral organs arc sepals, petals, stamens, and carpels. These four flower parts are in whorls on the receptacle, the expanded top of the flower stalk, or pedicel (fig. 6.1).
The sepals are the outermost floral organs. They are leaflike structures that cover the unopened flower bud. In most flowers, the sepals arc green and photosynthetic. The petals make lip the next whorl of flower parts. Petals are often brighdy colored and conspicuous; their function is to attract animal pollinators.
The male and female structures arc usually located in the center of the flower. The stamens are the male structures, and each stamen consists of an anther supported on a stalk, called the filament. The anther consists of four chambers, where meiosis occurs and where pollen develops. Each pollen grain is a male gametophvtc and is capable of producing sperm during the growth of the pollen tube just prior to fertilization.
The female structures are carpels, which are located in the middle of the flower. Flowers can have from one to many carpels. When there is only one carpel present, it is called a simple pistil. When a flower has many carpcis, they may either be fused together to form one compound pistil or remain as many separate simple pistils. Carpels, whether single or fused, consist of three parts: the stigma, the style, and the ovary. The stigma, which is at the tip of the carpel, receives pollen on its sticky, feathery, or hairy surfacc. One to man\ ovules develop within the ovary at the base of the carpel. The style connects the Stigma to the ovary.
The ovule includes the female gametophvtc, and when mature it contains an egg that can be fertilized by sperm from the pollen. Following fertilization, die ovary becomes the fruit and cach fertilized ovule becomes a seed (fig. 6.2). An ovary can have one ovule las in a peach or plum) or thousands of ovules (as in a watermelon or pumpkin). The resulting fruits can have one to thousands of seeds.
Monocots generally have floral organs in 3s or multiples of 3, whereas dicots have floral organs in 4s or 5s or
multiples of 4 or 5. For example, a lily, which is a mono-cot, has 3 sepals, 3 petals, 6 stamens, and a 3-part ovary formed from the fusion of 3 carpels. The flower of a wild geranium, a dicot, consists of 5 sepals, 5 petals, 10 stamens, and 5 fused carpels with separate stigmas.
Flowers that contain all four floral organs arc known as complete and perfect flowers; however, the basic flower structure is frequently modified. In some flowers, one or more flower parts are missing: such flowers arc incomplete, incomplete flowers that lack cither carpels or stamens are also called imperfect. If carpels arc missing, the flower is staminate (male); and if stamens are lacking, the flower is carpellatc or pistillate (female).
Although many flowers develop individually on a stalk, other flowers are grouped into a cluster called an inflorescence. Many times what looks like a single flower at first glance, such as a sunflower or a daisy, is actually an inflorescence. Another example is dogwood; the small flowers are in a cluster and surrounded by showy pink or wrhitc bracts (leaflike structures that often look like petals).
Pollination is the t ransfer of pollen from the anther to the stigma. In a flower that is sclf-pollinatcd, the trans fer occurs within a single flower. By contrast, cross-pol lination involves the transfer of pollen from one plant to another. A variety of animals, including insects, birds, and even mammals, serve as agents to transfer pollen for many flowers; wind carries the pollen for other plants. Animal-pollinated plants usually have large, showy flowers. The petals arc brightly colored, and essential oils impart scents ro attract the pollinator. These flowers often produce nectar, which is a reward for the animal. Pollen in the flowers is usually large, heavy, and sticky. By contrast, wind pollinated flowers tend to be small and inconspicuous. They are often formed in an inflorescence and lack petals. Wind-pollinated flowers produce copious amounts of small, lightweight pollen.
Dissecting microscope and compound microscope
Four different flowers. Select from those provided by the instructor.
Prepared slide of Lilium (lily) anther Prepared slide of Lilium (lily) ovary Razor blades, single-edged or scalpel and dissecting needles
1. Examine the flowers available in lab. You should be able to determine whether you are looking at a single flower or an inflorescence. You should also be able to identify sepals, petals, stamens, and carpels. Which flowers are incomplete? Which are imperfect: Based
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/ Megaspore / mother cell
Microspore mother cells
PoJ'en chamber with microspores
Three ^ megaspore: degenerate
Ovary develops into fruit: ovule de/stops mto seed
Zygote develops ! into embryo
Each microspore matures into a poflen grain
Endosperm forms when the two polar nucia and one sperm unite
On the stigma. the pollen germinates and produces two sperm
- Generative nucleus
Antipodats ft>!ien tube
The eight nuclei are produced by three successive divisions of the megaspore nucleus. They become rearranged ¡n what «s now called the embryo sac, or female gametophyle.
on the structures you see, predict whether each flower is wind pollinated or animal pollinated. Fill in your answers in worksheet 6-1 at the end of this laboratory topic.
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