Reproduction and Development in Flowering Plants

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Reproduction and Development in Flowering Plants Sexual Reproduction in Flowering Plants The flower functions in sexual reproduction of plants and precedes the development of seeds and fruits.

1 Reproduction and Development in Flowering Plants Sexual Reproduction in Flowering Plants The flower functions in sexual reproduction of plants and precedes the development of seeds and fruits. Flowers vary greatly in size, shape, color, pattern and number of floral parts. A complete flower has four major types of structures: 1. Sepals, the outer group of floral structures, collectively called the calyx 2. Petals that collectively form the corolla 3. Stamens, the male reproductive organs 4. Carpels, the female reproductive organs. The floral parts may vary in number and shape and may be separate or fused to each other or to other floral parts. In addition, a flower may be lacking one or more of its floral parts. Windpollinated flowers, for example, often lack a corolla. Flowers that have all four organs are said to be complete. All of the floral organs are produced on the receptacle, the expanded tip of the flower stalk, the peduncle or pedicel. Activity 1: Flower Structure Examine several flowers provided. For each flower: Is this a complete flower? Are all of the floral organs visible without disturbing the flower? Identify the calyx and the corolla of your flower. If the sepals and petals are similar in appearance, such as in the lily flower, they are called tepals. Observe the stamens. Each stamen consists of a thread-like stalk, the filament, and an anther, in which meiosis occurs producing the microspore that develops into the haploid male gametophyte, the pollen grain. Examine the stamens of your flower. Do you find any mature pollen? Mature pollen is released through slits or pores in the anther. The female reproductive structure of the flower is the carpel. The carpel is separated into three parts: the receptive stigma to which pollen grains adhere during pollination; the style, a stalk that elevates the stigma; and the basal ovary, which contains one to several ovules. Meiosis occurs in the ovule producing a megaspore that divides to form the haploid 7- celled embryo sac that contains one egg along with several other nuclei. Generalized Flower

Activity 2: Gametogenesis Obtain a prepared slide of mature pollen. Each mature pollen grain contains a tube nucleus and a generative nucleus that divides to produce two sperm.

Anther with four pollen chambers (locules) Anther with Mature Pollen (Male Gametophyte) Obtain a prepared slide of a cross section of the Lilium ovary.

2 Activity 2: Gametogenesis Obtain a prepared slide of mature pollen. Each mature pollen grain contains a tube nucleus and a generative nucleus that divides to produce two sperm. The pollen grain has a thickened and resistant pollen wall. Pollen grains are not activated unless successful pollination occurs with a genetically compatible female. Anther with four pollen chambers (locules) Anther with Mature Pollen (Male Gametophyte) Obtain a prepared slide of a cross section of the Lilium ovary. The ovary will have three locules (chambers), each of which contains 2 ovules. Do you see any evidence of a developing embryo sac in any of the ovules? (You may not see developing embryo sacs in all of the ovules as the section may have been cut through sterile portions of ovule tissue.) Now observe the slides of the developing embryo sac of the lily flower that show the 2- and 4- nucleate stages prior to the formation of the mature embryo sac. The mature embryo sac contains eight nuclei. The egg will be located at one end of the embryo sac surrounded by 2 synergids. Three antipodal nuclei will be at the opposite end and the central cell with its 2 polar nuclei will be located in the center of the embryo sac. Ovule with megaspore mother cell Megaspore Embryo Sac (Mature Female Gametophyte) Activity 3: Pollination, Fertilization Before the two sperm from the male pollen grain can reach the egg within the ovule, the pollen grain must be transported from one flower to another. Pollen may be carried by wind. Animals may also serve as agents of pollination. The colors, shapes, patterns and fragrances of petals serve to attract animal pollinators such as insects, birds or bats. Many flowers also produce a sugary nectar to reward loyal pollinators

Pollen Germination Observe a slide of germinating pollen. These pollen grains have been stimulated to germinate so that you should see several pollen tubes.

Mature Pollen Grains Germinating Pollen After successful pollination the pollen grain germinates and grows a pollen tube through the style of the carpel.

3 Pollen Germination Observe a slide of germinating pollen. These pollen grains have been stimulated to germinate so that you should see several pollen tubes. Can you find the two sperm nuclei within the tube? It is sometimes possible to stimulate fresh pollen to germinate and grow pollen tubes. Mature Pollen Grains Germinating Pollen After successful pollination the pollen grain germinates and grows a pollen tube through the style of the carpel. The two sperm migrate through the pollen tube to one of the degrading synergids of the embryo sac. One sperm fertilizes the egg to form the diploid zygote that develops into the embryo and ultimately the adult plant, the sporophyte. The second sperm unites with the 2 polar nuclei to form the endosperm, a tissue that will provide nutrients for the developing embryo. Activity 4: Embryo Development The early embryo development of a flowering plant can be seen in slides of the ovary of Capsella, shepherd's purse. The ovary of Capsella contains many ovules so that many embryos may be seen at one time. Obtain the four prepared slides of developing embryos of Capsella. You should be able to observe the stages of embryo development with the prepared slides, from the early embryo to the mature embryo. Some sections may also show the loosely packed endosperm tissue. Use the diagrams provided to help locate the different embryo stages. 1. The first slide will show the embryo before the cotyledons appear. Identify the basal cell of the suspensor, the suspensor stalk and the proembryo (the rounded mass of cells attached to the suspensor). 2. The second slide shows the Capsella embryo as the cotyledons are forming. This is often called the "heart" stage. Capsella is a dicotyledon, having two cotyledons. The embryo will also have its three primary meristems: protoderm, procambium and ground meristem visible at this stage. These meristems will produce the mature epidermis, vascular tissue and ground tissue respectively. 3. The third slide will show an elongated embryo with well-developed cotyledons. At this time you should locate the hypocotyl-root axis, the region of the embryo below the cotyledons. The hypocotyl may already show evidence of bending as the embryo rapidly elongates. You should also see the epicotyl, the embryonic shoot, located between the two cotyledons. 4. The fourth slide shows the mature embryo, with the two cotyledons bent over in the ovule. The hypocotyl with the root axis (radicle) at its tip and the epicotyl are clearly visible. In addition you will note that the embryo sac wall and ovule layers have formed a protective layer around the embryo. This layer is the seed coat

4 Embryo Development in Capsella, Shepard s Purse, a Dicot Fertilization Two-celled Embryo Young Embryo Cotyledon Stage Bending Embryo Mature Embryo Embryo Development in Lily, a Monocot - 4 -

The most important of these are the grains: wheat, rice and corn, which provide about two-thirds of the world's food supply.

5 Activity 5: Seed and Seed Germination The seed consists of the mature embryo, endosperm tissue and the seed coat. Seeds are the primary food source for the world, feeding more people and other animals than any other food source. The most important of these are the grains: wheat, rice and corn, which provide about two-thirds of the world's food supply. Many other seeds, such as rye, oats, soybeans, peanuts and other legumes are also important. Keep in mind that the seed is a mature embryonic plant, containing enough nutrients, supplied by the endosperm, for germination. The seed is protected from the environment by its seed coat, and often by the surrounding ovary tissue (the fruit). The nutrients of the seed are needed for adequate seedling growth when the young plant is most vulnerable to the environment. It is this high nutrient content of seeds that makes them such a valuable food source. Bean Seed Examine a seed of a bean that has been soaked in water for several hours. The bean is a dicot that has "fleshy" cotyledons. The endosperm tissue was absorbed into the cotyledons as the seed developed. This is unusual for seeds, but bean seeds are large and the embryo structures are easy to see. Locate the following in the bean seed Seed coat: The protective covering of the seed Hilum : A scar on the side of the seed where the seed was attached to the ovary Remove the seed coat and examine: Cotyledons: The two "halves' of the seed. Because the cotyledons have absorbed nutrients from the endosperm they are plump and nutrient rich. Embryonic axis: The remainder of the embryo will be attached to one of the cotyledons. You should note tiny leaves surrounding the embryonic shoot. This is called the plumule. The plumule surrounds the epicotyl, which is the primary shoot tip. The lower portion of the embryonic axis is the hypocotyl that terminates with the primary root, the radicle. The radicle of the axis is positively geotropic; the hypocotyl is negatively geotropic. Bean seed with seed coat and one cotyledon removed Bean Seed Diagram - 5 -

Corn Grain Now examine a corn grain that has been soaked.

Cut the corn grain longitudinally with a razor blade and add a drop of iodine to the cut surface.

the fleshy cotyledons. Since the endosperm is starchy, it should show a positive starch reaction with the iodine.

Coleoptile: A part of the modified cotyledon that forms a sheath around the embryonic shoot, protecting the shoot during

6 Corn Grain Now examine a corn grain that has been soaked. Technically the grain is a fruit, because the ovary tissue fuses to the seed coat. The bulk of the grain is the seed. Cut the corn grain longitudinally with a razor blade and add a drop of iodine to the cut surface. Observe: Endosperm: Most of the volume of the seed is endosperm in contrast to the bean where nutrients have been transferred to the fleshy cotyledons. Since the endosperm is starchy, it should show a positive starch reaction with the iodine. The embryo proper consists of the: a. Coleoptile: A part of the modified cotyledon that forms a sheath around the embryonic shoot, protecting the shoot during germination. b. Plumule: The embryonic shoot, consisting of the epicotyl (shoot meristem) and the first leaves c. Radicle: The primary root located at the tip of the embryonic axis. d. Hypocotyl: The embryonic axis joining the radicle to the plumule. It is surrounded by the coleorhiza portion of the modified cotyledon. Corn Grain With Adventitious Root Wheat Grain - 6 -

Germination Observe the various stages of

Cotyledon(s) Plumule Seed coat Bean Germination Pea

7 Germination Observe the various stages of germination and seedling development of the bean, corn and other seedlings on demonstration. Identify in each seedling: Primary root Hypocotyl Cotyledon(s) Plumule Seed coat Bean Germination Pea Germination Epigynous Hypogenous Corn Germination Onion Germination - 7 -

8 What structure emerges first as a seed germinates? What structure breaks through the surface of the soil in the germinating bean? What replaces the hypocotyl arch of the bean seed in the corn seedling as protection for the shoot during germination? What happens to the bean cotyledons as the seedling becomes established? Asexual Reproduction There are many forms of asexual reproduction in plants. Strawberries produce stolons (runners) that put out new shoots. Many grasses reproduce by similar underground rhizomes. Stolons and rhizomes are both modified stems. The leaves of some plants, like the African violet, develop roots when detached from the parent plant and the infamous dandelion sprouts new shoots from the root portion that remains in the ground after weeding. Some plants even produce "plantlets" along the edges of the leaves. Bulblets and cormlets, axillary buds of bulbs and corms are also common. Some of these methods of asexual propagation of plants will be discussed elsewhere in the term with modified structures of shoots and roots. The ability of many species of plants to reproduce asexually has been exploited to develop cultivated varieties for food or ornamental use. Such plants are, genetically identical to the parent stock so uniformity is assured. The grower may make root cuttings or may graft a cutting onto an already rooted woody plant. Many fruit trees and roses are propagated in this way. Many economically important plants are sterile and are propagated asexually; these include pineapples, bananas, seedless grapes, navel oranges and many ornamentals

Chapter 38 Angiosperm Reproduction and Biotechnology

Chapter 38 Angiosperm Reproduction and Biotechnology Concept 38.1 Pollination enables gametes to come together within a flower Diploid (2n) sporophytes produce spores by meiosis; these grow into haploid