Biology 213 Angiosperms. Introduction

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1 Biology 213 Angiosperms Introduction The flowering plants, the angiosperms, are the most recent plants to evolve and quickly became the dominant plant life on this planet. They are also the most diverse group of all the plants and include over 250,000 different species. The flowering plants used to be divided into two groups the monocots and dicots. While the classification system of dividing the flowering plants into only two groups no longer reflects our understanding of phylogenic relationships, these terms (or monocot and eudicot) are still used since the great majority of angiosperms do fit into these two groups The trend that we have already seen, the reduction of the gametophyte stage, continues. In the flowering plants the male gametophyte is a pollen grain and consists of several cells or even one cell with several nuclei while the female gametophyte typically consists of 7 cells with 8 nuclei. What one sees when one looks at a flowering plant is the sporophyte. Flowers are the sexual structures of the angiosperms. Objectives: 1. Know the distinguishing characteristics of the angiosperms and what distinguishes them from other plants that we have observed 2. Be able to identify the parts of a flower and its role in the life cycle 3. Be able to drawn/label and explain the life cycle of an angiosperm using correct terminology 4. Recognize different fruits and understand how the seed and fruit were formed Parts and Functions of Flowers. The flower can be dived into 4 whorls although a plant does not necessarily have all 4 whorls. The parts of a flower will be described starting at the lowest or outermost whorl. A flower that does not have all four whorls is called incomplete. If it is missing either the stamens (male part) or pistil (female part) it is called imperfect. Calyx- consists of the sepals. These are usually green though they can be the same color as the petals or completely lacking. Their function is to protect the inner parts of the flower or if colored to attract pollinators. Corolla- consists of the petals. These are often brightly colored. Their job is also to protect the inner parts of the flower when it is young and also they serve to attract pollinators. Stamens- the male reproductive structures. If the flower has male parts it will usually have 4 or more stamens. A stamen usually consists of the filament, a narrow stalk and at the top of the 1

2 filaments are the anthers. Inside the anthers, microsporocytes undergo meiosis and mitosis and produce the male gametophyte, the pollen grain. Pistils or Carpels- the female reproductive structures. A flower can have one or many pistils (or of course none if the flower is imperfect). A typical pistil consists of: Stigma- this produces a sticky substance that catches pollen grains Style-a region through which the pollen tube grows toward the ovules Ovary- the region which contains the ovules (megasporangia). An ovary can be made of one carpel or many carpels fused together. Inside the ovule a megasporocyte undergoes meiosis to make four megaspores. Usually three of the megaspores die. One megaspore will undergo mitosis to form the female gametophyte. The female gametophyte consists of approximately 8 nuclei with one of the nuclei functioning as the egg. Other parts found in some Flowers: Flowers often have specialized structures such as nectaries that produce food to attract pollinators. Individual plants can produce one flower, many flowers that are located individually or it can have an inflorescence, a cluster of flowers.. As you probably already know, flowers come in a huge variety of forms. Some such as those on grasses are not even easily recognized as flowers. Other flowers, such as a dandelion, are actually an inflorescence (cluster of flowers) and not one individual flower. Please label the stylized diagram of a flower with the following terms petals, sepals, stamens, anthers, filaments, stigma, style, ovary. pistil, calyx and corolla Examination of Various Flowers: Please examine the flowers provided. For four of these flowers please count the number of floral parts and fill in the table. When counting floral parts if you count more than 10, it is customary to say many (which is good since some plants have LOTS of stamens, pistils or ovules). Also cut open the ovary (cross-section) to see if the ovary (or pistil) has developed from one or several fused carpels. To see how many ovules (future seeds) are in each carpel, you will probably need to make a longitudinal cut. You will need to use the dissecting scope to see the carpels and ovules. Please write in the name of the flower and the number of its parts in the table provided. Determining the number and arrangement of the floral parts is the first piece of information that helps you identify unknown species of flowering plants. The angiosperm life cycle and double fertilization Please look at the diagram of the angiosperm life cycle and follow along as the life cycle is described. Inside the anther a sporocyte undergoes meiosis and mitosis and forms the male gametophyte, i.e., the pollen grain. In the ovary of the pistil, a megasporocyte undergoes meiosis. And three of the four haploid cells die. The remaining cell undergoes mitosis and cytokinesis and forms the female gametophyte. Pollen will be either wind blown or carried by a pollinator 2

3 and land on the stigma of the pistil. When pollen lands on the stigma, if it is compatible, it germinates and a pollen tube grows down to the female gametophyte One of the unique features of the angiosperm life cycle is the occurrence of double fertilization. The pollen grain has two nuclei that function as sperm. One of the nuclei fertilizes the egg of the female gametophyte and forms the zygote while the second nuclei fuses with the two polar nuclei to form triploid (3n) nuclei. This triploid nucleus undergoes many mitotic divisions and forms a triploid nutritive tissue called the endosperm. The other cells of the female gametophyte disintegrate as the zygote undergoes mitosis and begins to develop into the embryo. The endosperm will serve as food for the developing embryo. Surrounding the gametophyte, the nearest maternal tissue, the integument will form into the seed coat. The pistil or ovary (and sometimes other parts of the plant) will develop into the fruit that surrounds the seed in a flowering plant. In many plants after a certain amount of development the seed loses water and becomes dormant. Under correct environmental conditions it will finish development and germinate and grow into the sporophyte. Please label the life cycle of the angiosperm using the following terms. stamens, megasporocytes, pistil, ovary, female gametophyte, egg, polar nuclei, pollen grain, developing embryo, seed, germinating seed, fruit. Also label where meiosis and double fertilization takes place. You do not need to label everywhere where mitosis occurs but you should understand this. Fruits Fruits are also distinguishing characteristics of the angiosperms. A fruit always develops from the ovary but may also contain other floral parts or even other parts of a plant. Many fruits are fleshy such as peaches, apples or cucumbers. Other fruits are such as the winged fruit of the maple family or sunflower seeds are dry. Fruits along with the seed coat help protect the developing embryo and in many cases fruits help with the dispersal of the plant. Light fruits with parachutes or wings can be dispersed by the wind. Animals disperse others either by the fruit being eaten or by adhering to the coat of plants. Other fruits provide water protection and the ability for the seed to float. A simple fruit develops from a single pistil that contains one carpel (cherry) or several carpels fused together (cucumber). Aggregate fruits develop from a flower that had many separate pistils, each made of one carpel (blackberry). Accessory fruit develop from more than just the pistils of the flowers. One example would be a strawberry. The red part of the strawberry is actually the end of the flower stem, the receptacle, and each seed is actually the fruit. Complex fruits, such as pineapples come from a cluster of flowers or an inflorescence. Examination of Various Fruits: Please examine the fruits and try to determine what part of the flower/plant developed into the fruit. You should examine all the fruits but need to only fill in the table with information for five of the fruits. Please use the terms simple, aggregate, accessory and complex. Also state how many pistils each fruit had (again if more than 10 state many) and how many carpels were in each ovary/pistil of the flower that produced that fruit. 3

4 Biology 213 Name Please fill in the chart and answer the following questions. Also attach any drawings or diagrams that you labeled to this piece of paper. Flower Name Calyx # of sepals Corolla # of petals # of stamens # of pistils # of carpels per ovary # of ovules per carpel color color Name of Fruit Type of Fruit # of pistils per fruit # of carpels per pistil/ovary 4

5 Please answer the following questions: 1) Where in the flower are the megasporocytes? 2) How many sperm are found in the male gametophyte? 3) What is endosperm and how is it formed? 4) What is the name for a group of flowers sharing a common stalk? 5) Name two fruits that developed from flowers that appeared to have a pistil with more than one carpel. 6) Name one dry fruit. 7) What structure becomes a seed coat? What is the ploidy number of this structure? 8) What structure(s) becomes a fruit? What is the ploidy number of these structures? 5