Flowering Plant Reproduction

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Norah Hodge
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1 Lab Exercise Flowering Plant Reproduction Objectives - To be able to identify the parts of a flower - Be able to distinguish between dicots and monocots based on flower morphology - Become familiar with the structure of the ovule, seed and plant embryo - Understand how flowers can be arranged in an inflorescence - To be able to identify some of the principle types of fruits - Become familiar with the seed structure - Become familiar with the different patterns of germination Introduction One of the most important functions for biological organisms is reproduction. As a consequence, some of the structures that are most distinguishable for a group of organisms are those structures associated with the reproductive process. The flowering plants are no exception. In these lab activities, use the material in the Reproduction simulation within the Plant section of the BiologyOne DVD. 1

2 Activity.1 Floral Structures Flowers are reproductive organs composed of what are thought to be highly modified leaves. These organs function either directly in the reproductive process, function to promote pollination, or function to protect the flower. Go to the Reproduction simulation within the Plant section of the BiologyOne DVD. Review the structures of the flower shown there. In the center of the flower is found the structure(s) which is responsible for the production of egg cells. This structure is called the pistil and may be composed of one or more subparts called carpels. The number of carpels may be used to distinguish between the monocots and dicots. The pistils of monocots usually have 3 (or multiples of 3) carpels. Dicots usually have 4 or 5 carpels or multiples of 4 or 5 carpels. The pistil can also be divided into functional regions. At the base of the pistil is a swollen portion, the ovary, which contains the egg producing structures, the ovules. Above this is a thin, elongate portion, called the style, which elevates the third region to a position advantageous for pollination. This third region is called the stigma. It is frequently hairy or covered with a sticky fluid to help capture pollen grains. Outside the pistil(s) are the structures specialized to produce the pollen. These are the stamens. Like the carpels, in monocots the stamens are in 3s or multiples of 3. In dicots, the stamens are in 4s or 5s or multiples of 4 or 5. The stamens are divided into two regions. At their apex is swollen tip called the anther which is where the pollen is produced. The filament elevates the anther to a position where the pollen can be effectively dispersed. Surrounding the stamens are the petals. The petals are collectively referred to as the corolla. As with the above flower parts, 3 or multiples of 3 in the corolla indicate a monocot; 4 or 5 or multiples of 4 or 5 indicate a dicot. In some flowers the petals may be highly reduced and not evident. In others, the petals are very bright and showy. Many petals become highly modified to attract specific pollinators to the flower. After pollination, petals are usually shed. Floral Structures petal stigma pistil style stamen { anther filament ovary ovules sepal receptacle 2

3 Activity.2 Anthers & Pollen The outer most structures of the flower are the sepals, collectively referred to as the calyx. Again, the number of sepals is characteristic for monocots and dicots. The calyx covers and protects the flower during floral development. In some plants, the sepals of the calyx will become petallike and help to attract pollinators. One last feature to note: if the ovary is located above the other flower parts, it is said to be a superior ovary. If the lower portion of the stamen, petals, or sepals are fused to the ovary, the ovary is said to be inferior. In the Results Section, label the illustration of a flower. The male functions of the flower occur in structures at the tips of the stamen called anthers. Commonly, each anther consists of four chambers. Inside each chamber, microspore mother cells undergo meiosis to produce haploid pollen grains. In flowering plants, each pollen grain initially contains just two cells. On germination of the pollen grain, of these cells will produce the pollen tube while the second cell will divide once to produce two sperm nuclei. When pollen grains reach the stigma of a flower (pollination) the first event to occur is the germination of that pollen grain. This produces the pollen tube that will grow and digest its way down through the style. Eventually the pollen tube reaches the ovaries where it deposits two sperm nuclei to fertilize the egg and to form the endosperm. In the plant reproduction simulation on the BiologyOne DVD, examine the structure of the anther and pollen grains. Make your own drawing of these structures in the Results Section. Pollen Grain & Tube pollen grain sperm nuclei tube nucleus 3

4 Activity.3 Ovary & Ovule Activity.4 Inflorescence Pollen tubes deliver two sperm nuclei to a structure called the embryo sac. This structure is surrounded by layers of tissue (the integuments) that protect the sac and developing embryo. The sac and these tissue layers are called an ovule. The ovules are located in the swollen basal part of the pistil called and ovary. As these structures mature, the ovules develop into seeds and the ovary develops into the fruit. Inside the embryo sac, meiotic and mitotic cell divisions have produced eight nuclei (the cell membranes between nuclei are only weakly developed). These nuclei are usually arranged so that 3 are located at each end of the sac and two are in the center. The egg nucleus is the center nucleus at the end of the sac nearest where the pollen tube will enter. One sperm will fertilize the egg to produce the embryo. The two central nuclei (the polar nuclei) will fuse with the second sperm nucleus delivered by the pollen tube. This creates tissue that is triploid (3n), having three sets of chromosomes. This tissue is called the endosperm. Thus, in flowering plants, two fertilizations occur, a process called double fertilization. Double fertilization is unique to the flowering plants. The endosperm serves as a food reserve for the developing embryo. To the layman, a flower consists of a collection of petals and other structures at the tip of a stem. Included in this category are tulips, geraniums, and roses. These are simple flowers. However, for many plants closer examination reveals that what may at a casual glance appear to be a single flower is actually an arrangement of many small flowers in a cluster or group on a flower axis or on branches of such an axis. This cluster is termed an inflorescence and many types have been recognized and given distinctive names. The type of inflorescence is useful in the identification of plants, since it is constant for a species. The advantage of this type of floral grouping is that the small flowers are more easily available to insect pollinators. In the simulation, click on the names for some of the different types of inflorescences to see that arrangement of flowers. Observe the ovary and ovules in the plant reproduction simulation on the BiologyOne DVD. When you are able to identify the structures discussed above, make your own drawing of these in the Results Section. Ovule polar nuclei embryo sac egg nucleus integument layers 4

5 Activity.5 Fruits After pollination, the flower will develop fruit. The fruit s primarily function to aid in the dispersal of the seeds. Some are fleshy and eaten by animals that later defecate the seeds. Others produce special adaptations to cling to passing animals, some to be blown by the wind, or others to explosively expel their seeds. The fruit develops from the wall of the ovary. Thus, you can define a fruit as a mature ovary. The nature of the fruit is quite varied, from very fleshy to very hard and woody. In addition, other parts of the flower and even other parts of the plant may be incorporated into the fruit as it matures. In some plants, the fruits from separate flowers will fuse together to form a single, large fruit structure. All fruits may be classified into three major groups on the basis of the number of ovaries and the number of flowers involved in their formation. These are: Simple Fruits - Simple fruits develop from a single matured ovary in a single flower. Aggregate Fruits - Aggregate fruits consist of a number of matured, separate ovaries formed in a single flower and arranged over the surface of a single receptacle. Individual ovaries are called fruitlets. Multiple Fruits - Multiple fruits consist of the matured ovaries of several to many flowers more or less united into a mass. Multiple fruits are almost invariably accessory fruits. What does it mean to say a fruit is accessory? An accessory fruit is one that develops from tissues surrounding the ovary. Accessory fruits generally develop from flowers that have inferior ovaries, and the receptacle or hypanthium becomes a part of the fruit. Accessory fruits can be simple, aggregate or multiple fruits. A strawberry is a good example of an accessory fruit in which most of the flesh of the strawberry is from the flower s receptacle. The specific characteristics of fruits allow us to identify (and of course name) different types. Use the key on the next page to determine the botanical term for the five simple fruits (A through E) shown in the simulation. The following are some features about these fruits you may not be able to determine from the pictures. Fruit A: While this hickory nut is open now, the fruit is considered indehiscent because it does not open until well after it has be released from the parent plant. The wall of the fruit has a very woody texture. Fruit B: This fruit is entirely derived from the ovary wall, no other flower part are involved in this fruit s tissues. Fruit C: This fruit does not open to release its seed. Fruit D: Derived only from the ovary, this fruit develops a hard rind. Fruit E: Derived only from the ovary, the inner part of the ovary wall become very hard while the outer portion of the wall is fleshy. In the Results Section, record the path you follow in the key for each fruit (i.e. 1b, 9b, 10a, etc.) and the botanical term for the fruit type. 5

6 Classification o Fruits Fruit formed from several flowers which have fused to form what looks like a single fruit.... Multiple fruit Fruit formed from several separate pistils within a single flower.... Aggregate fruit Fruit formed from a single pistil within a single flower.... Simple fruit Conspicuous part of fruit formed from receptacle of flower, not from the pistil.... Accessory fruit Key to some simple fruits: 1a. Fruit dry at maturity a. Fruit indehiscent (does not break open) a. Fruit with wings... Samara 3b. Fruit without wings a. Ovary wall thick and woody... Nut 4b. Ovary wall thin and papery a. Seeds not attached to ovary wall... Achene 5b. Seeds attached to ovary wall... Caryopsis 2b. Fruit dehiscent (breaks open to disperse seeds) a. Pistil made from one carpel a. Fruit breaks open lengthwise on one side...follicle 7b. Fruit breaks open lengthwise on two sides...legume 6b. Pistil made from more than one carpel a. Fruit dehiscing to release free seeds by splitting where the carpel are fused... Septicidal Capsule 8b. Fruit dehiscing to release free seeds by forming a hole near the top of the fruit... Poricidal Capsule 1b. Fruit fleshy or pulpy at maturity a. Flesh of fruit formed from flower parts other than the ovary wall... Pome 9b. Flesh of fruit formed entirely from the ovary wall a. Inner portion of ovary wall becomes hard and stony... Drupe 10b. Inner portion of ovary wall remains fleshy a. Texture of fruit homogeneous, covered by a skin... Berry 11b. Texture not homogeneous, covered by leathery rind a. Fruit divided into section, contains juice sacs... Hisperidium 12b. Fruit not divided, no juice sacs... Pepo 6

7 Activity.6 Seeds When the two sperm from a pollen grain enter the embryo sac, one will unite with the egg nucleus to form the embryo while the second sperm will unite with the two polar nuclei to produce the triploid (3 sets of chromosomes) endosperm. As the embryo and endosperm mature, the integument layers also mature. These layers eventually become the seed coat. The seed coat varies between species. In some, the seed coat has layers that are thick and woody, in others, the seed coat is thin and papery. At this point, the ovule is now called a seed. A mature ovule is one way to define the seed. Examine some external features of a seed in the simulation. In the dicots, there are 2 cotyledons which quickly replace the endosperm as the site of food storage in the seed. In the monocots there is only a single cotyledon. Here, the cotyledon serves as an organ to absorb nutrients from the long-lasting endosperm. The cotyledon is sometimes called the scutelium in the monocots. Two additional tissue layers occur in the monocots. These are the coleoptile and the coleorhiza which surround the embryonic shoot and root. These structures protect the emerging shoot and root as they push through the soil. After studying the dicot and monocot seeds, label the illustrations of these in the Results Section. The mature embryo has a number of distinguishable features. Along the axis of the embryonic plant you can find the apical meristem at the shoot end and the radicle at the root end. These are the embryonic shoot and root respectively. Just below the apical meristem are 1 or 2 cotyledons. These are the seed leaves. The region of the embryonic stem below the point of cotyledon attachment is called the hypocotyl and the region above the cotyledon attachment point is called the epicotyl. 7

8 Activity.7 Germination Once a seed has been dispersed, hopefully it will germinate to produce a new plant, thus completing the reproductive cycle. Just as variation occurs in the other reproductive functions and structures of the flowering plants, the characteristics of germination also vary. Examine the illustration of germination patterns below. Note the position of the cotyledons as the seedling grows. Answer the questions concerning seedling development in the Results Section. Germination Patterns Bean Pea Corn cotyledon seed coat seed coat 8

9 Lab Exercise Name Results Section Activity.1 Floral Structures

10 Activity.2 Anthers & Pollen object object Activity.3 Ovary & Ovule object object 10

11 Activity.5 Fruits unknown fruit fruit path through key type A B C D E Activity.6 Seeds

12 Activity.7 Germination 1. What part of the seedling emerges from the seed first? 2. What are the differences between the cotyledon of a corn plant and a bean plant? Do they function differently? 12

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