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BIODIVERSITY LABORATORY

REPRODUCTIVE DIVERSITY OF PLANTS: FLOWERS, FRUITS, SEEDS Objectives Observe and understand the development and diverse morphology of flowers and fruits. A. Flowers Flowers are the sexual organs of plants. Pollen produced in flowers contain sperm cells, while egg cells form in the ovary portion of flowers. Though many plants are self-fertile (i.e., a sperm cell and an egg cell from the same plant can successfully unite), cross-pollination (union of gametes from different plants) is usually more advantageous for maintaining genetic diversity. Aquatic plants, and terrestrial plants living in moist environments, like mosses and ferns, can utilize water as a medium that allows male gametes to swim to female gametes. Most terrestrial plants, however, face special challenges in bringing sperm and egg together. The solution is to employ some agent to facilitate the transfer of gametes. These ‘agents of pollination’ accomplish the transfer of pollen from one plant to another. The earliest evolving terrestrial plants, e.g., conifers and grasses, utilized wind for pollination. While wind is generally available it is not particularly reliable in discriminating where the pollen is delivered. Thus a wind-pollinated plant must make a large investment in producing huge amounts of pollen in order to guarantee that some of its pollen finds receptive eggs. To circumvent this problem, plants evolved colorful, conspicuous modified leaves(flowers) offering rewards to animals that could function as more discriminating agents for pollination. These animals include bees, butterflies, other insects, bats, and birds.

When animals serve as pollinators, a plant needs to: 1) attract the pollinator to the flower; 2) accommodate the pollinator physically so that its body or head and mouthparts can fit into the flower; and 3) reward the visiting animal so that it will learn to "expect" similar rewards when it visits another flower of the same species (thus completing pollen transfer). You have already seen some of the more interesting, bizarre, and spectacular examples of plant attractants, morphologies, and rewards (or false rewards!) in the video "Sexual Encounters of the Floral Kind". In this part of today's lab you will dissect two different types of flowers to familiarize yourself with basic floral anatomy. Parts of a flower

Flowers of two different types of plants, monocot and dicot, will be available in lab. You will need to compare and contrast characteristics of the two types. In your comparison, focus upon how the two types differ in terms of multiples of flower parts.

1) To become familiar with floral parts it is helpful to note that a flower consists of concentric whorls of modified leaves (Fig. 1). Obtain a fresh flower from one of the types provided. Starting at the outermost whorl working towards the center, identify the sepals, petals, stamens and carpels (because they are often fused, the carpels may appear as a single structure rather than a whorl of modified leaves). Make a longitudinal section of the flower (cut it in half from top to bottom).

2) Examine a stamen of your flower using the hand lens. The stalk is called the filament and the head is called the anther--it contains the pollen grains.

3) Next examine a pistil from your flower using the hand lens. A pistil is composed of one or more carpels (modified leaves) and is differentiated into three parts. The upper portion, called the stigma, often has a sticky surface to which pollen will adhere. Below the stigma is the neck-like style. At the base of the pistil is a swollen portion, the ovary, which houses the ovules, the immature seeds.

Note that as the ovule develops into a seed, the ovary will mature into a fruit.

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4) Repeat the above examination for the other flower type. 5) How do these examples of monocot and dicot flowers compare to one another? What features

do they share in common? How are they different?

Figure 1. A generalized flower.

B. Fruit Development

The stimuli of pollination and fertilization initiate changes in the ovary of a flower that result in the formation of a fruit. A fruit may be defined as the ripened ovary (or ovaries), with contents, of a flower, and often including closely associated parts. There are numerous kinds of fruit, ranging from small, dry, seed-like fruits to the larger, fleshy types. The correct botanical classification of a fruit may at first be confusing. For instance, a tomato is really a berry, while a raspberry is really an aggregate simple fruit formed from drupelets. There can be no argument about what is a fruit and what is a vegetable; if it is derived from the ovary of a flower, it is a fruit. The parts of a fruit are the ripened ovary wall, or pericarp (which may be differentiated into outer exocarp, inner endocarp, and intermediate mesocarp); seeds; placental tissues; partitions; receptacle (sometimes called the floral tube); and axis of the stem (Figs 2a and 2b).

Figure 2a. Examples of dry and fleshy fruits.

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The making of a fruit: the tomato

The tomato is type of fruit known as a berry. As such, it is fleshy at maturity and is derived entirely from the ovary of a flower. The tomato has a “superior” ovary that develops above the whorls of sepals and petals.

Carefully slice a tomato in half along its horizontal axis. Many flowers have compound carpels, single structures that were formed by the fusion of several separate carpels. (Some texts use the general term pistil to describe a single carpel or several fused carpels.) Is there any evidence to suggests that the tomato flower has a compound carpel? How many seeds (generally speaking) does the tomato contain? How many ovules did the ovary of the tomato flower contain?

The making of a fruit: the apple The apple, although fleshy like the tomato, is classified as an accessory fruit. Accessory fruits develop from the ovary of the flower, but also include other floral parts as well. In the case of the apple, the fleshy part of the fruit arises from the floral tube (receptacle)--only the “core” arises from the original ovary. The "core line" marks the location of a group of vascular bundles. The leathery endocarp portion of the ovary is closest to the seeds.

In contrast to berries, accessory fruits typically develop from epigenous flowers (those with an “inferior” ovary--one that is situated below the whorls of sepals and petals.) In the apple flower, the ovary is buried in the floral tube (receptacle) tissue. Where on the apple do you find the remnants of sepals, stamens, and stigma?

Figure 2b illustrates changes in the floral parts that accompany the processes of pollination, fertilization, and subsequent development of the apple fruit. Study the diagram carefully. Note in particular that each ovule must be fertilized if that ovule is to develop into a mature seed.

Carefully slice an apple in half along its horizontal axis (equator). Does the apple have a compound carpel? If so, how many carpels fused to form the ovary of the flower from which the apple fruit developed? How many ovules were in each carpel?

Figure 2b. Development of an apple fruit.

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C. Outline of the Classification of Fruits I. SIMPLE FRUIT (Figs. 3, 5, and 6) - arises from the single ovary of one flower.

A. SIMPLE FLESHY FRUIT(Fig. 3) - a portion or all of the PERICARP is fleshy at maturity. 1a. True Berry - always develops from a superior ovulary; entire pericarp fleshy at maturity. Tomato,

grape, avocado, date, eggplant, persimmon. 1b. Hesperidium - pericarp leathery, pulpy, juice sacs develop from inner carpel walls. Citrus 2. Drupe - Pericarp divided into three parts: outer exocarp, middle mesocarp (fleshy), and inner

endocarp (a stone or pit enclosing a seed). Plum, peach, apricot, almond, coconut. 3. Simple Accessory Fruit - fruit includes ovulary wall and other flower parts or closely-related

structures. Most develop from inferior ovularies, include ripened floral tubes. a. False berry - floral tube and pericarp (entire fruit) ripens fleshy. Banana, gooseberry, currant,

cranberry, and pepo-type fruits(pumpkin, cucumber, melons). b. Pome - fleshy receptacle, exocarp, and mesocarp; endocarp leathery. Apple. B. SIMPLE DRY FRUIT (Figs. 5 and 6) - Pericarp is leathery, papery, or woody. 1. Dehiscent (Fig.5) - opens when ripe.

a. Follicle - develops from a simple pistil, opens along one side only. Milkweed, columbine b. Legume - develops from a single pistil, opens along two sides. Bean, pea.

c. Capsule - develops from a compound pistil with two or more carpels. Opens in a variety of ways. Poppy, iris, daylily, jimson weed, horse-chestnut, Brazil nut.

2. Indehiscent (Fig. 6) - fruit does not open at maturity. a. Achene - small, one-seeded; seed coat does not adhere to pericarp. Dandelion, sunflower,

buttercup. b. Grain, or Caryopsis - seed coat is fused with pericarp. Corn, rye, oats, barley, rice. c. Samara, - usually one-seeded, has a wing formed by an extension of the ovary wall. Maple, elm. d. Nut - a rather large, one-to-two-seeded fruit, bracts usually wholly or partially enclose the hard,

stony or woody pericarp. Hazelnut, beech, oak, hickory. II. AGGREGATE FRUIT (Fig. 4) - arises from a number of separate ovaries attached to the single receptacle of

one flower.

A. AGGREGATE-SIMPLE - individual ovularies form druplets, these adhere to one another and separate from the dry receptacle. Raspberry

B. AGGREGATE-ACCESSORY - receptacle becomes fleshy, remains part of the fruit. 1. Drupelets adhere to elongated, fleshy receptacle. Blackberry. 2. Simple pistils ripen into achenes on fleshy, greatly enlarged receptacle. Strawberry. III. MULTIPLE FRUIT (Fig. 4) - develop from a number of individual ovaries of flowers grouped closely together. A fruit is produced from each flower; these fruits remain together in a single mass. Pineapple, mulberry, osage orange, breadfruit, fig.

D. Observations of Fruit Types Stations featuring the various fruit types are set up around the perimeter of the lab. Visit these stations to make observations and to answer the questions posed below. 1. Legume. (Fig.5) Examine the pod of a pea. Along how many sides does this fruit split? Identify the floral structures that are represented. The pod is composed of one carpel. The fact that this carpel may represent a modified leaf is more apparent in the pea than in many fruits, since the venation is not obscured. Remove one of the peas and examine the hilum, or scar of the seed stalk, and close by it the micropyle, a tiny opening through which the pollen tube entered the ovule and which admits water as a preliminary to germination.

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2. Follicle. (Fig.5) A follicle differs from a legume in that it splits along one side only. Examine the follicles of milkweed. In the milkweed each seed has a large tuft of hairs by which it may be carried by air currents. Does the milkweed ‘parachute’ arise from part of the seed or part of the fruit? 3. Capsule. (Fig.5) Examine capsules of poppy and Iris. How do these differ from follicles and legumes? How many carpels are there in the Iris capsule? How many in a poppy capsule? (Dissect the poppy capsule along its ‘equator’ to reveal the answer.) 4. Achene. (Fig.6) Examine achenes of buckwheat and sunflower. Crack the outer coat and remove the seed. Are the seed coats attached to any part of the pericarp? Peel off the seed coats and remove the embryo. Are these plants monocotyledons or dicotyledons? Explain. Does the dandelion ‘parachute’ arise from part of the seed or part of the fruit? 5. Samara. (Fig.6) The broad-winged fruits of elm and maple are known as samaras. Drop one of these fruits from the height of six feet. Explain how the behavior of this fruit as it falls might benefit the plant. Does the samara ‘wing’ arise from part of the seed or part of the fruit? 6. Nuts. (Fig.6) Acorns, filberts, and hazelnuts are examples of true nuts, and are not to be confused with such commercial "nuts" as almonds, walnuts, and hickory nuts, which are really like the stones of drupes, or with pine nuts and Brazil nuts, which are hard-walled seeds. The pericarp is stony in a true nut. Examine an acorn, note the cup-like involucre (a series of bracts) at the base. How does a nut differ from an achene? 7. Caryopsis. (Fig.6) The caryopsis is the typical fruit of the grasses. Examine a grain of corn (one of the largest grasses), and find the embryo or germ. Section the grain longitudinally, perpendicular to the flat sides. Identify the pericarp, the large endosperm (divided into hard and starchy portions), and the embryo. How does this fruit differ from an achene? 8. Berry. (Fig.3) Examine the fruit of grape or tomato. Cut it into sections, and determine the number of carpels and the arrangement of seeds. Such a fruit is a true berry. The hesperidium is a type of berry with a leathery rind. The orange and the other citrus fruits fall into this category. From what is the pulp derived? The pepo, fruit type peculiar to the melon family, is a false berry. Study a cucumber, squash, pumpkin, melon, or gourd. Cut it into sections, and observe the arrangement of the seeds. A fruit of this type is developed from a flower with an inferior ovary, so the fruit wall and rind are composed in part of tissues from the receptacle. The flesh is mostly mesocarp and endocarp, but the placental tissues may also be well developed (especially in watermelon), which forms much of the pulp. 9. Drupe. (Fig.3) Cut longitudinally through a cherry, peach, plum, or olive. Remove the flesh from one side, leaving the stone in place. Identify the exocarp or skin, the fleshy mesocarp, and the bony endocarp. Is the seed easily visible? Why not? What is necessary to obtain the seed? 10. Pome. (Fig.3) Examine an apple. Cut longitudinally and another cut transversely. Identify the sepals, ripened ovary, receptacle (fused floral tube) tissues, and vestiges of stamens. How many carpels are there? Was the ovary inferior or superior? 11. The strawberry is an aggregate fruit (Fig.4) in which the fleshy part consists of the enlarged receptacle. The small, seed-like structures (fruitlets) scattered over the surface of the receptacle are small achenes. Raspberries and blackberries are also aggregate fruits (i.e., not true berries). How do raspberries and blackberries differ from one another? 12. The pineapple is a multiple fruit (Fig.4). How does this differ from an aggregate fruit? To help with your investigation, ask your instructor to demonstrate the proper method for carving a pineapple. 13. Fruit type unknowns. Attempt to identify to fruit type all the fruits at this station. Have your instructor check your identifications.

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Figure 3. Simple fleshy fruits Figure 4. Aggregate and Multiple fruits

Figure 5. Dry dehiscent fruits Figure 6. Dry indehiscent fruits