OEB 59 – Plants and Human Affairs Plant Anatomy Lab 1: Flowers, Fruits and Seeds
Objectives of this lab: 1) Explore the structure and function of flowering plant reproductive organs from flower development through fruit maturation 2) Examine the relative placement of the four flower whorls (sepals, petals, carpels, and stamens) in different flowers and their influence on fruit development 3) Distinguish between simple, compound, aggregate fruits, and false or accessory fruits 4) Examine in detail fruits of cultivated members of the rose family (Rosaceae); identify the embryo, endosperm, seed coat, endocarp, mesocarp, epicarp in the mature fruit 5) Understand the botanical definition of a fruit (many common vegetables are fruits, botanically speaking)
Part I: Floral morphology
The basic structure of a flower is depicted in the figure. There are four series of floral parts usually arranged in whorls, each with its own set of functions. Evolutionary botanists regard a flower as a highly modified stem axis around which are a series of modified leaves. In some floral parts it is easy to visualize their leaf like nature, but in others it takes some extra study to understand the origin of these organs from leaf-like ancestors. The sepals (collectively the calyx) are the lowermost whorl. They are most commonly green and leaf-like, and function to protect the entire flower in the bud as their other parts are developing. As the flower opens the calyx often folds out of the way. The petals (collectively the corolla) constitute the second whorl of parts of a flower. Their primary function is to attract animal pollinators via a visual signal, such as color, pattern, and shape. Neither the calyx nor corolla is involved directly in production of gametes; however, their roles are critical to the success of the reproductive process in flowering plants.
The stamens (the male part of the flower) constitute the third whorl of floral parts. Each generally consists of a stalk-like filament tipped by an anther. The anthers are the structures in which the pollen grains are produced. The female part of the flower (the pistil) consists of three parts: the ovary, the chamber that contains the ovules/seeds; the style, the tissue through which the pollen tube cell grows to the vicinity of the ovules; and the stigma, the receptive tissue that recognizes legitimate pollen and promotes the germination of the pollen grain.
The terms used above (ovary, style, stigma) refer to functional aspects of the pistil. There is, however, another set of terms that are used to describe the female components of a flower, and these terms refer to the evolutionary interpretation of flower parts as modified leaves.
The ancestral female flower part is regarded as a leaf-like organ with ovules attached at its margins. Now, imagine what would happen to this primordial leaf if the margins were brought together by folding the leaf along its central vein, along with the ovules being tucked into the chamber formed by the folded leaf. The result would be an enclosed chamber that contained ovules. Use the fresh peas provided in lab as a model of this structure. This modified leaf is known as a carpel. In some species, the carpels are separate from each other, and each one consists of an ovary, style, and stigma. In other species, the gynoecium (the collective term for all female parts of a flower) consists of only a single carpel. In both of these cases, it is easy to distinguish individual carpels. In most species of flowering plants, however, the gynoecium consists of two or more carpels that are fused into a compound structure. When this occurs you must determine the number of carpels by examining whatever evidence is available. For example, if the gynoecium is composed of one ovary but two styles (or even style branches), assume that it represents a two-carpelled gynoecium. You can also count stigmas or the number of chambers within the ovary to determine the number of carpels.
Variation on the theme With over 250,000 species of flowering plants (Angiosperms), you wouldn’t expect each one to have identical flowers. Indeed, there are numerous variations on the general theme described above, yet it is still amazing that all flowers can be viewed as a modification of this typical pattern.
(1) Number of parts The number of parts of each organ series can vary considerably among species, but rarely varies within a species. Note that the numbers of parts in different series vary independently from each other (e.g., most mustards have 4 petals and sepals, but 6 stamens).
(2) Fusion of parts Fusion of parts is a common source of floral variation. Like parts may be fused (e.g., sepals fused to sepals, petals fused to petals), and unlike parts can be fused (e.g., stamens can be fused to the corolla). Furthermore, the extent to which parts are fused to each other can also vary considerably. Note that the fusion of parts can alter the shape of the flower.
(3) Loss of series Another “option” for floral variation is the complete loss of one or more series of flora organs. For example, many flowers lack a corolla of any kind.
(4) Sexuality A special variation of a loss of series is the lack of one or the other gamete- producing series. This produces unisexual flowers, and has profound implications on the reproductive biology of species possessing this pattern.
TO DO: Dissect the flowers provided and identify the sepals, petals, stamens and pistils. Divide the stamens into filaments and anthers. Divide the pistils into stigma, style and carpel. Fill out the table below for three flowers. What are likely to be the pollinator rewards on these flowers?
Flower #1 Flower #2 Flower #3 Name Number of petals Number of sepals Number of stamens Number of carpels Carpels fused or free? Free / Fused Free / Fused Free / Fused Symmetry? Bilateral / Radial Bilateral / Radial Bilateral / Radial Ovary position? Superior / Half / Inferior Superior / Half / Inferior Superior / Half / Inferior Petals showy? Sepals showy? Placentation type Number of ovules/ovary Part II: Fruit
Fruit is a mature (i.e. ripened) ovary of a flower along with all of its contents and any adhering accessory structures. Fruits usually contain seeds resulting from the maturation of fertilized ovules, however, seed and fruit maturation can both occur in the absence of fertilization.
The exocarp (or epicarp) is the outermost layer or "skin" of a fruit. The endocarp is the innermost layer that surrounds the seed. The endocarp may be hard as in a peach pit or soft and papery as in an apple, or it may be fleshy and not differentiated from the mesocarp, which is the (often) fleshy tissue between exocarp and endocarp. Exocarp + mesocarp + endocarp = pericarp.
Fleshy Fruits: those fruits that have a mesocarp that is at least partially fleshy at maturity. I. Simple fleshy fruits: develop from a flower with a single pistil (ovary, style & stigma), whose ovary may have 1 to several carpels. a. Drupe: a simple fleshy fruit with a single seed enclosed by a hard, stony endocarp (ex. the pit of a peach). Drupes generally develop from flowers with a single ovule. Examples: peach, olive, coconuts (most sold in markets have had the mesocarp and endocarp removed), cherries, almonds (mesocarp and exocarp are removed before selling in market - we remove the endocarp to get the seed). b. Berry: a simple fleshy fruit that develops from a compound ovary (more than 1 carpel) and commonly contains more than 1 seed. The entire pericarp is fleshy, and it is difficult to distinguish between the endocarp and mesocarp. Examples: grapes, tomatoes, bell peppers, blueberries, gooseberries, bananas, dates, avocados. c. Pome: simple fleshy fruits whose fleshy portion is derived mainly from an enlarged receptacle that grows up around the ovary. The pericarp forms the inner “core,” with the endocarp forming a papery or leathery structure around the seeds. Examples: apples, pears.
II. Aggregate fruits: those fleshy fruits derived from a single flower with more than one pistil. The individual pistils develop into tiny drupes or other fruitlets, but they mature as a clustered unit on a single receptacle. Examples: blackberries, raspberries, and strawberries.
III. Multiple Fruits: those fleshy fruits that are derived from several to many individual flowers on a single inflorescence. As each flower matures individually into a fruitlet, they coalesce into a single fruit. Examples: pineapples and figs.
Dry Fruits: those fruits whose mesocarp is definitely dry at maturity. I. Dehiscent Fruits: dry fruits that split open at maturity. a. Legume: splits along two sides or seams. Examples: members of the Legume family (peas, beans, kudzu). b. Follicle: splits along one side or seam. Examples: milkweed, columbine, peony, larkspur. c. Siliques: also split along two sides or seams, but the seeds are borne on a central portion that is exposed when the two halves separate. Examples: broccoli, cabbage, radish (all are members of the family Brassicaceae). d. Capsule: consist of al least two carpels and split in a variety of ways. Examples: poppies, lilies, orchids. II. Indehiscent Fruits: dry fruits that do not split open at maturity. a. Achene: dry fruit in which the seed is attached to the pericarp at the base. Examples: strawberries, sunflower “seeds” (actually sunflower fruits). b. Nuts: dry, one-seeded fruits with a very thick, hard pericarp. Examples: acorns, hazelnuts, chestnuts. Note: many of the fruits we traditionally think of as nuts are actually fleshy fruits: coconuts, almonds, walnuts, and pecans (all drupes). c. Samara: dry fruits in which the pericarp forms a wing or membrane, which aids in wind dispersal. Examples: maples, ashes, elms. d. Caryopsis (grain): dry fruits in which the pericarp is tightly fused with the testa of the seed and cannot be separated. Examples: rice, corn, wheat, barley (all are members of the family Poaceae).
TO DO: Examine the examples of cultivated fruit of the rose family (Rosaceae). Dissect the fruits provided and identify the exocarp, mesocarp and endocarp. Determine what fruit type each represents, and fill in the rest of the chart below.
Fruit #1 Fruit #2 Fruit #3
# of carpels
# of seeds per carpel
What is the fleshy tissue?
What are the attractive structures for dispersers?
Part III: Fruits you know as vegetables
Botanists define fruit precisely (vegetable does not have a botanical meaning, though vegetative indicates any part of a plant not directly associated with reproduction). We will look at several edible plants found in the grocery store that are typically referred to as vegetables, but are botanically defined as fruits.
There are three families that have fruits commonly thought of as vegetables: Cucurbitaceae: the cucumber family. We basically only use the flowers (e.g. squash blossom soup) and fruits of this family. Some of these fruits (e.g. melons, cantaloupe) are recognized as such while others (e.g. squash, chayote, zucchini, cucumber) are not. The luffa "sponge" is derived from the fibrous tissue in the fruit of a squash.
Fabaceae: the bean family. We eat only the seeds and fruits of this family. Commonly eaten members of the family include beans, lentils, chickpeas, and peanuts. Tamarind is made from the fruit wall of a legume pod.
Solanaceae: the potato family. We eat the fruits of this family (tomatoes, eggplants, peppers) but also the stem tubers (potatoes). We also smoke the leaves (tobacco). In 1893, the US Supreme Court ruled that the tomato was a vegetable, not a fruit, based upon the commonly used definition by the public. (Google Nix v. Hedden for details.)
TO DO: Examine the examples of these fruits on display in the lab: tomatoes, peppers, squash, cucumber, eggplant, avocados, olives, and beans.