INTRODUCTION WHAT IS A FRUIT? A fruit is a ripened ovary or group of ovaries containing the seeds produced from the process of sexual reproduction. The seeds are produced from the ovules contained within the ovary tissue. The number of seeds will vary based on the fruit type or the specific species of plant. The fruit classification may include adjacent tissue that fuses with the ovary. This tissue is typically termed a floral tube or hypanthium. The classification of fruit will fall into three very broad classifications and will include simple, aggregate, or multiple fruit. Another general difference that is also used in classifying fruit is whether the mature fruit is fleshy or dry at maturity. A simple fruit forms from the ripening of the ovary of a single pistil. The fruit may contain one to many seeds. Examples of simple fruits include peanuts, sunflower, grains (corn, barley, rice), acorns, walnuts, tomatoes, grapes, eggplants, citrus and apples. Aggregate fruits form from a single flower with several ovaries . Examples of aggregate fruits include blackberries, raspberries and strawberries. Multiple fruit forms from a cluster of several flowers. A fruit is produced by each flower and these fruits merge into a multiple fruit. Pineapple is an example of a multiple fruit. LEARNING OBJECTIVES 1. Understand the development of fruit and how this differs from a seed. 2. Be able to identify the various types of fruit and the variation in structure that differs in each type. 3. Be able to use the terminology associated with the various fruit types. MATERIALS You will be provided with the following materials: • Various types of fruit • Dissecting microscope and/or a hand lens • Knife or razor blade SIMPLE FRUITS Simple fruits are the most widely found type of fruit. They are formed from a single ovary. The terms fleshy and dry refers to the condition of the ovary tissue at maturity. The outer layer (ovary wall) is termed the pericarp. The number of ovules (potential seed) varies based on the fruit type and the plant species.
A drupe is a fleshy fruit that is typically called a ‘stone’ fruit (Figure1). It is classified as a fleshy fruit with a thin outer layer (exocarp) and a thick and fleshy middle layer (mesocarp). The hard inner layer (endocarp or stone) surrounds the seed. In most cases, we typically use the exocarp and mesocarp. Examples of these types of fruits include the peach, plum, nectarine, apricot, cherry, olive, and mango. One unique variation is the almond, where we consume the seed contained within the endocarp. A B
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Figure 1A.Drupe X-section (peach – Prunus persica) shows the outer layer (exocarp), the fleshy inner layer (mesocarp) and the hard inner layer (endocarp) which is also referred to as the stone. The seed is found within this structure.1B. Example of a peach on a tree. 1C. Endocarp of an almond (Prunus dulcis) with the seed exposed. The classification of berry is another example of a fleshy fruit (Figure 2). With a berry, we find that the fruit is formed from a single ovary. The entire pericarp and ovary tissue is succulent or fleshy, although the outer layer (pericarp) is sometimes tough. Variation in ovule numbers produce fruit that are one to many seeded. Examples include grape, tomato, papaya, pomegranate, banana and avocado.
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Figure 2, Various types of berries including (A) tomatoes (Lycopersicon eculentum) , (B) peppers, (Capsicum annuum) (C) banana, (Musa acuminata) and (D) grapes (Vitis spp.). The hesperidium is a berry-like fruit that has a leathery rind and parchment-like partitions between sections (Figure 3). These partitions contain juice sacs. The rind typically has a high level of essential oils. This represents the typical fruit of the citrus family (Rutaceae). Examples of hesperidiums include oranges, lemons, grapefruit and limes.
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Figure 3. Example of several citrus (Citrus spp.)plants and the production of a hesperidium: (A) cross section and a (B) whole orange , An example of a grapefruit and a tangerine (C), an example of a lime (D). A pepo is a modified berry that has a tough or hard rind that can vary in thickness (Figure 4). The rind is a product of the receptacle, and all of the tissue is fleshy. All pepos produce a high number of seeds with the exception of the plant types classified as seedless (these have become popular ). This is the typical fruit of the gourd family (Cucurbitaceae) and include numerous horticultural species including watermelon, cucumber, squash, cantelope and pumpkin.
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Figure 4, Examples of a pepo: (A) muskmelon (Cucumis melo) (B) pumpkin (Cucurbita spp.) and a (C) cucumber C. sativus). The pome is a widely produced type of fruit and is characteristic of the members of the rose family (Rosaceae) which would include apples, pears, quince and loquat (Figure 5). One of the key characteristics of a pome is that the ovary or core is surrounded by an edible, fleshy receptacle tissue (hypanthium or fleshy floral tube) that is really not part of the pericarp. The actual ovary or core is usually not eaten, at least by most humans.
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Figure 5. Examples of fruits classified as a pome: (A) apple (Malus X domestica) with the ovary within the highlighted area. The rest of the tissue is a floral tube or hypanthium. (B) Pear (Pyrus spp.), (C) Examples of apples showing variability in color. Legumes are typically classified as a dry dehiscent fruit formed from a single carpel. They are typically dehiscent and usually open along two suture lines (Figures 6 & 7). This dehiscing characteristic has been significantly reduced through genetic selection. This is to reduce a negative trait called shattering. This fruit typically has more than one seed and is harvested at maturity. However, several legumes are harvested in an immature stage of development, and the entire fruit is consumed. A B A
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Figure 6. Examples of several legumes: (A & B) Examples of green beans (Phaseolus vulgaris) at an immature stage of development. Peanuts (Arachis hypogaea) at a mature (C) and an immature (D) stage of development. Figure 7. Examples of legume (continued): (A) English peas (pisum sativum) with a dehisced and an intact legume. (B) An intact and (C) a dehisced soybean (Glycine max) legume . A fruit that is sometimes referred to as a seed is the achene (Figure 8). The achene is a simple, dry, one-seeded indehiscent fruit (lacking special seams that split to release the seed ) with the seed distinct from the fruit wall. A common achene is that of the sunflower (Helianthus annuus).
Figure 8. Example of an achene is the sunflower with the intact fruit (left) and the seed with a papery testa and the pericarp removed (right) in the above image. The caryopsis (or what is more typically called a grain) is a very small, dry, one-seeded, indehiscent fruit in which the actual seed coat is completely fused to the ovary wall or pericarp (Figure 9). The outer pericarp layer or husk is referred to as the bran, while the inner seed layer is called the germ. The majority of the fruit’s weight is comprised of the endosperm which is primarily for the storage of carbohydrates.
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Figure 9. Examples of a caryopsis: (A) dent corn and (B) sweet corn (Zea mays), (C) rice (Oryza sativa), and wheat (Triticum aestivum). A capsule is a simple, dry, dehiscent fruit formed from more than one carpel (Figure 10). The individual chamber is typically referred to as a locule which contains the seed. Examples of plants producing this type of fruit are cotton (Gossypium hirsutum) and okra (Abelmoschus esculentus). A C
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Figure 10. Examples of several fruit types classified as a capsule. Closed cotton (capsule) boll (A) and an open boll (B). Fruit of an okra plant showing the capsules and inset is a cross section where you can see the carpel (or locule) structure. The nut is another fruit that we find associated with perennial plants (Figure 11). A nut is a larger, one-seeded fruit with a very hard pericarp usually enclosed in a husk or cup-like involucre. Examples include a A walnut (Juglans spp.), oak (Quercus spp.), and pecans (Carya illinoinensis ). A B
Figure 11. Fruit classified as nuts including a walnut (A) and an acorn (B).
A fruit classified as a silique is a slender, dry, dehiscent fruit that superficially resemble a legume (Figure 12). The exception is that the silique is composed of two carpels with a partition or septum down the center (i.e. between the two carpels or valves). The partition is also translucent. This fruit is very typical of the genus Brassica (or the mustard family). A
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Figure 12. Siliques typical of rapeseed (Brassica napus) showing the whole fruit (A) and a dehisced fruit (B). Another fruit type is the samara (Figure 13). A samara is a small, winged, one-seeded fruit, usually produced in clusters on trees. The winged structures are primarily for dissemination of the seed. These are typically associated with woody perennial plants. Examples include the maple (Acer spp.), ash (Fraxinus spp.), and the elm (Ulmus spp.).
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Figure 13. Examples of the fruit type classified as a samara: (A) maple, (B) elm with a closer view of the samara.
A follicle is a single ripened ovary (representing a single modified carpel) that splits open along one seam (dehiscing to release the seed) (Figure 14). A follicle will typically contain numerous seed. A B
Figure 14. A follicle from a common milkweed (Asclepias spp.) with an intact fruit (A) and a follicle splitting along the suture line exposing the seed (B). Schizocarp is another type of fruit that is found in the family Apiaceae (what used to be called the Umbelliferae). It is a small, dry fruit composed of two or more sections that break. Several common plants that produce a Schizocarp are carrots (Daucus carota), dill (Anethum graveolens) and parsley (Polytaenia nuttallii).
Figure 15. Examples of several types of schizocarp with both intact and ‘split’ fruit.
Aggregate Fruits
Aggregate fruits are clusters or aggregations of many ripened ovaries (fruits) produced from a single flower. In blackberries and raspberries (Rubus), the individual fruits are tiny, one-seeded drupes or drupelets. In strawberries (Fragaria), the individual fruits are tiny, one-seeded achenes imbedded in a sweet, fleshy receptacle.
Figure 16. Examples that are classified as being aggregate fruit. (A) Black- berries. (B) Strawberry). MULTIPLE FRUITS
Multiple fruits are defined as a fruit having a cluster of many ripened ovaries (fruits) produced by the coalescence of many flowers crowded together in the same inflorescence and are typically surrounded by a fleshy stem axis. An example of this fruit type is the pineapple (Ananas comosus). A B
Figure 17. A pineapple which is an example of a multiple fruit. (A) whole fruit and (B) an individual ovary. REFERENCES Forbes, J.C., and K.D.Watson. 1992. Plants in Agriculture. University Press, Cambridge.
Parker, R. 2004. Introduction to Plant Science. Thomson, Delmar Learning.
McMahon, M.J., A.M. Kofranek, and V.E. Rubatzky. 2011. Plant Science: Growth, Development, and Utilization of Cultivated Plants, Fifth Edition. Prentice Hall.