Levetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles6. Plant Life Cycle: Fruits and Seeds© The McGraw−Hill Companies, 200888CHAPTER OUTLINE Fruit Types 89Simple Fleshy Fruits 89 Dry Dehiscent Fruits 89 Dry Indehiscent Fruits 89 Aggregate and Multiple Fruits 89 Seed Structure and Germination 89Dicot Seeds 89 Monocot Seeds 92 Seed Germination and Development 92 Representative Edible Fruits 92Tomatoes 92 A CLOSER LOOK 6.1 TheInfluence of Hormones on Plant Reproductive Cycles 94Apples 96 Oranges and Grapefruits 98 Chestnuts 99 Exotic Fruits 100 Chapter Summary 101 Review Questions 101 Further Reading 102 KEY CONCEPTS 1. Fruits are ripened ovaries that are the end products of sexual reproduction in angiosperms and are a major vehicle for the dispersal of their enclosed seeds. 2. Protected by a tough outer coat, seeds are ripened ovules that contain an embryonic plant plus some nutritive tissue and are the starting point for the next generation. 3. Edible fruits of various types play a major role in the human diet. 6Plant Life Cycle: Fruits and Seeds Pineapple, a multiple fruit, forms from the fusion of the ripened ovaries of each flower in the spike. CHAPTERLevetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles6. Plant Life Cycle: Fruits and Seeds© The McGraw−Hill Companies, 2008CHAPTER 6Plant Life Cycle: Fruits and Seeds 89 Fruits, as are flowers, are unique aspects of sexual reproduction in angiosperms; they protect the enclosed seeds and aid in their dispersal. Not only are fruits essential in the angiosperm life cycle; they also are widely utilized as significant food sources. FRUIT TYPES The fruit wall that develops from the ovary wall is known as the pericarp and is composed of three layers: the outer exocarp, the middle mesocarp, and the inner endocarp. Thethickness and distinctiveness of these three layers vary among different fruit types. Simple Fleshy Fruits Simple fruits are derived from the ovary of a single carpel or several fused carpels and are described as fleshy o r dry.When ripe, the pericarp of fleshy fruits is often soft and juicy. Seed dispersal in the fleshy fruits is accomplished when ani-mals eat the fruits. The following describes the most common types of fleshy fruits ( fig. 6.1 ): A berry has a thin exocarp a soft fleshy mesocarp, and an endocarp enclosing one to many seeds. Tomatoes, grapes, and blueberries are familiar berries. A hesperidium is a berry with a tough leathery rind such as oranges, lemons, and other citrus fruits. A pepo is a specialized berry with a tough outer rind (consisting of both receptacle tissue and exocarp); the mesocarp and endocarp are fleshy. All members of the squash family, including pumpkins, melons, and cucumbers, form pepos. A drupe has a thin exocarp, a fleshy mesocarp, and a hard stony endocarp that encases the seed; cherries, peaches, and plums are examples. Apples and pears are pomes; most of the fleshy part of pomes develops from the enlarged base of the perianth that has fused to the ovary wall. As described for the pepo and pome, some fruits develop from flower parts other than the ovary; fruits of these types are termed accessory fruits.Dry Dehiscent Fruits The pericarp of dry fruits may be tough and woody or thin and papery; dry fruits fall into two categories, dehiscent andindehiscent. Dehiscent fruits split open at maturity and so release their seeds. Dehiscent fruits usually contain more than one seed and often many seeds. When the fruit wall opens, the seeds can be dispersed individually rather than en masse. Wind often aids the dispersal of seeds from dehiscent fruits. Three common types of dehiscent fruit— follicles, legumes,and capsules—are characterized by the way in which they open. Follicles, as found in magnolia and milkweed, split open along one seam while legumes such as bean pods and pea pods split along two seams ( fig. 6.1 ). The most common dehiscent fruit is a capsule that may open along many pores or slits; cotton and poppy are representative capsules. Dry Indehiscent Fruits Indehiscent fruits do not split open. Instead, they use other means of dispersing the seeds. Achenes, samaras, grains, and nuts are examples of indehiscent fruits. Sunflower “seeds” are, in fact, achenes, one-seeded fruits in which the pericarp is free from the seed ( fig. 6.1 ). Carried by the wind, the winged fruits of maple, elm, and ash trees are familiar types of samaras. Samaras are usually described as modified achenes. The fruits of all our cereal grasses are grains, single-seeded fruits in which the peri-carp is fused to the seed coat. Also called a caryopsis, this type of fruit is found in wheat, rice, corn, and barley. Botanically, nuts are one-seeded fruits with hard stony pericarps such as hazelnuts, chestnuts, and acorns. In common usage, however, the term nut has also been applied to seeds of other plants; pea-nuts, cashews, and almonds are actually seeds, not nuts. Aggregate and Multiple Fruits Aggregate fruits develop from a single flower with many separate carpels , all of which ripen at the same time as in raspberries and blackberries. Strawberries, another aggregate fruit, also contain accessory tissue. The brownish yellow spots on the surface are actually achenes inserted on the enlarged, fleshy, red receptacle ( fig. 6.1 ). Multiple fruits result from the fusion of ovaries from many separate flowers on an inflorescence. Figs and pine-apples are examples of multiple fruits ( fig. 6.1 ). S EED S TRUCTURE AND G ERMINATIONA seed contains the next generation and so completes the life cycle of a flowering plant. The seed develops from the ferti-lized ovule and includes an embryonic plant and some form of nutritive tissue within a seed coat. Differences between dicots and monocots are apparent within seeds. The very names, dicot and monocot, refer to the number of seed leaves, or cotyledons, present in the seed. Dicot seeds usually have two cotyledons that are attached to and enclose the embry-onic plant. The cotyledons, which are often large and fleshy, occupy the greatest part of the dicot seed and have absorbed the nutrients from the endosperm. Thus, the endosperm in many dicot seeds either is lacking entirely or is very much reduced. Monocots have a single thin cotyledon that