Botany-202-224 (3) PDF

© John Sohlden/Visuals Unlimited Ragweed. The spikes at the tips of ragweed (Ambrosia) contain numerous clusters of inconspicuous, pollen-producing flowers. 176 CHAPTER 9 Flowers, Fruits, and Seeds Most people are bo...

© John Sohlden/Visuals Unlimited Ragweed. The spikes at the tips of ragweed (Ambrosia) contain numerous clusters of inconspicuous, pollen-producing flowers. 176 CHAPTER 9 Flowers, Fruits, and Seeds Most people are born with some resistance to pollen This chapter examines sexual reproduction in flower- allergies, but many become sensitized by repeated contact. ing plants, including floral adaptations that are important For that reason, a move to a different geographic location in pollination; seed and fruit structure and dispersal; and often temporarily halts the suffering from one pollen al- germination and early growth. lergy, although it frequently gives rise to another! Reproductive Flexibility stalk, or peduncle ( Figure 9-1). The peduncle may ter- minate in a single flower or a cluster of flowers known as of Flowering Plants an inflorescence ( Figure 9-2). In flowers with all four Flowering plants, which include about 300,000 species, organs, the normal order of whorls from the flower’s pe- are the largest, most successful group of plants. One rea- riphery to the center (or from the flower’s base upward) son for the success of flowering plants, or angiosperms, is the following: is that they reproduce both sexually and asexually. You may have admired flowers for their fragrances as well Sepals ¡ petals ¡ stamens ¡ carpels as their appealing colors and varied shapes. The biologi- cal function of flowers, however, is sexual reproduction. The tip of the peduncle enlarges to form a recep- Their varied colors, shapes, and fragrances are adap- tacle that bears some or all of the flower parts. All four tations (evolutionary modifications) that increase the floral parts are important in the reproductive process, likelihood that pollen grains will be carried from plant but only the stamens and carpels participate directly in to plant. sexual reproduction. Sepals and petals are sterile. Sexual reproduction entails the fusion of reproduc- A flower that has all four parts—sepals, petals, sta- tive cells: eggs and sperm cells, collectively called gam- mens, and carpels—is said to be a complete flower; an etes. The union of gametes, which is called fertilization, incomplete flower lacks one or more of these four parts. occurs within the flower’s ovary. A flower that has both stamens and carpels is described The offspring of plants that reproduce sexually as a perfect flower; an imperfect flower has stamens or show considerable genetic variation. They may resemble carpels, but not both. Thus, an imperfect flower is also one of the parent plants, both of the parents, or neither an incomplete flower. However, a perfect flower may be of the parents. Sexual reproduction offers the advantage complete (if it has both sepals and petals) or incomplete of new combinations of genes (the units of heredity) not (if it lacks sepals or petals). found in either parent. These new gene combinations Sepals, which constitute the outermost and lowest may make an individual plant better suited to its envi- whorl on a floral shoot, are leaflike in shape and form ronment. (Chapters 12 to 14 give the details of how this and are often green. Some sepals, such as those in lily variation occurs.) flowers, resemble petals. Sepals cover and protect the Many flowering plants also reproduce asexually. flower parts when the flower is a bud ( Figure 9-3a). As Asexual reproduction does not usually involve the for- the blossom opens from a bud, the sepals fold back to mation of flowers, seeds, and fruits. Instead, offspring reveal the more conspicuous petals. The collective term generally form asexually when a vegetative structure— for all the sepals of a flower is calyx. stems, leaves, or roots—of an existing plant expands, The whorl just inside and above the sepals consists grows, and then becomes separated from the rest of the of petals, which are broad, flat, and thin (like sepals and plant, often by the death of tissues. This part then forms leaves) but tremendously varied in shape and frequently a complete, independent plant. Because asexual repro- brightly colored, which attracts pollinators. As dis- duction involves only one parent and no fusion of gam- cussed later, petals play an important role in ensuring etes occurs, the offspring of asexual reproduction are that sexual reproduction will occur. Sometimes petals virtually genetically identical to each other and to the fuse to form a tube (for example, trumpet honeysuckle parent plant from which they came. flowers, Figure 9-3b) or other floral shape (for exam- ple, snapdragons). The collective term for all the petals of a flower is corolla. Just inside and above the petals are the stamens Flowers ( Figure 9-3c). Each stamen has a thin stalk called a fil- A flower is a reproductive shoot usually consisting of ament, at the top of which is an anther, a saclike struc- four kinds of organs—sepals, petals, stamens, and car- ture in which pollen grains form. For sexual reproduc- pels—arranged in whorls (circles) on the end of a flower tion to occur, pollen grains must be transferred from  Flowers 177 the anther to the carpel, usually of another flower of the In the center or top of most flowers is one or more same species. At first, each pollen grain consists of two carpels. Carpels bear ovules, which are structures that cells surrounded by a tough outer wall. One cell gener- have the potential to develop into seeds. The carpels of a ates two male gametes, known as sperm cells, and the flower may be separate or fused into a single structure. other produces a pollen tube through which the sperm A single carpel or a group of fused carpels is sometimes cells travel to reach the ovule. called a pistil. A pistil may consist of a single carpel (making it a simple pistil) or a group of fused carpels (making it a compound pistil) ( Figure 9-4). In most flowers, each carpel or group of fused carpels has three sections: a stigma, on which the pollen grains land; a style, a necklike structure through which the pollen FERTILIZATION Fusion of male and female gametes. After fertilization, flowering plants produce seeds inside fruits. SEPAL One of the outermost parts of a flower, usually leaf- like in appearance, that protect the flower as a bud. PETAL One of the often conspicuously colored parts of a Darwin Dale/Photo Researchers, Inc. flower attached inside the whorl of sepals. STAMEN The pollen-producing part of a flower. CARPEL The ovule-bearing reproductive unit of a flower. OVULE The structure in the ovary that develops into a seed after fertilization. (a) An Arabidopsis thaliana flower. Female floral parts Male floral parts Pollen grain (each will produce two sperm cells) Stigma Style PISTIL (consisting of one or more carpels) Anther STAMEN Ovary Filament Ovules (each producing Petal one egg cell) Sepal Receptacle Peduncle (b) Cutaway view of an Arabidopsis flower. Each flower has four sepals (two are shown ), four petals (two are shown ), six stamens, and one pistil. Four of the stamens are long, and two are short (two long and two short are shown). Pollen grains develop within sacs in the anthers. In Arabidopsis, the pistil consists of two fused carpels that each contain numerous ovules. FIGURE 9-1 Flower structure. 178 CHAPTER 9 Flowers, Fruits, and Seeds (a) Head (b) Umbel (c) Raceme Common sunflower Water pennywort Fireweed (Helianthus annuus) (Hydrocotyle ranunculoides) (Epilobium angustifolium) (d) Spike (e) Panicle (f) Corymb Elephant-heads False spike-nard Common tansy (Pedicularis groenlandica) (Smilacina racemosa) (Tanacetum vulgare) FIGURE 9-2 Examples of inflorescences. Many arrangements of inflorescences are recognized. tube grows; and an ovary, a juglike structure that con- discussed in Chapter 25, the egg and both polar nuclei tains one or more ovules and can develop into a fruit participate directly in fertilization. (see Figure 9-3c). Each ovule contains a female gameto- An ovary is designated as superior or inferior de- phyte, also known as an embryo sac, in which develop pending on its location relative to other flower parts; one female gamete (an egg) and two polar nuclei. As this character is used a great deal in the classification of  Petals Sepals James Mauseth, University of Texas Stamen Pistil (a) The leaflike sepals of a rose flower cover and protect the inner flower parts. Marion Lobstein (c) A twinleaf (Jeffersonia diphylla) flower has eight stamens. Note the rounded green ovary in © Jeffrey Howe/Visuals Unlimited the center of the flower. (b) The five petals of each trumpet honeysuckle (Lonicera sempervirens) flower are fused to form a tubular corolla. FIGURE 9-3 The four parts of a flower. One carpel Ovules Stigma Stigma Ovules Style Style Ovary wall Ovary wall Ovary Ovary (a) This simple pistil consists of a single carpel. (b) This compound pistil has two united carpels. In most flowers with single pistils, the pistils are compound, consisting of two or more fused carpels. FIGURE 9-4 Simple and compound pistils. 180 CHAPTER 9 Flowers, Fruits, and Seeds flowering plants. A superior ovary is one that has the cell delivery to the ovule. Self-incompatibility, which is other floral organs (sepals, petals, and stamens) free from more common in wild species than in cultivated plants, the ovary and attached at the ovary’s base. An inferior ensures that reproduction occurs only if the pollen ovary is one that is located below the point at which the grains come from a genetically different individual. other floral organs are attached (see Figure 25-16a). PROCESS OF SCIENCE The molecular basis of self-incom- patibility in Arabidopsis and related plants is an area of active botanical interest. Research suggests that self- Pollination incompatibility in these plants is the ancestral (nor- mal) condition. Self-fertile individuals contain muta- Before fertilization occurs, pollen grains must travel tions (changes in the deoxyribonucleic acid, or DNA) in from the anther (where they form) to the stigma. The one or more genes involved in self-incompatibility.
transfer of pollen grains from anther to stigma is known as pollination. Plants are self-pollinated if pollination occurs within the same flower or within a different EVOLUTION Flowering plants and their flower on the same individual plant. Cross-pollination LINK animal pollinators have coevolved occurs when pollen grains are transferred to a flower on Animal pollinators and the plants they pollinate have another individual of the same species. had such a close, interdependent relationship over time Flowering plants accomplish pollination in a va- that they have affected the evolution of certain physical riety of ways. Beetles, bees, flies, butterflies, moths, and behavioral features in one another. The term coevo- wasps, and other insects pollinate many flowers. Other lution describes such reciprocal adaptation, in which animals, such as birds, bats, snails, and small nonflying two species interact so closely that they become increas- mammals (rodents, primates, and marsupials) also pol- ingly adapted to each other as each undergoes evolu- linate plants. Wind is an agent of pollination for certain tionary change by natural selection (see Chapter 16). flowers, whereas water transfers pollen grains in a few Let us examine some of the features of flowers and their aquatic flowers. pollinators that may be the products of coevolution. Flowers pollinated by animals have various features Many plant species have mechanisms to attract them, including showy petals (a visual attrac- to prevent self-pollination tant) and scent (an olfactory attractant). One reward In plant sexual reproduction, the two gametes that unite for the animal pollinator is food. Some flowers produce to form a zygote (fertilized egg) may be from the same nectar, a sugary solution, in special floral glands called plant or from two different parents. The combination of nectaries. Pollinators use nectar as an energy-rich food. gametes from two different parents increases the varia- Pollen grains are also a protein-rich food for many ani- tion in offspring, and this variation may confer a selec- mals. As they move from flower to flower searching tive advantage. Some offspring, for example, may sur- for food, pollinators inadvertently carry pollen grains vive environmental changes better than either of their on their body parts, facilitating sexual reproduction in parents. plants. Plants have evolved a variety of mechanisms that Botanists estimate that insects pollinate about prevent self-pollination and thus prevent inbreeding, 70 percent of all flowering plant species. Bees are par- which is the mating of genetically similar individuals. ticularly important as pollinators of crop plants ( Fig- Inbreeding is generally undesirable because it can in- ure 9-5a). Bee-pollinated crops provide about 30 percent crease the concentration of harmful genes in the off- of human food. Plants pollinated by insects often have spring. To avoid inbreeding, some species have individ- blue or yellow petals. The insect eye does not see color uals with staminate flowers that lack carpels and other the same way the human eye does. Most insects see well individuals with carpellate flowers that lack stamens. in the violet, blue, and yellow ranges of visible light but Other species have flowers with both stamens and car- do not perceive red as a distinct color. Consequently, pels, but the pollen grains are shed from a given flower insect-pollinated flowers are not usually red. Insects before or after the time that the stigma of that flower is also see ultraviolet radiation, wavelengths that are invis- receptive to pollen grains. ible to the human eye. Insects see ultraviolet radiation as Many species have genes for self-incompatibility, a a color called bee’s purple. Many flowers have dramatic genetic condition in which pollen grains are ineffective in ultraviolet markings called nectar guides that may or fertilizing the same flower or other flowers on the same may not be visible to humans but that direct insects to individual plant. In other words, an individual plant can the center of the flower where the pollen grains and nec- identify and reject its own pollen grains. Genes for self- tar are ( Figure 9-6). incompatibility usually inhibit the growth of the pollen Insects have a well-developed sense of smell, and tube in the stigma and style, thereby preventing sperm many insect-pollinated flowers have strong scents that  Pollination 181 Alan & Sandy Carey/Photodisc/Getty Images © Malcolm Schuyl/Alamy (a) A honeybee pollinates an Indian blanket (Gaillardia (b) A broad-billed hummingbird obtains nectar from a desert pulchella) flower. The bee’s body is covered in hairs that flower, ocotillo (Fouquieria splendens), in Arizona. The pollen pick up pollen grains. grains on the bird’s feathers are carried to the next plant. FIGURE 9-5 Animal pollinators. this region of visible light. Because birds do not have a strong sense of smell, bird-pollinated flowers usually lack a scent. Hummingbird-pollinated flowers have a long, tubular corolla with nectar glands at the bottom. The bird hovers beside a flower and inserts its beak and long tongue inside to lap up the nectar. Hummingbird- © Michael & Patricia Fogden/CORBIS pollinated flowers produce more nectar than insect- pollinated flowers, because hummingbirds are larger animals than insects and therefore require more food. Bats, which feed at night and do not see well, are important pollinators, particularly in the tropics, where they are most abundant ( Figure 9-5c). Bat-pollinated flowers are night blooming and often have dull white (c) An Underwood’s long-tongued bat obtains nectar from a petals and a strong scent, usually of fermented fruit. cluster of macuna (Macuna rostrata) flowers. Macuna grows Nectar-feeding bats are attracted to the flowers by in tropical rain forests. The flowers hang down so the bats can easily get to them while in flight. As the bat moves from flower their scent; they lap up the nectar with their long, ex- cluster to flower cluster, it transfers pollen. tensible tongues. As they move from flower to flower, they transfer pollen grains. At least one bat-pollinated may be pleasant or foul to humans. The carrion flower, flower (the tropical vine Mucana holtonii ) has evolved for example, is pollinated by flies and smells like the rot- an unusual adaptation to encourage pollination by bats. ting flesh in which flies lay their eggs ( Figure 9-7). As When the pollen grains in a given flower are mature, a flies move from one reeking flower to another looking concave petal lifts up. The petal bounces the echo from for a place to lay their eggs, they transfer pollen grains. the bat’s echo-locating calls back to the bat, helping it (Should a fly lay her eggs on the carrion flower, the lar- find the flower. vae will starve to death when they hatch.) Birds such as hummingbirds are important polli- POLLINATION In seed plants, the transfer of pollen grains nators ( Figure 9-5b). Bird-pollinated flowers are usu- from the anther to the stigma. ally red, orange, or yellow because birds see well in  182 CHAPTER 9 Flowers, Fruits, and Seeds Thomas Eisner, Cornell University Thomas Eisner, Cornell University (a) An evening primrose (Oenothera parviflora) flower as seen by (b) The same flower viewed under ultraviolet radiation provides the human eye is solid yellow. clues about how the insect eye perceives it. The outer, light- appearing portions of the petals appear purple to a bee’s eyes, whereas the dark blue inner parts appear yellow. FIGURE 9-6 Nectar guides. Many insect-pollinated flowers have nectar guides that are invisible to humans but conspicu- ous to insects. These differences in color draw attention to the center of the flower, where the pollen grains and nectar are located. During the time when plants were coevolving spe- the bee departs and tries to copulate with another or- cialized features such as petals, scent, and nectar to at- chid flower, pollen grains are transferred to that flower. tract pollinators, animal pollinators also coevolved. Interestingly, once a flower has been pollinated, it emits Specialized body parts and behaviors adapted animals a scent like that released by female bees that have al- to obtain nectar and pollen and, coincidently, to assist ready mated. Male bees have no interest in visiting flow- the plant in transferring pollen grains. ers that have been pollinated (just as they lose interest in For example, coevolution may have led to the long, female bees that have already been inseminated).
curved beaks of the ‘i‘iwi, one of the Hawaiian honey- creepers. An ‘i‘iwi inserts its beak into tubular flowers to obtain nectar ( Figure 9-8). The long, tubular corollas of the flowers that ‘i‘iwis visit probably also came about through coevolution. During the 20th century, some of the tubular flower species (such as lobelias) became rare, largely as a result of grazing by nonnative cows and feral goats, and about 25 percent of lobelia species have be- come extinct. The ‘i‘iwi now feeds largely on the flow- ers of the ‘ohi’a tree, which lacks petals, and the ‘i‘iwi bill may be slowly adapting to this change in feeding preference. Comparison of the bills of ‘i‘iwi museum specimens collected in 1902 with the bills of live birds captured in the 1990s showed that the ‘i‘iwi bills were about 3 percent shorter in the 1990s than in 1902. Animal behavior has also coevolved, sometimes in bizarre and complex ways. The flowers of certain Linda R. Berg orchids (Ophrys) resemble female bees in coloring and shape (see Figure 16-8a). The unpollinated flowers also secrete a scent similar to that produced by female bees, and the males are irresistibly attracted to it. The resem- FIGURE 9-7 Carrion flower. blance between Ophrys flowers and female bees is so This desert plant is sometimes called the carrion flower (Sta- strong that male bees mount the flowers and try to cop- pelia) because of its flower color, which resembles dried blood, ulate with them. During this pseudocopulation, a pollen and its putrid scent. Photographed at the Fairchild Tropical sac usually becomes attached to the bee’s back. When Garden in Miami, Florida.  Fertilization and Seed and Fruit Development 183 Dr. Jeremy Burgess/Photo Researchers, Inc. FIGURE 9-9 Wind pollination. Clusters of male oak (Quercus) flowers, which lack petals, FIGURE 9-8 Coevolution. dangle from a tree branch and shed a shower of pollen grains The gracefully curved bill of the ‘i‘iwi, one of the Hawaiian hon- when the wind blows. eycreepers, enables it to sip nectar from flowers of the lobelia (Lobelia). The ‘i‘iwi bill fits perfectly into the long, tubular lobelia flowers. Some flowering plants depend tube penetrates the ovule, the attracting signals cease. on wind to disperse pollen grains As a result, only one pollen tube enters each ovule. The second cell (the generative cell ) within the pol- Some flowering plants, such as grasses, ragweed, ma- len grain divides to form two male gametes (the sperm ples, and oaks, are pollinated by wind. Wind-pollinated cells), which move down the pollen tube and enter the plants produce many small, inconspicuous flowers ovule. After fertilization has occurred, the ovule devel- ( Figure 9-9). They do not produce large, colorful pet- ops into a seed, and the ovary surrounding it develops als, scent, or nectar. Some have large, feathery stigmas, into a fruit. (Chapter 25 considers the fertilization pro- presumably to trap wind-borne pollen grains. Because cess and the flowering plant life cycle in greater detail.) wind pollination is a hit-or-miss affair, the likelihood of a pollen grain landing on a stigma of the same species Embryonic development in seeds of flower is slim. Wind-pollinated plants produce large quantities of pollen grains, which increases the likeli- is orderly and predictable hood that some pollen grains will land on the appropri- Flowering plants package a young plant embryo com- ate stigma. plete with stored nutrients in a compact seed, which de- velops from the ovule after fertilization. The nutrients in seeds not only are used by germinating plant em- Fertilization and Seed bryos but are eaten by animals, including humans (see Plants and People: Seed Banks). Development of the and Fruit Development embryo following fertilization is possible because of Once pollen grains have been transferred from anther to the constant flow of nutrients from the parent plant into stigma, the tube cell, one of the two cells in the pollen the developing seed. grain, grows a thin pollen tube down through the style and into an ovule in the ovary ( Figure 9-10). How does The mature seed contains an embryonic the pollen tube “know” where to grow? Botanists have plant and storage materials found that molecular signals from the ovule guide the A mature seed contains an embryonic plant and food growing pollen tube toward the ovule. Once a pollen stored in either the endosperm or the cotyledons. 184 CHAPTER 9 Flowers, Fruits, and Seeds FIGURE IN FOCUS For fertilization to take place, a pollen grain must germinate on the stigma, and a pollen tube must grow through the style and ovary to the female gametophyte (embryo sac) in the ovule. Anther Pollen grains Stigma 1 Pollination Generative cell occurs. Pollen tube Tube cell 2 Pollen tube Pollen Style grain grows through style to ovule in ovary. Three nuclei Ovary fuse to form Two polar endosperm Ovule (containing Meiosis nuclei female and mitosis gametophyte) Megaspore Integuments mother cell Female Zygote gametophyte Sperm cells Egg 3 Two sperm cells Tube nucleus move down pollen 4 Double tube and enter ovule. fertilization occurs. FIGURE 9-10 Pollination, pollen tube growth, and fertilization. See Figure 25-3 for additional details. Endosperm is the nutritive tissue that surrounds the the epicotyl). After the radicle, hypocotyl, cotyledon(s), embryonic plant in a seed. The seed, in turn, is sur- and plumule have formed, the young plant’s develop- rounded by a tough, protective seed coat, derived from ment is arrested, usually by desiccation (drying out) or the outermost layers (the integuments) of the ovule, and dormancy. enclosed within a fruit. When conditions are right for continuing the devel- The mature embryo within the seed consists of a opmental program, the seed germinates, or sprouts, and short embryonic root, or radicle; an embryonic shoot; the embryo resumes growth. Because the embryonic and one or two seed leaves, or cotyledons. Monocots plant is nonphotosynthetic, it must be nourished dur- have a single cotyledon, and eudicots have two ( Fig- ing germination until it becomes photosynthetic and ure 9-11). The short portion of the embryonic shoot therefore self-sufficient. The cotyledons of many plants connecting the radicle to one or two cotyledons is the function as storage organs and become large, thick, and hypocotyl. The shoot apex above the point of attach- fleshy as they absorb the food reserves (starches, oils, ment of the cotyledon(s) is the plumule (also known as and proteins) initially produced as endosperm. Seeds  Fertilization and Seed and Fruit Development 185 PLANTS AND PEOPLE | Seed Banks To preserve diverse varieties of plants, in seed banks are safe from habitat many countries are collecting plant destruction, climate changes, diseases, germplasm, which is any plant material predators, and general neglect. There used in breeding. Germplasm includes have even been some instances of seeds, plants, and plant tissues of seeds from seed banks being used to traditional crop varieties. The Interna- reintroduce a plant species that has be- tional Plant Genetics Resource Institute come extinct in the wild. For example, in Rome, Italy, is the scientific organi- Bromus, a grass native to Belgium, zation that oversees plant germplasm became extinct in the mid-1900s but collections worldwide. More than 100 was reintroduced from seeds stored in seed collections, called seed banks, a seed bank in 2005. Courtesy of Nordiska Genbanken, Alnarp, Svenge, photo by Stellan Stebe exist around the world and collectively Seed banks have some disadvan- hold more than 3 million samples tages, as well. Seeds do not remain of thousands of kinds of plants (see alive indefinitely and must be germi- figure). The U.S. National Plant Germ- nated periodically so that new seeds plasm System in Fort Collins, Colorado, can be collected. Growing, harvesting, stores seeds of about 250,000 differ- and returning seeds to storage is the ent species and varieties. most expensive aspect of storing plant Most seed banks help preserve the material in seed banks. According to genetic variation within different variet- the UN Food and Agricultural Organi- ies of crops and their wild relatives. zation, many countries establish seed Farmers typically discontinue planting banks but do not provide the funds to local varieties when newer, improved pay for periodic replanting. varieties become available. The newer Many types of plants—avocados varieties have desirable genetic char- and coconuts, for example—cannot be acteristics, such as a greater yield, but stored as seeds. The seeds of these
Seeds from a seed bank. These small vials the discontinued local varieties also plants do not tolerate being dried out, of seeds are from the seed bank in Svalbard, Norway. contain valuable genes. Each local which is necessary before they are variety’s combination of genes gives sealed in moisture-proof containers it distinctive nutritional value, size, for storage at ⫺18⬚C (⫺0.4⬚F). If 20 in an evolutionary sense. Removed color, flavor, resistance to disease, percent or more moisture remains in from their natural habitats, they are no and adaptability to different climates a seed when it is frozen, it will prob- longer subject to the forces of natural and soil types. Maintaining the genetic ably die. Some seeds cannot be stored selection. As a result, they may be less diversity present in local crop varieties successfully because they remain fit for survival when they are reintro- and their wild relatives helps preserve viable for only a short period, such as duced into the wild. genes we may need in the future. The a few months or even just a few days. Despite their shortcomings, seed gene combinations of local varieties Cryopreservation at ⫺160⬚C (⫺256⬚F) banks are increasingly viewed as an im- are potentially valuable to agricultural in liquid nitrogen is a new method portant method of safeguarding seeds breeders because these genes can be being developed for certain kinds of for future generations. For example, the transferred to other varieties, either seeds. Seeds stored at this temperature Millennium Seed Bank Project at the by traditional breeding methods or by survive for longer periods than seeds Royal Botanic Gardens (Kew Gardens) genetic engineering. stored at warmer temperatures. in Great Britain is currently collecting Seed banks offer the advantage of Perhaps the most important disad- and storing seeds from 10 percent of storing a large amount of plant genetic vantage of seed banks is that plants the world’s plant species, including all material in a small space. Seeds stored stored in this manner remain stagnant species native to Great Britain. that store nutrients in cotyledons have little or no en- ENDOSPERM The nutritive tissue that is formed at dosperm at maturity. Examples of such seeds are peas, some point in the development of all flowering plant beans, squash seeds, sunflower seeds, and peanuts. seeds. Other plants—wheat and corn, for example—have thin COTYLEDON The seed leaf of a plant embryo that often cotyledons that function primarily to help the young contains food stored for germination. plant digest and absorb food stored in the endosperm.  186 CHAPTER 9 Flowers, Fruits, and Seeds THE ENVIRONMENT | Relating Seed Size to a Plant’s Habitat PLANTS AND Assuming that a given plant species contain more stored nutrients. For stored in a large seed lets a young invests a fixed amount of its energy in example, plants that grow in widely seedling establish an extensive root sys- reproduction, is it more advantageous scattered open sites (such as old fields) tem quickly, enabling it to survive the to produce many small seeds or a few usually produce smaller seeds, perhaps dry climate. Island plants also produce big ones? After studying the seed sizes because they can disperse more easily larger seeds than similar species on the that predominate in different habitats, over large areas than can larger seeds. nearby mainland. Botanists hypothesize botanists have suggested that in some However, wide dispersal is probably less that large seeds are less likely to be environments a smaller seed size may important for plants adapted to densely widely dispersed and therefore less be advantageous, whereas in others a vegetated areas such as forests. These likely to fall into the ocean. Seeds that larger seed size may be better. plants generally produce bigger seeds remain dormant in the soil for a long Seed size for each plant spe- with an ample food reserve that may time tend to be smaller; the exact rea- cies probably represents a trade-off confer a greater likelihood of success- son for this tendency is not known. between the requirements for dispersal fully establishing seedlings in a shaded Despite many associations of seed and for successful establishment of environment. The stored energy may size with specific environments, no seedlings. Small-seeded plants can pro- allow the young seedling to grow tall general rule concerning the adaptive duce more seeds with a given amount enough to reach adequate sunlight for advantages of large seeds versus small of energy than can large-seeded plants. photosynthesis. seeds has emerged. Botanists continue On the other hand, seedlings growing Other ecological factors are associ- to study the wide divergence in seed from large seeds may be more likely ated with seed size. Larger seeds are sizes that has evolved in present-day to survive environmental stresses they typical of many plants that live in arid plants. encounter because the large seeds habitats, possibly because the food In flowering plants, seed size varies consider- ably, from the microscopic, dustlike seeds of orchids to the giant seeds of the double coconut (Lodoicea mal- divica), which weigh as much as 27 kilograms (almost 60 pounds). Despite this variation among species, seed Foliage leaf size is a remarkably constant trait within a species (see James Mauseth, University of Texas Cotyledon Plumule Plants and the Environment: Relating Seed Size to a Plant’s Habitat). Hypocotyl Radicle Fruits are mature, ripened ovaries After fertilization takes place within the ovule, the ovule develops into a seed, and the ovary surrounding it de- velops into a fruit. For example, a pea pod is a fruit, and FIGURE 9-11 Parts of a bean seed. the peas within it are seeds. A fruit may contain one or This dissected bean (Phaseolus vulgaris) seed shows the radicle, more seeds; some orchid fruits contain several thousand hypocotyl, cotyledon, plumule, and foliage leaf. (The seed coat to a few million seeds! Fruits provide protection for the and one of its two cotyledons were removed). This particular enclosed seeds and sometimes aid in their dispersal. seed has begun germinating, so its radicle is larger than that of There are several types of fruits; their differences a nongerminating seed. result from variations in the structure or arrangement of the flowers from which they were formed. The four basic types of fruits are simple fruits, aggregate fruits, FRUIT In flowering plants, a mature, ripened ovary that often multiple fruits, and accessory fruits ( Figure 9-12). provides protection and dispersal for the enclosed seeds. Most fruits are simple fruits. A simple fruit de- velops from a single carpel or several fused carpels. At SIMPLE FRUIT A fruit that develops from one or several maturity, simple fruits may be fleshy or dry. Two ex- united carpels. amples of simple, fleshy fruits are berries and drupes  Seed Berry (simple fruit) Fused fruit Caryopsis (simple fruit) A simple, fleshy fruit in which the wall and A simple, dry fruit in seed fruit wall is soft throughout. coat which the fruit wall is fused to the seed coat. Tomato (Lycopersicon lycopersicum) Wheat (Triticum) Single seed Single Fruit Achene (simple fruit) seed wall A simple, dry fruit in which the Drupe (simple fruit) fruit wall is separate from the A simple, fleshy fruit in which seed coat. the inner wall of the fruit is a hard stone. Sunflower (Helianthus annuus) Seed Peach (Prunus persica) coat Scale-covered Nut (simple fruit) Single seed inside cup A simple, dry fruit that has a stone Woody stony wall, is usually large, and fruit does not split open at maturity. wall Follicle (simple fruit) Oak (Quercus) A simple, dry fruit that splits open along one suture to release its Seed Single seeds; fruit is formed from ovary seed that consists of a single carpel. Milkweed (Asclepias syriaca) Seed Aggregate fruit A fruit that develops from a single flower with several to many pistils (i.e., carpels are not fused into a single pistil). Legume (simple fruit) Blackberry (Rubus) A simple, dry fruit that splits open along two sutures to release its seeds; fruit is formed Seed Multiple fruit from ovary that consists of a A fruit that develops from the single carpel. ovaries of a group of flowers. Seed Green bean (Phaseolus vulgaris) Mulberry (Morus) Split-open Capsule (simple fruit) suture A simple, dry fruit that splits Enlarged Accessory fruit open along two or more sutures floral tube A fruit composed primarily or pores to release its seeds; Ovary Seed of nonovarian tissue (such as wall fruit is formed from ovary that the receptacle or floral tube). consists of two or more carpels. Seed Apple (Malus sylvestris) Iris (Iris) FIGURE 9-12 Representative fruit types. Fruits are botanically classified into four groups—simple, aggregate, multiple, and acces- sory fruits—based on their structure and mechanism of seed dispersal. 188 CHAPTER 9 Flowers, Fruits, and Seeds ( Figure 9-13). A berry is a fleshy fruit that has soft tis- sues throughout and contains few to many seeds; a to- mato is a berry, as are grapes, blueberries, cranberries, and bananas. Many so-called berries do not fit the bo- tanical definition. Strawberries, raspberries, and mul- berries, for example, are not berries; these three fruits are discussed shortly. A pepo is a modified berry in which the fruit wall is a leathery rind. Pumpkin, squash, cucumber, and watermelon fruits are pepos. Another modified berry Dennis Drenner is a hesperidium (pl., hesperidia), which has a leathery fruit wall with numerous oil glands surrounding the succulent cavities where the seeds occur. Citrus fruits (a) The tomato (Lycopersicon lycopersicum) is a berry (lemons, limes, oranges, and grapefruits) are hesperidia. composed of soft tissues throughout. A drupe is a simple, fleshy or fibrous fruit that con- tains a hard stone surrounding a single seed. Examples of drupes include peaches, cherries, avocados, olives, and almonds. The almond shell is actually the stone, which remains when the rest of the fruit has been removed. Many simple fruits are dry at maturity. Some of these are dehiscent and split open, usually along seams called sutures, to release their seeds ( Figure 9-14). A milkweed pod is an example of a follicle, a simple, dry fruit that splits open along one suture to release its seeds. A legume is a simple, dry fruit that splits open along Marion Lobstein two sutures (the top and bottom). Pea pods are legumes, as are lima bean pods, although both are generally har- vested before the fruit has dried out and split open. Pea seeds are usually removed from the fruit and consumed, (b) The blueberry (Vaccinium) is another representative berry. whereas in green beans the entire fruit and seeds are eaten. A capsule is a simple, dry fruit that splits open along multiple sutures or pores. Iris, poppy, buckeye, and cotton fruits are capsules. Other simple, dry fruits—caryopses (sing., caryop- sis), or grains, for example—are indehiscent and do not split open at maturity ( Figure 9-15a). Each caryopsis C Squared Studios/Photodisc/Getty Images contains a single seed. Because the seed coat is fused to the fruit wall, a caryopsis looks like a seed rather than a fruit. Kernels of corn and wheat are fruits of this type. Nuts are simple, dry fruits that have a stony wall and do not split open at maturity ( Figure 9-15b). Nuts are usually large and one-seeded. Examples of nuts in- clude chestnuts, acorns, and hazelnuts. Many so-called nuts do not fit the botanical definition. Peanuts, al- (c) The avocado (Persea americana ), a drupe, has a hard stone monds, and Brazil nuts, for example, are seeds, not nuts. that surrounds the single seed. An achene is similar to a caryopsis in that it is sim- ple and dry, does not split open at maturity, and con- FIGURE 9-13 Simple, fleshy fruits. tains a single seed ( Figure 9-15c). However, the seed coat of an achene is not fused to the fruit wall. Instead, the single seed is attached to the fruit wall at one point Aggregate fruits are a second main type of fruit. An only, permitting an achene to be separated from its seed. aggregate fruit is formed from a single flower that con- The sunflower fruit is an example of an achene. One tains several separate (free) carpels ( Figure 9-16). Af- peels off the fruit wall (the shell) to obtain the sunflower ter fertilization, each ovary from each individual carpel seed within. enlarges. As they enlarge, the ovaries may fuse to form  Fertilization and Seed and Fruit Development 189 © Evan Sklar/Jupiterimages Carlyn Iverson (a) The milkweed (Asclepias syriaca) follicle, (b) A lima bean (Phaseolus limensis) fruit is a legume which is dry at maturity, splits open along one that splits open along two sutures at maturity. suture to release its seeds. a single fruit. Raspberries, blackberries, and magnolia fruits are examples of aggregate fruits. A third type of fruit is the multiple fruit, which forms from the carpels of many flowers that grow close to one another on a common floral stalk. The carpel from each flower fuses with nearby carpels as it devel- © WoodyStock (Alfred Schauhuber)/Alamy ops and enlarges after fertilization. Pineapples, figs, and mulberries are multiple fruits ( Figure 9-17). Accessory fruits are the fourth type. They differ from other fruits in that other plant tissues in addition to ovary tissue make up the fruit. For example, the ed- ible portion of a strawberry is the red, fleshy receptacle. (Each tiny “seed” on a strawberry is actually a fruit—an achene—that contains a single seed.) Apples and pears are accessory fruits called pomes; the outer part of each (c) These red buckeye (Aesculus pavia) capsules split along pome is an enlarged floral tube, consisting of receptacle several sutures. tissue, along with portions of the calyx, that surrounds the ovary ( Figure 9-18). FIGURE 9-14 Simple, dry fruits that split open to release their seeds. Seed dispersal is highly varied Wind, animals, water, and explosive dehiscence disperse AGGREGATE FRUIT A fruit that develops from a single flower the various seeds and fruits of flowering plants. Effec- with several separate carpels that fuse, or grow together. tive methods of seed dispersal have made it possible for MULTIPLE FRUIT A fruit that develops from the carpels of certain plants to expand their geographic range. In some closely associated flowers that fuse, or grow together. cases, the seed is the actual agent of dispersal, whereas ACCESSORY FRUIT A fruit whose fleshy part is composed in others the fruit performs this role. In tumbleweeds, primarily of tissue other than the ovary. such as Russian thistle, the entire plant is the agent of 190 CHAPTER 9 Flowers, Fruits, and Seeds dispersal because it detaches and Caryopsis — corn Nut — acorn blows across the ground, scat- Plumule tering seeds as it bumps along. Endosperm Radicle Tumbleweeds are lightweight Cotyledon Cotyledons and are sometimes blown many kilometers by the wind. Plumule Seed coat Wind disperses the seeds Fruit wall fused Fruit wall of many plants ( Figure 9-19a). to seed coat Cup of fused Plants such as maple trees have bracts winged fruits adapted for wind Radicle dispersal. Light, feathery plumes enable other seeds or fruits to be transported by wind, often for (a) The corn (Zea mays) fruit is a caryopsis, (b) An oak (Quercus) acorn is a nut. A nut considerable distances (see Fig- or grain. In grains, the fruit wall is fused to has a hard fruit wall that surrounds a single ure 9-14a). Both dandelion fruits the seed coat. seed. and milkweed seeds have this type of adaptation. Achene — sunflower Some plants have special structures that aid in the dispersal of their seeds and fruits by animals ( Fig- Fruit wall ure 9-19b and c). The spines and barbs of burdock burs and similar fruits often get caught in animal fur and Seed coat are dispersed as the animal moves about. Fleshy, edible fruits are also adapted for animal dispersal. As an animal Cotyledon eats these fruits, it either discards or swallows the seeds. Many seeds that are swallowed have thick seed coats Radicle and are not digested; instead, they pass through the di- gestive tract and are deposited with the animal’s feces Attachment of seed to fruit wall some distance from the parent plant. In fact, some seeds will not germinate unless they have passed through an (c) A sunflower (Helianthus annuus) fruit is an achene. animal’s digestive tract. The animal’s digestive juices Its seed coat is attached to the fruit wall at one spot only, probably aid germination by partially digesting the and it is possible to peel off the fruit wall, to separate it seed coat. Some edible fruits apparently contain chemi- from the seed. cals that function as laxatives to speed seeds through an FIGURE 9-15 Simple, dry fruits that do not split open. Stigmas and styles Stamens Remnants of stigmas and styles Ovaries (in a) become tiny drupes (in b) Receptacle Remnants of stamens Dennis Drenner Petal Sepal Ovaries of separate carpels (a) (b) (c) FIGURE 9-16 Aggregate fruit. (a) Cutaway view of a blackberry (Rubus) flower, showing the many separate carpels in the center of the flower. (b) A developing blackberry fruit is an aggregate of tiny drupes. The little “hairs” on the blackberry are remnants of stigmas and styles. (c) Developing fruits at various stages of maturity.  Single female S. Alden/PhotoLink/Getty Images flower Inflorescence (a cluster of flowers on a common floral stalk) (a) FIGURE 9-17 Multiple fruits. Both (a) pineapple (Ananas comosus) and (b) mulberry (Morus) are formed from the ovaries of many separate flowers that fused to become a multiple fruit. Mulberry flowers are imperfect and Multiple contain either stamens or carpels. The inflorescence of car- fruit pellate flowers from which the mulberry fruit develops is also shown. (b) Petal Stigma Sepals Stamens Style Ovary Floral tube Sepal Ovule (in a) becomes seed (in b) (a) (b) FIGURE 9-18 An accessory fruit. (a) Note the floral tube surrounding the ovary in the pear (Pyrus) flower. This tube becomes the major edible portion of the pear. (b) Longitudinal section through a pear, showing the fruit tissue derived from both the floral tube and the ovary.  192 CHAPTER 9 Flowers, Fruits, and Seeds Fritz Polking/Peter Arnold, Inc. © Bob Gibbons/Jupiterimages Seed Wing (a) The fruits of maple (Acer ) have wings for (b) Great burdock (Arctium lappa) flowers develop wind dispersal. into hooked fruits that, when mature, may be carried away from the parent plant by becoming matted in animal fur or clothing. FIGURE 9-19 Methods of seed and fruit dispersal. animal’s digestive tract. The less time these seeds spend in the gut, the more likely they are to germinate. Animals such as squirrels and many bird species also help disperse acorns and other fruits and seeds by burying them for winter use. Many buried seeds are never used by the animal and germinate the following spring. (Also see Plants and the Environment: Seed Dis- persal by Ants.) The coconut is an example of a fruit adapted for © Gary Meszaros/Visuals Unlimited dispersal by water. The coconut has air spaces that make it buoyant and capable of being carried by ocean cur- rents. When it washes ashore, the seed may germinate and grow into a coconut palm tree. Some seeds are not dispersed by wind, animals, or water. Such seeds are found in fruits that use explosive dehiscence, in which the fruit bursts open suddenly, (c) Fleshy fruits such as blackberries (Rubus) are eaten by animals such as this white-footed and quite often violently, to forcibly discharge its seeds. mouse. The seeds are frequently swallowed Pressures due to differences in turgor or to drying out whole and pass unharmed through the animal’s cause these fruits to burst open suddenly. The fruits of digestive tract. plants such as touch-me-not (Impatiens noli-tangere)  Seed Germination and Early Growth 193 THE ENVIRONMENT | Seed Dispersal by Ants PLANTS AND For a plant species to survive, it must Both ants and flowering plants disperse its seeds to places where they benefit from their association. The ants will successfully germinate and grow. ensure the reproductive success of the Plants lend themselves to a variety plants whose seeds they bury, and the of dispersal methods (such as wind, plants supply food to the ants. Many animals, and water) that increase the of the seeds that ants collect and take chances of their seeds landing in suit- underground have a special protrud- able locations. Regardless of how seeds ing structure called an elaiosome, or are dispersed from the parent plant, oil body (see figure). Ants carry the most seeds either land in places that seeds underground before removing are unsuitable for growth or are eaten the elaiosomes, which are a nutritious by animals, such as mice and squirrels, food for them. Once an elaiosome is shortly after being dispersed. removed from a seed, the ants discard The survivability of the seeds of the undamaged seed in an underground some plants is enhanced by a dispersal refuse pile, which happens to be rich in method in which the seeds are bur- organic material (such as ant droppings Marion Lobstein ied underground, where they are less and dead ants) and thus contains the likely to be eaten by animals. For many minerals (inorganic nutrients) required plants, the role of burying seeds is per- by young seedlings. Hence, ants not formed by ants, which collect the seeds only bury seeds away from animals that
Elaiosomes. Bloodroot (Sanguinaria and take them underground to their might eat them but also place them in canadensis) seeds have nutrient-rich elaio- nests. Ants disperse and bury seeds rich soil that is ideal for seed germina- somes, or oil bodies. The golden brown part for hundreds of plant species in almost tion and seedling growth. is the seed proper, and the white part is the every terrestrial environment, from elaiosome. northern coniferous forests to tropical rain forests to deserts. and bitter cress (Cardamine pratensis) split open so ex- plosively that their seeds are scattered a meter or more ( Figure 9-20). Seed Germination and Early Growth You have seen that pollination and fertilization are fol- lowed by seed and fruit development. Each seed devel- ops from an ovule and contains an embryonic plant and food to provide nourishment for the embryo during ger- mination, when the seed sprouts. A mature seed—that is, a seed in which the embryo is fully developed—is of- (a) (b) ten dormant (not actively growing) and may not germi- nate immediately, even if growing conditions are ideal. FIGURE 9-20 Explosive dehiscence. Numerous factors influence whether or not a seed (a) An intact fruit of bitter cress (Cardamine pratensis) before it has split open. In this long, dry, dehiscent fruit, called a silique, germinates. Many of these are environmental cues, in- the two halves split open, leaving a central partition. (b) The bit- cluding the presence of water and oxygen, proper tem- ter cress fruit dehisces with explosive force, propelling the seeds perature, and sometimes the presence of light penetrat- some distance from the plant. ing the soil surface. No seed germinates, for example,  194 CHAPTER 9 Flowers, Fruits, and Seeds some germinate at each temperature in a broad range of temperatures. Each plant species, however, has an opti- mal, or ideal, temperature at which the largest number of seeds germinates. For most plants, the optimal ger- mination temperature is between 25⬚C and 30⬚C (77⬚F and 86⬚F). Some seeds, such as those of apples, require prolonged exposure to cold before they germinate at any temperature. Some of the environmental factors necessary for seed germination help ensure the survival of the young Marion Lobstein plant. The requirement of a prolonged cold period en- sures that seeds germinate in the spring rather than in the autumn. Some plants—especially those with tiny seeds, such as lettuce—require light for germination. A FIGURE 9-21 Pinto bean (Phaseolus vulgaris) seeds (left ) before light requirement ensures that a tiny seed germinates and (right) after imbibition. only if it is close to the surface of the soil. If such a seed germinates several inches below the soil surface, it may not have enough food reserves to grow to the surface. unless it has absorbed water. The embryo in a mature On the other hand, if this light-dependent seed remains seed is dehydrated, and a watery environment in cells dormant until the soil is disturbed and it is brought to is necessary for active metabolism. When a seed germi- the surface, it has a much greater likelihood of survival. nates, its metabolic machinery is turned on, and numer- In certain seeds, internal factors, which are un- ous materials are synthesized and degraded. Therefore, der genetic control, prevent germination even when all water is an absolute requirement for germination. external conditions are favorable. Many seeds are dor- The absorption of water by a dry seed that precedes mant either because the embryo is immature and must germination is known as imbibition. As a seed imbibes develop further or because certain chemicals are pres- water, it often swells to several times its original, dry ent. The presence of such chemical inhibitors helps en- size ( Figure 9-21). Cells imbibe water by osmosis, which is the movement of water across a membrane from an area of high concentration to an area of low concen- tration, and by adsorption of water onto and into materials such as cellulose, pec- tin, and starches within the seed. Water is attracted and bound to these materials by adhesion, the attraction between unlike materials. Cotyledons Seed germination and subsequent growth also require a great deal of en- ergy. Because plants obtain this energy Hook Shriveled by converting the energy of fuel mol- cotyledon ecules stored in the seed