Flowers, Fruits, & Seeds PDF

Flowers, Fruits, & Seeds Asexual Reproduction Reproductive Flexibility of Flowering Plants does not usually involve the formation of...

Flowers, Fruits, & Seeds Asexual Reproduction Reproductive Flexibility of Flowering Plants does not usually involve the formation of flowers, seeds, and fruits. Angiosperm (flowering plants) are the largest, most successful plant group. One reason is that offspring generally form asexually when a they reproduce both sexually and asexually. vegetative structure— stems, leaves, or roots—of an existing plant the biological function of flowers is sexual expands, grows, and then become reproduction. Their varied colors, shapes, and separated from the rest of the plant, often by fragrances are adaptations (evolutionary the death of tissues. This part then forms a modifications) that increase the likelihood that complete, independent plant. pollen grains will be carried from plant to plant. asexual reproduction involves only one Sexual reproduction parent and no fusion of gametes occurs, the the fusion of gametes (eggs and sperm offspring of asexual reproduction are cells). The union of cells is called virtually genetically identical to each other fertilization, which occurs within the and to the parent plant from which they flower’s ovary. came. the offspring of plants that reproduce sexually show considerable genetic variation. Sexual reproduction offers the advantage of new combinations of genes (the units of heredity). These new gene combinations may make an individual plant better suited to its environment. FLOWERS A flower is a reproductive shoot usually PETALS - whorls just inside and above the consisting of four kinds of organs—sepals, sepals, which are broad, flat, and thin (like petals, stamens, and carpels—arranged in sepals and leaves) but tremendously varied in Flowers, Fruits, & Seeds 1  whorls (circles) on the end of a flower stalk, or shape and frequently brightly colored, which peduncle. attracts pollinators. The collective term for all the petals of a slower is corolla. The peduncle may terminate in a single flower or a cluster of flowers known as an sometimes petals fuse to form a tube (for inflorescence. example, trumpet honeysuckle flowers) or other floral shapes (for example, the normal order of whorls from the flower’s snapdragons). periphery to the center (or from the flower’s base upward): STAMENS - located just inside and above the sepals — petals — stamens — carpels petals. Each stamen has a thin stalk called a filament, at the top of which is an anther, a COMPLETE FLOWER - a flower that has all four saclike structure in which pollen grains form. parts—sepals, petals, stamens, and carpels For sexual reproduction to occur, pollen grains INCOMPLETE FLOWER- lacks one or more of must be transferred from the anther to the the four parts. carpel, usually of another flower of the same species. PERFECT FLOWERS - has both stamens and carpels. each pollen grain consists of two cells surrounded by a tough outer wall. IMPERFECT FLOWERS - has stamens or carpels, but not both. one cell generates two male gametes, known as sperm cells, and the Thus, an imperfect flower is also an other produces a incomplete flower. However, a perfect flower pollen tube through which the sperm cells may be complete (if it has both sepals and travel to reach the ovule. petals) or incomplete (if it lacks sepals or petals). CARPELS - located in the center or top of most flowers. They bear ovules — are RECEPTACLE - bears some or all of the flower structures that have the potential to parts. develop into seeds. A single carpel or a group of fused carpels is sometimes only the stamens and carpels participate called a pistil. A pistil may consist of a directly in sexual reproduction. Sepals and single carpel (making it a simple pistil) or petals are sterile. a group of fused carpels (making it a SEPALS - the outermost and lowest whorl of a compound pistil). floral shoot, are leaflike in shape and form and are often green. Some sepals resemble petals (ex. lily sepals). The collective term for all the sepals of a flower is calyx. each carpel or group of fused carpels has three sections: Flowers, Fruits, & Seeds 2  1. stigma — on which the pollen grains land; 2. style — a neck-like structure through which the pollen tube grows 3. ovary — a jug-like structure that contains one or more ovules and can develop into a fruit each ovule contains a female gametophyte, also known as an embryo sac, which develops one female gamete (an egg) and two polar nuclei. INFLORESCENCE OF FLOWERS An ovary is designated as superior or inferior depending on its location relative to other flower parts; A SUPERIOR OVARY has the other floral organs (sepals, petals, and stamens) free from the ovary and attached at the ovary’s base An INFERIOR OVARY is located below the point at which the other floral organs are attached POLLINATION the transfer of pollen grains from the anther to Mechanisms that prevent self-pollination the stigma Plants have evolved a variety of SELF-POLLINATION — pollination occurs within mechanisms that prevent self-pollination and thus the same flower or within different flower on the prevent inbreeding — the mating of genetically same individual plant. similar individuals. Inbreeding is generally Flowers, Fruits, & Seeds 3  CROSS-POLLINATION — occurs when pollen undesirable because it can increase the grains are transferred to a flower on another concentration of harmful genes in the offspring. individual of the same species a. To avoid inbreeding, some species have Flowering plants accomplish pollination in a individuals with staminate flowers that lack variety of ways. Beetles, bees, flies, carpels and other individuals with carpellate butterflies, moths, wasps, and other insects flowers that lack stamens. pollinate many flowers. Other animals, such b. Other species have flowers with both stamens as birds, bats, snails, and small nonflying and carpels, but the pollen grains are shed from mammals (rodents, primates, and a given flower before or after the time that the marsupials) also pollinate plants. Wind is an stigma of that flower is receptive to pollen agent of pollination for certain flowers, grains. whereas water transfers pollen grains in a few aquatic flowers. Many species have genes for self-incompatibility, a genetic condition in which pollen grains are ineffective in fertilizing the same flower or other flowers on the same individual plant. In other words, an individual plant can identify and reject its own pollen grains. Genes for self-incompatibility usually inhibit the growth of the pollen tube in the stigma and style, thereby preventing sperm cell delivery to the ovule. Coevolution of Plants and their Pollinators coevolution — describes such reciprocal Bird-pollinated flowers: adaptation, in which two species interact so a. are usually red, orange, or yellow because closely that they become increasingly adapted birds see well in this region of light. to each other as each undergoes evolutionary change by natural selection b. usually lacks a scent because birds do not have a strong sense of smell. nectaries — a sugary solution, in special floral glands. c. (in general) Hummingbird-pollinated flowers produce more nectar than insect-pollinated Botanists estimate that insects pollinate about flowers, because hummingbirds are larger 70% of all flowering plant species. Bees are animals than insects and therefore require particularly important as pollinators of crop more food. plants. Bee-pollinated crops provide about 30% of human food. Bat-pollinated flowers: Insect-pollinated flower: a. are night blooming, often have dull white petals, and a strong scent, usually of a. have blue or yellow petals because fermented fruit. most insects see well in the violet, blue, and yellow ranges of visible light but do b. they lap up the nectar with their long, not perceive red as a distinct color. extensible tongues Consequently, insect-pollinated flowers c. At least one bat-pollinated flower (the are not usually red. tropical vine Mucana holtonii) has evolved b. Insects see ultraviolet radiation as a an unusual adaptation to encourage color called bee’s purple. pollination by bats. When the pollen grains in a given flower are mature, a concave petal c. have dramatic ultraviolet markings called lifts up. The petal bounces the echo from the nectar guides — may or may not be bat’s echo-locating calls back to the bat, visible to humans but that direct insects helping it find the flower. to the center of the flower where the pollen grains and nectar are. WIND POLLINATION Flowers, Fruits, & Seeds 4  wind-pollinated plants produce many small, inconspicuous flowers. They do not produce large, colorful petals, scent, or nectar. some have large, feathery stigmas, presumably to trap wind-borne pollen grains. wind-pollinated plants produce large quantities of pollen grains, which increases the likelihood that some pollen grains will land on the appropriate stigma. SEEDS Once pollen grains have been transferred from anther to stigma, the tube cell, one of the two cells in the pollen grain, grows a thin pollen tube down through the style and into an ovule in the ovary. Molecular signals from the ovule guide the growing pollen tube toward the ovule. Once a pollen tube penetrates the ovule, the attracting signal ceases. As a result, only one pollen tube enters each ovule. the second cell (the generative cell) within the pollen grain divides to form two male gametes (sperm cells), which move down to the pollen tube and enter the ovule. after fertilization has occurred, the ovule develops into a SEED, and the ovary surrounding it develops into a FRUIT. a mature seed contains an embryonic plant and food stored in either the endosperm or the cotyledons. ENDOSPERM — the nutritive tissue that surrounds the embryonic plant in seed. The seed, in turn, is surrounded by a tough, protective seed coat, derived from the outermost layers (the integuments) of the ovule, and enclosed within a fruit. The mature embryo within the seed consists of a short embryonic root, or radicle; an embryonic shoot; and one or two seed leaves, or COTYLEDONS. hypocotyl — the short portion of the embryonic shoot connecting the radicle to one or two cotyledons. plumule (epicotyl) — the shoot apex above the point of attachment of the cotyledon(s). Flowers, Fruits, & Seeds 5  After the radicle, hypocotyl, cotyledon(s), and plumule have formed, the young plant’s development is arrested, usually by desiccation (drying out) or dormancy. When conditions are right for continuing the developmental program, the seed germinates, or sprouts, and the embryo resumes growth. Because the embryonic plant is non- photosynthetic, it must be nourished during germination until it becomes photosynthetic and therefore self-sufficient. The cotyledons of many plants function as storage organs and become large, thick, and fleshy as they absorb the food reserves (starches, oils, and proteins) initially produced as endosperm. Seeds that store nutrients in cotyledons have little or no endosperm at maturity. Examples of such seeds are peas, beans, squash seeds, sunflower seeds, and peanuts. Other plants—wheat and corn, for example—have thin cotyledons that function primarily to help the young plant digest and absorb food stored in the endosperm. monocots like corn and wheat rely more on the endosperm for nutrient storage, while their cotyledons serve to absorb these nutrients during germination. In contrast, dicot cotyledons often directly store the nutrients needed for seedling development. In flowering plants, seed size varies considerably, from the microscopic, dustlike seeds of orchids to the giant seeds of the double coconut (Lodoicea maldivica), which weigh as much as 27 kilograms (almost 60 pounds). Despite this variation among species, seed size is a remarkably constant trait within a species. FRUITS fruits are mature, ripened ovaries after fertilization takes place within the ovule, the ovule develops into a seed, and the ovary surrounding it develops into a fruit. fruits provide protection for the enclosed seeds and sometimes aid in their dispersal. 4 Basic Types of Fruits 1. simple fruits 2. aggregate fruits 3. multiple fruits 4. accessory fruits Flowers, Fruits, & Seeds 6  An achene is similar to a caryopsis in that it is SIMPLE FRUITS simple and dry, does not split open at maturity, develops from a single carpel or several fused and contains a single seed. However, the seed carpels. At maturity, simple fruits may be fleshy coat of an achene is not fused to the fruit wall. or dry. Ex: fleshy — berries and drupes. Instead, the single seed is attached to the fruit berry - fleshy fruits that has soft tissues wall at one point only, permitting an achene to throughout and contain few to many seeds; be separated from its seed. The sunflower fruit a tomato is a berry, as are grapes, is an example of an achene. One peels off the blueberries, cranberries, and bananas. fruit wall (the shell) to obtain the sunflower seed within. pepo — a modified berry in which the fruit wall is a leathery rind. Pumpkin, AGGREGATE FLOWERS squash, cucumber, and watermelon formed from a single flower that contains fruits are pepos. several separate (free) carpels. hesperidium — has a leathery fruit wall after fertilization, each ovary from each with numerous oil glands surrounding individual may fuse to form a single fruit. Ex: the succulent cavities where the seeds Raspberries, blackberries, and magnolia occur. Citrus fruits (lemons, limes, oranges, and grapefruits) are hesperidia. MULTIPLE FRUITS drupe — a simple, fleshy, or fibrous fruit that forms from the carpels of many flowers that contains a hard stone surrounding a single grow close to one another on a common floral seed. Ex: peaches, cherries, avocados, stalk. olives, and almonds. The almond shell is Flowers, Fruits, & Seeds 7  actually stone, which remains when the rest the carpel from each flower fuses with nearby of the fruit has been removed. carpels as it develops and enlarges after fertilization. Ex: Pineapples, figs, and mulberries. Many simple fruits are dry at maturity. Some of these are dehiscent and split open, usually ACCESSORY FRUITS along seams called sutures, to release their seeds. a fruit whose fleshy part is composed primarily of tissue other than the ovary. A milkweed pod is an example of a follicle, a simple, dry fruit that splits open along one differ from other fruits in that other plant tissues suture to release its seeds. in addition to ovary tissue make up the fruit. A legume is a simple, dry fruit that splits For example, the edible portion of a strawberry open along two sutures (the top and is the red, fleshy receptacle. (Each tiny “seed” bottom). ex: pea pods and lima bean pods on a strawberry is actually a fruit—an achene— that contains a single seed.) A capsule is a simple, dry fruit that splits open along multiple sutures or pores. Iris, Apples and pears are accessory fruits called poppy, buckeye, and cotton fruits are pomes; the outer part of each pome is an capsules. enlarged floral tube, consisting of receptacle tissue, along with portions of the calyx, that Other simple, dry fruits—caryopses (sing., surrounds the ovary. caryopsis), or grains, for example—are indehiscent and do not split open at maturity. 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. Nuts are usually large and one-seeded. Examples of nuts include chestnuts, acorns, and hazelnuts. Many so-called nuts do not fit the botanical definition. Peanuts, almonds, and Brazil nuts, for example, are seeds, not nuts. Seed Germination and Early Growth Each seed develops from an ovule and contains Light. Some plants—especially those with tiny an embryonic plant and food to provide seeds, such as lettuce—require light for nourishment for the embryo during germination. A light requirement ensures that a germination, when the seed sprouts. tiny seed germinates only if it is close to the surface of the soil. If such a seed germinates A mature seed—that is, a seed in which the several inches below the soil surface, it may not embryo is fully developed—is often dormant have enough food reserves to grow to the (not actively growing) and may not germinate surface. On the other hand, if this light- immediately, even if growing conditions are dependent seed remains dormant until the soil is ideal. disturbed and it is brought to the surface, it has Numerous factors influence whether or not a a much greater likelihood of survival. seed germinates. Many of these are environmental cues, including the presence of Temperature. temperature is another water and oxygen, proper temperature, and environmental factor that affects germination. In sometimes the presence of light penetrating the a group of seeds of the same species, some soil surface. germinate at each temperature in a broad range of temperatures. Each plant species, however, Water. The embryo in a mature seed is has an optimal, or ideal, temperature at which dehydrated, and a watery environment in the largest number of seeds germinates. cells is necessary for active metabolism. Flowers, Fruits, & Seeds 8  The absorption of water by a dry seed that In certain seeds, internal factors, which are precedes germination is known as under genetic control, prevent germination even imbibition. Cells imbibe water by osmosis. when all external conditions are favorable. Many Water is attracted and bound to these seeds are dormant either because the embryo is materials by adhesion, the attraction immature and must develop further or because between unlike materials. certain chemicals are present. The presence of such chemical inhibitors helps ensure the Oxygen. Seed germination and subsequent survival of the plant. growth also require a great deal of energy. Because plants obtain this energy by the seeds of many desert plants, for converting the energy of fuel molecules example, often contain high levels of stored in the seed’s endosperm or abscisic acid. Abscisic acid is washed out cotyledons to ATP by aerobic respiration, only when rainfall is sufficient to support the oxygen is usually needed during plant’s growth after the seed germinates. germination. Some seeds, such as those of legumes, for most plants, the optimal germination have extremely hard, thick seed coats that temperature is between 25 C and 30 C (77 F prevent water and oxygen from entering, and 88 F). Some seeds, such as those of thereby inducing dormancy. After these apples, require prolonged exposure to cold seeds are dispersed into the natural before they germinate at any temperature. environment, exposure to the elements gradually weakens their seed coats so that germination eventually occurs. Scarification, the process of scratching or nicking the seed coat (physically with a knife or chemically with an acid) before sowing it, induces germination in these plants. Eudicots and monocots exhibit characteristics patterns of early growth Once conditions are right for seed germination, the first part of the plant to emerge from the seed is the radicle, or embryonic root. The plant shoot is next to emerge from the seed. the stem of a bean seedling (a eudicot), for instance, curves over to form a hook so that the stem tip and cotyledons are actually pulled up through the soil. Corn and other grasses (monocots) have a special sheath of cells called a coleoptile that surrounds the young shoot This indeterminate growth—the ability to grow indefinitely—is characteristic of stems and roots, both of which arise from apical meristems. Other parts of a plant, such as leaves and flowers, have determinate growth that is, they stop growing after reaching a certain size. Flowers, Fruits, & Seeds 9