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  1. Discuss the function of each structure found in the following.
  1. Roots
  2. Stems
  3. Leaves
  1. Roots have a simpler structure than stems. Roots lack external features such as nodes, leaves, and buds. They also lack pith Instead, the vascular bundles are at the center of a root. The vascular tissue of a root is surrounded by a thick layer of cortex that is covered by an outer sheath of dermal tissue. Epidermal cells cover the end of a root and are replaced by layers of cork cells as the root grows. The epidermal cells near the root tip produce slender projections called root hairs. By extending the cell membranes of epidermal cells, root hairs greatly increase the surface area of a root and play a critical role in the absorption of water and minerals. The actively growing tip of a root is covered by a protective layer called a root cap. Based on branching patterns, there are two different types of root systems – taproot systems and fibrous root systems. Plants with a taproot system, such as carrots and radishes, have a large central root called a taproot. The lateral roots that branch from the taproot are usually much smaller than the taproot. Most dicots have a taproot system. The roots of a highly branched fibrous root system are all about the same size. Most monocots, such as grasses, have a fibrous root system. Many plants, such as orchids, also have adventitious roots, which grow from aboveground parts such as stems and leaves. Just like underground roots, adventitious roots provide support and absorb water and minerals.
  2. Functions of the stem are to produce and support new leaves, branches, and flowers; to place them in positions where they can function most efficiently; and to transport materials to and from the roots. Frequently, stems serve to store food, carry on photosynthesis, and reproduce new plants.
Support is provided by various thick-walled cells found in the xylem or in strands outside the xylem. In herbaceous stems, turgor, or internal water pressure, is also important, as evidenced by the limp shape of a wilted plant.
 

 

Water and minerals are transported in the xylem and manufactured food in the phloem. In monocot stems the conducting tissues occur in separated, usually scattered, bundles, whereas in dicot stems the vascular tissues are arranged in a ring, with the primary xylem on the inside, the primary phloem on the outside, and a layer of dividing cells, called the vascular cambium, between them. The term wood in its commercial sense refers to secondary xylem. Secondary xylem is produced by the vascular cambium inward toward the center of the stem between itself and the primary xylem, increasing the thickness of the stem. The yearly production of secondary xylem usually forms a ring around that of the previous year, and these rings can be used to determine the age of the tree. In a similar manner secondary phloem is produced by the vascular cambium outward toward the surface of the stem between itself and the primary phloem; this, too, contributes to the thickness of the stem.

The places where leaves attach to a stem are called nodes. Internodes are the areas of a stem between nodes. Also located at the nodes are lateral buds that grow into the branches of a stem. The bud at the tip of a stem is called the terminal bud. A typical stem consists of bundles of vascular tissues embedded in ground tissue. The outer layers of ground tissue in a stem are called the cortex, and the inner layers are called the pith. Other features of a stem vary, depending on whether the stem is woody or non-woody.

    Herbaceous stems Flexible, relatively soft, and usually green stems, like those of violets and petunias, are called herbaceous stems. An epidermis forms the outermost layer of a herbaceous stem. Stomata in this epidermis enable the stem to exchange gases. The vascular tissues of a herbaceous(non-woody) stem are distributed within the ground tissue and arranged in vascular bundles that contain both xylem and phloem, Woody stems Stiff, usually nongreen stems that contain layers of wood, like the trunks of trees, are called woody stems. In woody stems, the vascular tissue are arranged in solid cylinders. Xylem cells form the innermost cylinder. A new belt of xylem cells form each year, adding to the width of the stem. Wood consists primarily of these xylem cells. The darker wood in the center of a tree trunk is called hardwood. Xylem cells in heartwood no longer conduct water because they have been filled with substances that help to straighten the stem. The lighter wood in a tree trunk, which contains xylem cells that still conduct water, is called sapwood. Phloem cells form then outer most vascular cylinder. As a woody stem grows, a layer of cork cells replaces the epidermis. Cork cells produce the stem from physical damage and help prevent water loss. Together, the phloem and cork layers of a woody stem make up its bark. Gas exchange occurs through tiny openings in loosely organized groups of cork cells called lenticels.
Stems are distinguished from roots in that stems have buds. Buds called terminal buds occur at the tip of the stem and lateral buds grow on the sides of the stem. The buds develop into leaves, side branches, and flowers or cones. Annuals, most biennials, and a few perennials have naked buds that are covered only by the flower parts or elementary leaves. Perennials, which must survive the hardships of winter, have protected buds that are covered with waterproof waxy bud scales. When the buds begin to swell in the spring, the bud scales fall off, leaving scars. The amount of annual growth of a plant can be measured by the distance between these scars.
 

 

Active buds are those that are growing and producing new plant parts. Most buds are latent that is, they do not grow unless the plant suffers injury, as from fire, insects, or frost. Latent buds lie in reserve and are stimulated to growth only when necessary to restore the plant to good health.

In addition to housing vascular tissues and providing a supporting framework for the leaves, stems often perform other functions for plants. The table below contains example of modified stems:

Name  Type of Stem Description Function
Strawberry Stolon Horizontal, aboveground stem Spreading growth, asexual reproduction
Potato Tuber Enlarged underground stem Food Storage
Cactus Succulent Flesh, often leafless stem Water Storage

C. The leaves intercept light, exchange gases, and provide a site for photosynthesis. Some leaves also store food and water, provide support, or form new plants.

A flat, broad, thin structure gives more surface area for light interception and penetration. Where high light intensities are harmful, leaves may reduce the effects of the light by orientating themselves vertically; by becoming thickened or covered with hairs; or by having a highly reflective surface.

Intake of carbon dioxide and release of oxygen occurs through small pores (stomata) in the leaf surface. The stomata are mostly on the lower surface and are able to close at midday. The cells within the leaf may be formed into two layers, the upper, tightly packed with elongated palisade cells, and the lower, loosely packed with spongy tissue. Photosynthesis occurs mostly in the palisade cells. A stoma is surrounded by a pair of guard cells that are shaped like two cupped hands. The stoma opens and closes because of changes in the water pressure within the guard cells.

Most leaves consist of a flattened surface, the blade, that is often attached to the stem by a slender stalk, the petiole. Leaves with an undivided blade are called simple leaves. Those with a blade divided into two or more section, or leaflets, are called compound leaves. Veins, which are bundles containing strands of both xylem and phloem tissue, are the pluming system of a leaf. These veins are extensions of vascular bundles that run from the tips of roots to the edges of leaves. Veins in the leaves of most monocots run parallel to one another, while the veins in the leaves of most dicots branch and form a network. Many plants have highly modified leaves that are specialized for particular purposes, such as protection, water conservation, and climbing. The table below compares the structure and function of three types of modified leaves:
 
Name Function
Cactus spines Protection, water conservation
Garden pea tendrils Climbing
Venus’ flytrap leaves Photosynthesis, trapping insects to obtain nitrogen

A typical leaf is a mass of ground tissue that has veins running through it and that is encased in an envelope of epidermis. The ground tissue in a leaf is called the mesophyll, which comes from the Greek words mesos, meaning "middle, and phyllon meaning "leaf". Two kinds of mesophyll are found in most plants. Just beneath the upper epidermis of many kinds of leaves is the palisade layer, which consists of one or more rows of closely packed, columnar cells. The lower portion of the mesophyll usually consists of loosely packed, spherical cells and is called the spongy layer. The cells of the palisade and spongy layers are packed with chloroplasts, in which photosynthesis takes place. Scattered throughout the spongy layer are large air spaces, through which gases and water vapor travel. Stomata, the tiny holes that dot leaf surfaces, connect the air spaces of the mesophyll to the outside air.

The leaves of many plants have a modified internal structure that is an adaptation for a type of photosynthesis that operates very efficiently in hot climates. Plants fix carbon during photosynthesis by means of the Calvin cycle. Because the first detectable product of the Calvin cycle is a three-carbon compound, plants that fix carbon only within the Calvin cycle are called C3 plants. In some plants, carbon is also fixed by an alternative pathway in which the first detectable product is a four-carbon compound. These plants are called C4 plants. Because they fix carbon efficiently in high temperatures and intense light, C4 plants are plentiful in the tropics.

A number of plants are capable of eating small animals, especially insects. These plants are called insectivorous plants. Examples include the Venus's-flytrap, pitcher plants, sundews, and bladderworts. These unusual plants are most often found in moist and nutrient-poor habitats, such as bogs. The insects that the plants trap are not a major source of organic food rather, they provide mineral nutrients such as nitrogen and phosphorus in these infertile habitats.

Insectivorous plants employ a variety of mechanisms to catch their prey. The sundew has sticky glands located on the ends of hairs on its leaves. Insects become stuck and eventually entangled in these hairs and are then digested by chemicals released from the leaf . Pitcher plants have tubular leaves that produce chemicals attractive to insects. Once the insect has crawled inside the leaf, it is unable to escape. The Venus's-flytrap has perhaps the most elaborate mechanism for catching insects. Its leaves form a snap-trap that is triggered when an insect touches hairs on the leaf's surface. The leaf quickly folds around and traps the insect.

2. Explain all ways materials move in plants.

Transpiration Pulls Water Up a Plant

The leaves of a plant have many tiny holes – the stomata. When they are open, stomata enable gas exchange. Water is also free to diffuse through stomata in the for of water vapor. The passage of air across the surface of a leaf carries away much of this water vapor before it can reenter the leaf. The loss of water vapor from as plant through its stomata is called transpiration.

Water is pulled up a plant because the loss of water by transpiration creates a suction that draws water out of the tracheids and vessels of the plant’s xylem. This water extends in an unbroken column down through the stems and into the roots, where water is absorbed from the soil. As long as the column of water in the xylem remains unbroken, water will continue to move upward because of the pull of transpiration. More than 90% of the water taken in by the roots of a typical plant is ultimately lost through the plant’s leaves in this way.

According to the tension-cohesion theory, two properties of water itself and a simple element of a plant structure assist the pull of transpiration in moving water up a plant. Water is a polar substance; as a result, water molecules readily form hydrogen bonds that enable them to stick to each other (cohere) and stick to other polar substances (adhere). The cohesion of water molecules, gives a column of water great tensile strength. In other words, it can withstand a lot of tension (pull) without breaking. Thus, cohesion of water molecules helps to maintain an unbroken column of water in xylem tissue. Because of adhesion, water is able to move up the sides of a narrow tube by capillary action. Water absorbed by the roots of a plant moves through xylem cells that are elongated and very narrow, like straws. Thus, the adhesion of water molecules to the walls of the very narrow xylem cells helps to draw water to the top of a plant.

Guard Cells Regulate the Rate of Transpiration

The rate of transpiration must be regulated so that a plant does not loose too much water. Water loss by transpiration can be prevented only by the closing of a plant’s stomata. However, stomata must be open at least part of the time so that the carbon dioxide needed for photosynthesis can enter the plant. Therefore, every plant must strike a balance between the conflicting demands of water conservation and photosynthesis.

A stoma is surrounded by a pair of guard cells that are shaped like two cupped hands. The stoma opens and closes because of changes in the water pressure within the guard cells. When guard cells take in water by osmosis, they become turgid (plump and swollen) and bowed in shape. The bowed shape results because the inner wall of a guard cell is thicker than the rest of the cell wall and can not stretch when the guard cell swells. If you were to use a piece of tape to thicken one side of a long balloon, it would also bend when inflated. As the inner walls of a pair of guard cells separate, the stoma opens. When water leaves the guard cells, they lose turgor, their inner walls come back together, and the stoma closes. Thus, loss of water from the guard cells for any reason causes stomata to close and stops further water loss. This is homeostasis in action.

Although the exact mechanism is not well understood, potassium ions, K+, play a critical role in opening and closing stomata. An active transport process that is triggered by light causes potassium ions to move into guard cells. The increased concentration of potassium ions inside the cells causes water to enter them by osmosis. When the movement of potassium ions is reversed and the potassium ion concentration becomes high in the surrounding cells, water diffuses out of the guard cells. The resulting loss of turgor by the guard cells causes the stomata to close.

Sugar Is Pushed Through a Plant

In a plant, sugar moves where it is made or stored to where it is needed through the sieve tubes of phloem. Botanists use the term source to refer to a part of a plant that provides sugar for other parts of the plant. For example, a leaf is a source because it makes sugar during photosynthesis. A root is also a source when sugar is stored there is moved to other parts of the plant. Botanists use the term sink to refer to a part of a plant to which sugar is delivered. Areas of active growth where sugar is needed for metabolism, such as root tips and developing fruits, are examples of sinks. The movement of sugar within a plant from a source to a sink is called translocation.

The movement of sugar in a plant is more complex than the movement of water. First, water flows freely through empty xylem elements, but sugar must pass through the cytoplasm of living cells. Second, water only moves upward within the xylem, while sugar moves both upward and downward in the same sieve tube, but at different times. Last, water diffuses freely through a plasma membrane, but sugar cannot. How, then, is sugar distributed throughout a plant? Many attempts have been made to answer this question. The model of translocation that most botanists favor was proposed in 1924 by the German botanist Ernst Munich. Munich’s model was first tested by an experiment. Using a concentrated sugar solution to represent the phloem near a source, water can be made to enter a tube and flow through it. The pressure created by water entering the tube pushes the water and some of the sugar in the solution to the other end of the tube. Therefore, Munich’s model of translocation is often called the pressure-flow model. Once the tube is completely filled with water, sugar is also able to diffuse from one end to the other, as long as the sugar concentration remains higher at one end than the other.

3. Explain how the following processes work in plants

  1. Growth
  2. Germination
  3. Photoperiodism
  4. Dormancy
  5. Responses to the environment
A. One of the general characteristics of plants, compared to animals, is that they tend to grow continuously throughout their lives. Growth serves not only to increase a plant's size but also to provide the plant with a limited means of movement and orientation for placing itself in a more favorable position with regard to light, nutrients, reproduction, and dispersal. The growth of plants involves both the production of new cells and their subsequent enlargement. Following enlargement, a cell undergoes differentiation to become a part of a specific tissue.

There are two aspects of plant growth: primary and secondary. Primary growth takes place in young, herbaceous organs, resulting in an increase in length of shoots and roots. Secondary growth follows primary growth in some plants and results in an increased girth as layers of woody tissue are laid down. Monocots and herbaceous dicots typically exhibit only primary growth.

The formation of new cells takes place in regions known as meristems. At the tip, or apex, of each stem and root is an apical meristem, where cells are actively dividing. Each apical meristem produces three other meristems (protoderm, ground meristem, and procambium) called primary meristems. Tissues derived from the primary meristems are called primary tissues and include epidermis, cortex, pith, and primary xylem and phloem (vascular tissues). The elongation of cells produced by the primary meristems accounts for most of the increased length of stems and roots.

The important factors affecting plant growth and development include heredity, hormones, nutrition, and environment.

Hereditary, or genetic, factors control the general species characteristics of the individual and set limits on size and rate of growth. The genetic structure, through DNA and RNA patterns, acts by regulating protein synthesis, especially the manufacture of enzymes, as well as cell division, cell enlargement, the incorporation of substances into the cell walls, and the production and activity of the hormones. The gene action, in turn, is controlled by various growth regulators, particularly hormones and nutrients.

So-called plant hormones are organic chemicals produced in small amounts at one place in the plant that cause some physiological action in another. The several classes of plant hormones include the auxins, cytokinins, gibberellins, abscisic acid, and ethylene. Cytokinins are especially important in cell division; elongation is promoted by auxins and gibberellins. The bending of stems toward light is caused by auxins in higher concentration on the dark side, inducing more cell elongation.

Cell and organ differentiation are usually regulated by the interaction of several hormones. The initiation of roots by auxins and of buds by cytokinins depends on the presence of opposing hormones in the proper amounts. Other growth-related activities regulated by hormones include seed germination, flower and fruit development, and leaf enlargement.

Plants require all the essential ingredients of photosynthesis to construct the necessary compounds and structures. Water is especially important, because cell enlargement is a result of internal water pressure (turgor) extending the walls. In periods of drought plants tend to have smaller leaves. Calcium interacts with auxins and cytokinins in regulating cell divisions and elongation. Nitrogen is involved in the structure of chlorophyll, proteins, auxins, and cytokinins.

The intake and use of nutrients and the activities of hormones and other regulators are affected greatly by the external environment, particularly temperature and light. Certain wavelengths of light affect the activity of a pigment called phytochrome, which in turn interacts with hormones in regulating flowering, leaf expansion, stem elongation, sleep movement of leaves, and seed germination.

All physiological activities are directly related to temperature, with warmer temperatures favorable to more growth. Cold temperatures are required for some seeds to germinate, some buds to begin growing, and some plants to flower.

B. The embryo has all of the basic plant parts. Its epicotyl or plumule will form the plant shoot as the seed begins to grow. The cotyledons quickly unfold into leaves and begin producing food. The radicle gives rise to the root system. The region that connects the radicle and plumule is called the hypocotyl.

In most plants, the nutritive tissue in the seed is endosperm, formed during the fertilization process. Seeds with large amounts of endosperm include those of corn, castor beans, and pumpkins. The "milk" contained in coconuts is actually endosperm. The seeds of other plants, such as beans and peas, contain very little endosperm. In these plants the cotyledons of the embryo are quite large and provide nourishment to the embryo during germination.

Seed germination requires moisture, oxygen, and a suitable temperature, but there are sufficient food and minerals stored in the seed so that these factors are not necessarily essential during the very early stages of germination. Many seeds germinate best in the dark. Initially they can grow using food reserves from the endosperm or cotyledons. Within a few days of germination, however, the developing seedling must have light in order to manufacture its own food.

Seed germination begins when the seed absorbs water. This causes the inner tissue layers to swell enough to rupture the seed coat. Water also hastens chemical reactions that occur very slowly in dormant dry seeds. These chemical reactions provide food directly to the embryo, causing it to begin its growth.

The rapid growth of the embryo results in very high rates of respiration. This is why oxygen is so important for the germination of most seeds. Seeds that are deprived of oxygen once they begin to germinate soon die. This sometimes happens when planted seeds receive too much water oxygen cannot diffuse easily into very wet soil.

Once germination of the seed begins, the radicle usually emerges first. The radicle grows rapidly downward through the soil to establish the root system. In some plants, the tissues that make up the hypocotyl stretch, pushing the cotyledons above the soil. The cotyledons can then unfold and begin producing food. In plants with cotyledons that store food, the cotyledons may remain in the soil. Once the root system is established, the epicotyl rapidly develops into a system of shoots and leaves.

Before germination, dry seeds are very resistant to environmental stresses such as drought or unfavorable temperatures. This portion of the plant's life cycle allows the plant to survive during periods when plant growth is impossible. In order to prevent seeds from germinating when conditions are unfavorable, many seeds are dormant when they are produced. This means that they will not germinate even if there is sufficient moisture and oxygen and suitable temperatures. Such seeds are nevertheless alive. If allowed to "afterripen" for a period of weeks or months, they will germinate normally. Many plants that grow in cold winter regions produce dormant seeds. Such seeds germinate in the spring, often only after they have been exposed to cold, moist conditions.

Some seeds require special conditions in order to germinate. Such requirements often guarantee that the seed will germinate only when conditions are most favorable for seedling growth. Seeds of the pin cherry, for example, may remain dormant in forest soils for decades. When the soil is disturbed and the seeds are exposed to light, they will germinate. It is only under these conditions that a pin cherry seedling is likely to survive to become a tree. In desert regions the seeds of many plant species germinate only following very heavy rains, when sufficient moisture will be available for the plants to complete their life cycles. The seed coats of many such plants contain chemical inhibitors that prevent normal germination. Heavy rains remove these inhibitors, permitting germination. Some plants germinate and grow best in areas that have recently been burned by wildfire. The heat of the fire is the stimulus that breaks the seed's dormancy.

C. Plant responses are strongly influenced by seasonal environmental changes. For example, many trees shed their leaves in autumn and most plants flower only during certain times of the year. How do plants sense seasonal changes? Contrary to what you might suspect, most plants do not mark the seasons by changes in temperature. Instead they respond to changes in day length. This response is called photoperiodism.

Biologists do not know exactly how plants monitor changes in day length, but evidence suggests that this response involves a pigment called phytochrome. Phytochrome exists in two forms: red light – sensitive phytochrome, Pr, and far-red light—sensitive phytochrome, Pfr. The Pr absorbs red light of wavelengths of about 660 nm and in doing is so converted to Pfr.. When Pfr absorbs far-red light of wavelengths of about 730 nm, it is converted to Pr. Since sunlight contains proportionally more red light than far-red light, Pr is converted to Pfr during the day. At night Pfr is slowly converted to Pr . Accordingly, the ratio of Pr to Pfr may be a chemical measure of the relative length of day and night. Biologists believe that the changing proportions of the two phytochromes play a role in starting the hormonal changes that cause flowering, though other factors may be involved.

Flowering

Flowering is influenced by several factors, including temperature and the availability of moisture. However, in some plants the most important factor controlling flowering is day length, Plants vary in their response to the duration of light. Many species have a critical length, which is the length of daylight above or below which these species of plants will flower. The adaptive value of photoperiodism is that members of a species bloom at the same time each year, when conditions for pollination are most favorable.

Long-day plants flower only when exposed to day lengths longer than the critical length of the plant. For example, wheat plants flower only when days are longer than 10 hours – during the lengthening days of late spring and early summer. Radishes, clover, irises, and beets are other long-day plants.

Short-day plants flower only when exposed to day lengths shorter than the critical length for the plant. For example, ragweed flowers only when days are shorter than 14.5 hours. Most short-day plants flower during the shortening days of the fall, through some bloom in early spring. Chrysanthemums, golden rods, soybeans, and poinsettias are short day plants.

Day-neutral plants are not affected by the length of days and nights. Tomatoes, dandelions, and roses are day-neutral.

D. Dormancy is a state of reduced physiological, or metabolic, activity in organisms. It may be regarded as a mechanism enabling them to survive unfavorable periods. Dormancy in plants is discussed here. For animals. Many factors are involved in dormancy; environmental features, such as temperature, light, and moisture, may interact with internal factors to induce or to break dormancy. Dormancy may affect the entire plant or just parts of it. It may occur sporadically, or, more often, it may be a regular phase in a plant's life. Two regularly occurring types of dormancy are those affecting seeds and buds.

The major internal factors in seed dormancy are the seed coat, chemical inhibitors, and the embryo. The seed coat may maintain dormancy and prevent growth by blocking the passage of water or oxygen to the embryo or by physically preventing the embryo from enlarging. The seed coat must rot or weaken before germination can occur. For agricultural purposes, however, the seed coat is removed or damaged (scarified) mechanically or chemically to speed up the process.

Chemical growth inhibitors may be present in the seed coat or other parts of the seed. Antiauxins are substances that counteract auxins, or growth hormones. One antiauxin, called coumarin, has been found in some seeds. Another group of regulators are the dormins, such as abscisic acid, which has been found to inhibit the alpha-amylase activity induced by the growth factor gibberellin in barley seeds. These chemical inhibitors must be leached from the seed by water, or the tissues containing them must be destroyed before growth can occur.

The embryos of most seed plants are not self-dormant and will grow unless prevented from doing so by the seed coat or inhibitors. Some embryos, however, are dormant and must pass through an after-ripening process, which is not clearly understood, before growth can occur. Another form of embryo dormancy is that due to the immaturity of the embryo itself; germination must await its full development.

Environmental factors that affect dormancy include temperature, light, and moisture. The seeds of many cold-region plants will not germinate unless exposed to freezing or to a period of low temperatures. The presence or absence of light (or of light of specific wavelength) may exert an effect as can the presence or absence or quality of moisture.

Overwintering leaf buds, containing some or all of next season's leaves in an embryonic state, are produced by many temperate-region trees and shrubs. In spring the bud scales open and the bud shoot and leaves renew growth. After a time, spring growth ceases, new overwintering buds are produced, and a new cycle begins. The ending of spring growth and the formation of buds is promoted by a number of factors, among them cheminhibitors, similar to those involved in seed dormancy, and photoperiod, or length of daylight. Many trees begin the dormancy cycle when exposed to short photoperiods of artificial light (12 hours or less to simulate the short days of fall).

To break bud dormancy, factors such as an increase in photoperiod length, a chilling period, or an increase in growth hormones (auxins, gibberellins, cytokinins)--or a combination of these--may be required.

E. Plants respond to environmental stimuli such as light, moisture, chemicals, gravity, and mechanical disturbances. These stimuli cause adaptive movements by influencing growth. Plant growth involves the elongation of cells, and most plant movements result from elongation triggered by environmental factors

Tropism’s

A tropism is a plant movement toward or away from an environmental stimulus. Each kind of tropism is named for its stimulus. For example, movements in response to light are called phototropism’s. Movement toward an environmental stimulus is called a positive tropism. Conversely, movement away from a stimulus is a negative tropism

Phototropism- Phototropism is plant growth response to light coming from one direction. Scientificanalysis has shown that light coming from one direction causes auxin to move to the shaded side of the stem. In most cases the auxin causes cells on the shaded side to elongate more rapidly than cells on the lighted side of the stem. As a result the stem curves toward the light. By orienting the plant toward its light source, phototropism maximizes the amount of light that reaches the photosynthetic cells of the plant.

Thigmotropism- Thigmotropism is a growth response to contact with a solid object. Auxin and ethylene control coiling, which occurs when plants such as morning glories come into contact with an object. Thigmotropism allows a plant to climb: over and cling to objects, increasing its chances of intercepting light for photosynthesis.

Gravitropism- Gravitropism is a plant growth response to gravity. Usually a root grows down and a stem grows up-–that is, roots are positively gravitropic and stems are negatively gravitropic. Gravitropism is also known as geotropism. Like phototropism, gravitropism appears to be regulated primarily by auxin. If a seedling is placed horizontally, auxin accumulates along the lower sides of both the root and stem. The concentration of auxin stimulates cellular elongation along the lower side of the stem, and the stem grows upward. A similar concentration of auxin inhibits cellular elongation in the lower side of the root, and as a result the root grows downward.

To learn how plant cells sense gravity, scientists have studied the position of inclusions in root cap cells called amyloplasts. Gravity causes amyloplasts, which contain grains of starch, to settle on the lower side of a cell. When a plant is turned sideways, amyloplasts fall to the lower cell wall. Amyloplasts are thought to cause a movement of calcium ions, which in turn influences the transport of auxins that cause the root to curve in response to gravity.

Chemotropism and Hydrotropism- Plant growth in response to a chemical is called a chemotropism. For fertilization to occur, for example, the pollen tube must grow into the ovule through the micropyle. The pollen tube grows in response to chemicals produced by the pistil of a compatible flower,. The growth of roots in response to water is called hydrotropism. One example occurs when roots encounter water-filled pipes and grow into rather than around them. Negative hydrotropism is an adaptation for avoiding flooded soil.

Nastic Movements

Plant movements that occur in response to environmental stimuli but that are independent of the direction of the stimuli are called nastic movements. For instance, when Minosa pudica, the sensitive plant , is touched, its leaflets fold and its petioles lower within a few seconds. This
response is triggered by rapid movements of potassium ions between parenchyma cells at the base of the leaflets and petiole. Stimulation causes ions to be pumped out of cells along the lower side of the petiole. Water then rapidly moves out of these cells by osmosis, and the cells shrink. Many nastic movements, such as the closing up of mourning glory petals in the evening, have adaptive significance. Nastic movements allow plants to conserve water and food energy when conditions are not favorable for photosynthesis and growth.

4. Discuss all the ways that plants affect our lives
 
        A. All types of plant parts—roots, stems, leaves, flowers, fruits, and seeds—are eaten as food
            a.  Most of the foods that people eat come directly or indirectly from the fruits of cereals, which are grasses that are grown as food for     humans and livestock
                1. Rice is the main part of every meal
                2. For 1/3 of the world population, wheat is the primary source of food
                3. Corn is the most widely cultivated crop in the United States
              b. Other significant foods are derived from stems, roots and seeds for vitamins, minerals, and proteins.
                1. Legumes from peas are protein-rich seeds in long pods
                2. Potatoes are an important food staple in many regions of the world
        B. Plants make up wood and lumber. Wood builds home and can be used for cooking, and heating. Paper is also made of trees for             newspapers, etc.
        C. Medicines are obtained from plants to cure illnesses and ease pains.
        D. Plant fiber is used to make cloth
        E. The milky white sap of trees makes rubber, an important resource.