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Plant Biology

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PLANT ORGANS 1. THE BASIC PLANT ORGANS Plants draw resources from two very

different environments: below-ground and above-ground. Plants must absorb water and minerals from below the ground and carbon dioxide and light from above the ground.

Therefore, they have three basic organs: roots, stems, and leaves.

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Roots are not photosynthetic and would starve without the organic nutrients imported from the stems and leaves.

Conversely, the stems and leaves depend on the water and minerals that roots absorb from the soil.

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ROOTS The root is an organ that anchors a vascular plant,

usually to the soil. It absorbs minerals and water, and often stores organic nutrients.

A taproot system consists of one main vertical root which gives rise to lateral roots. The taproot often stores organic nutrients that the plant consumes during flowering and fruit production. For this reason, root crops such as carrots, turnips, and sugar beets are harvested before they flower.

Taproot systems generally penetrate deeply into the ground.

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Taproots

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ROOTS In seedless vascular plants and grasses,

many small roots grow from the stem in what is called a fibrous root system. No roots stand out as the main one.

These roots are said to be adventitious. A fibrous root system is usually shallower

than a taproot system. Grass roots are fibrous root systems that hold

the top soil in place, preventing erosion.

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Fibrous Roots vs.

Tap Root

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ROOTS The entire root system helps

anchor a plant, but the absorption of water and minerals occurs primarily near the root tips, where vast numbers of tiny root hairs increase the surface area of the root enormously.

A root hair is an extension of a root at the dermal cell. Absorption is often enhanced by symbiotic relationships between plant roots and fungi and bacteria.

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Root Hairs from Dermal Cells

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STEMS

A stem is an organ system consisting of nodes (the points at which leaves are attached), and internodes (the stem segments between nodes).

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STEMS In the angle

formed by each leaf and the stem is an axillary bud, a structure that has the potential to form a lateral shoot, commonly called a branch.

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STEMS Most axillary buds of a young shoot are

dormant. Thus, elongation of a young shoot is

usually concentrated near the shoot apex (tip), which consists of a terminal bud with developing leaves.

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STEMS The resources of a plant are concentrated at the apex

for elongation growth to increase the plant's exposure to light. But what if an animal eats the end of the shoot? Or what if light is obstructed there?

Under such conditions, axillary buds began growing. A growing axillary bud gives rise to a lateral shoot with its own terminal bud, leaves, and axillary buds.

Removing the terminal bud usually stimulates the growth of axillary buds resulting in more lateral shoots.

That is why pruning trees and shrubs and pinching back houseplants will make them bushier.

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Removal of the growing tip will produce a short and bushy plant.

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STEMS Modified stems with different functions

have evolved in many plants as an adaptation to the environment.

These modified stems, which include stolons, rhizomes, tubers, and bulbs, are often mistaken for roots.

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A stolon is a horizontal stem that grows along the surface of the soil. These runners enable a plant to reproduce asexually, as plantlets form at nodes along each runner. An example is found in the strawberry plant.

19 A rhizome is a horizontal

stem that grows just below the surface of the soil. An example is the edible base of a ginger plant.

20 A tuber is an enlarged

end of a rhizome that has become specialized for storing food. An example is a potato. The eyes of a potato are clusters of axillary buds that mark nodes.

21 A bulb is a vertical,

underground shoot consisting mostly of the enlarged bases of leaves that store food. An example is an onion.

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Garlic Bulb

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LEAVES The leaf is the main photosynthetic organ of

most plants, although green stems also perform photosynthesis.

Leaves generally consist of a flattened blade and a stalk (the petiole), which joins the leaf to a node of the stem.

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Plants differ in the arrangement of veins, which are the vascular tissue of leaves.

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LEAVES Most monocot leaves (like grass) have

parallel major veins that run the length of the leaf blade.

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LEAVES In contrast, eudicot leaves (like trees

and most other plants) generally have a multi-branched network of major veins.

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Plants are sometimes classified according to the shape of the leaves and the pattern of the veins.

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Plants are sometimes classified according to the shape of the leaves and the pattern of the veins.

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LEAVES Most leaves are specialized for photosynthesis.

However, some plant species have leaves that have become adapted for other functions, such as support, protection, storage, or reproduction.

Tendrils Spines Storage leaves Bracts Reproduction

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LEAVES Tendrils are modified leaves which

allow a pea plant to cling for support.

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LEAVES The spines of a cactus are modified

leaves which serve as protection.

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LEAVES Succulent plants, such as the ice plant,

have storage leaves for storing water.

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LEAVES The red parts of a poinsettia plant are often

mistaken for petals but are actually modified leaves called bracts that attract pollinators.

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LEAVES Some leaves are modified for

reproduction, such as those which produce tiny plantlets, which fall off the leaf and take root in the soil.

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PLANT TISSUES Each plant organ (root, stem, or leaf) has

dermal, vascular, and ground tissues. A tissue system consists of one or more

tissues organized into a functional unit connecting the organs of a plant.

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37DERMAL TISSUE SYSTEM The dermal tissue system is the outer protective covering of a

plant. Like our skin, it forms the first line of defense against physical

damage and pathogenic (disease causing) organisms. In non-woody plants, the dermal tissue usually consists of a

single layer of tightly packed cells called the epidermis.

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In woody plants, protective tissues known as periderm replace the epidermis in older regions of the stems and roots.

39DERMAL TISSUE SYSTEM In addition to protecting the plant from water loss

and disease, the epidermis has special characteristics in each organ.

For example, at the tip of roots, the epidermis has extensions called root hairs which absorb water and minerals.

In the epidermis of leaves and most stems, a waxy coating called the cuticle prevents water loss.

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Cuticle

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VASCULAR TISSUE SYSTEM

The vascular tissue system carries out long distance transport of materials between roots and shoots.

The two vascular tissues are xylem and phloem. Xylem conveys water and dissolved minerals

upward from roots in to be shoots. Phloem transports nutrients such as sugars

from where they are made (usually the leaves) to where they are needed (usually the roots, developing leaves, and fruits).

The vascular tissue of a root or stem is collectively called the stele.

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Stele

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45GROUND TISSUE SYSTEM

Tissues that are neither dermal nor vascular are part of the ground tissues system.

Ground tissue that is internal to the vascular tissue is called pith, and ground tissue that is external to the vascular tissue is called cortex.

The ground tissues system includes various cells specialized for functions such as storage, photosynthesis, and support.

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TYPES OF GROWTH Unlike most animals, plant growth

occurs throughout the life of the plant. Except for periods of dormancy, most

plants grow continuously. Eventually of course, plants die. Based on the length of their lifecycle,

flowering plants can be categorized as annuals, biennials, or perennials.

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Annuals Annuals complete their lifecycle (from

germination to flowering to seed production to death) in a single year or less.

Many wildflowers are annuals, as are the most important food crops, including the cereal grains and legumes.

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Biennials Biennials generally live two years, often

including a cold period (winter) between vegetative growth (first spring/summer) and flowering (second spring/summer).

Beets and carrots are biennials but are rarely left in the ground long enough to flower.

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Perennials Perennials live many years and include trees,

shrubs, and some grasses.

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Perennials Some buffalo grass of the North American plains is

believed to have been growing for 10,000 years from seeds that sprouted at the close of the last ice age.

When a perennial dies, it is usually not from old age, but from an infection or some environmental trauma, such as fire or severe drought.

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Meristems

Plants have embryonic tissues called meristems that allow the plant to grow indefinitely.

Apical meristems Lateral meristems Vascular cambium Cork cambium

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Meristems Apical meristems, located at the tips of

roots and in the buds of shoots, enable a plant to grow in length, a process known as primary growth.

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Meristems Lateral meristems allow for growth in

thickness, known as secondary growth. In woody plants, the lateral meristems are called the vascular cambium and the cork cambium.

Lateral meristem

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Meristems The vascular cambium adds layers of

secondary xylem (wood) and secondary phloem.

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Meristems The cork cambium replaces the epidermis

with periderm which is thicker and tougher.

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PRIMARY GROWTH Primary growth lengthens roots and

shoots. The new growth produced by apical meristems affects the entire plant if it is herbaceous.

In woody plants, it only affects the youngest parts which have not yet become woody.

Although apical meristems lengthen both roots and shoots, there are differences in the primary growth of these two systems.

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PRIMARY GROWTH OF ROOTS The root tip is

covered by a root cap, which protects the delicate apical meristem as the root pushes through the abrasive soil during primary growth.

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PRIMARY GROWTH OF ROOTS

Growth occurs just behind the root tip, in three zones of cells at successive stages of primary growth.

Moving away from the root tip, they are the zones of cell division, elongation, and maturation.

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PRIMARY GROWTH OF ROOTS The primary growth of roots produces the

epidermis, ground tissue, and vascular tissue. Water and minerals absorb from the soil must enter

through the epidermis. Root hairs enhance this process by greatly

increasing the surface area of epidermal cells. In most roots, the stele is a vascular cylinder, a solid core of xylem and phloem.

However, in many roots, the vascular tissue consists of a central core of parenchyma cells surrounded by alternating rings of xylem and phloem.

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PRIMARY GROWTH OF SHOOTSLeaves arise as leaf primordia, which are finger-like projections along both sides of the apical meristem.

Axillary buds can form lateral shoots as well.

Within a bud, leaf primordia grow in length due to both cell division and cell elongation.

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SECONDARY GROWTH Secondary growth adds girth (width) to stems and

roots in woody plants. Secondary growth is produced by lateral

meristems. The vascular cambium adds secondary xylem and

secondary phloem. Cork cambium produces a tough, thick covering

consisting mainly of cork cells. Primary and secondary growth occurs

simultaneously like in different regions. While an apical meristem elongates a stem or root,

secondary growth commences where a primary growth has stopped.

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SECONDARY GROWTH The vascular cambium is a cylinder of

meristematic cells one layer thick. It increases in circumference and also lays

down successive layers of secondary xylem to its interior and secondary phloem to its exterior.

In this way, it is primarily responsible for the thickening of a root or stem.

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XYLEM In plants, vascular tissue made of dead

cells that transport water and minerals from the roots is called xylem.

Water and minerals ascend from roots to shoots through the xylem.

The xylem sap flows upward from the roots throughout the shoot system to veins that branch throughout each leaf.

Leaves depend on this delivery method for their supply of water.

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XYLEM Plants lose an astonishing amount of water by

transpiration, the loss of water vapor from leaves.

A single plant can lose 125 L of water during a growing season.

Unless the water is replaced, the leaves will wilt in the plant will eventually die.

The upward flow of xylem sap also brings mineral nutrients to the shoots.

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XYLEM Xylem sap needs to rise more than 100 m in the

tallest trees. To get to this height, it is either pushed up from the

roots or pulled upward by the leaves. Root pressure pushes the xylem sap upward,

especially at night. The root pressure at night sometimes causes more

water to enter the leaves then is transpired, resulting in exudation of water droplets that can be seen in the morning on tips of grass blades or the margins of leaves.

This is not the same thing as dew, which is condensed moisture produced during transpiration.

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Transpiration vs. Dew

Transpiration

Dew

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XYLEM Root pressure can only force water upward a few meters,

and it cannot keep pace with transpiration after sunrise. For the most part, xylem sap is pulled upward by the

leaves themselves. This is accomplished by the transpiration-cohesion-

tension mechanism, like sucking liquid through a straw.

As moisture escapes the leaves by transpiration, one water molecule sticks to the other water molecules by cohesion, and the entire column of water rises. This transpiration pull can extend down to the roots only if the chain of water molecules is unbroken.

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XYLEM If an air pocket forms, such as when xylem sap freezes in the winter,

the resulting air bubbles will break the chain. Air bubbles can also occur if there is an excess rate of evaporation of

water from the leaves. This is common when the leaves are exposed to windy conditions,

such as when plants are transported in the back of a truck. A plant can be killed in as little as 20 minutes of exposure to these

conditions if the soil is not thoroughly watered before the trip.

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PHLOEM In plants, vascular tissue that consists of

living cells that distribute sugars throughout the plant is called phloem.

Organic nutrients (the products of photosynthesis) are translocated through the phloem.

Phloem is arranged in sieve tubes that are positioned end to end.

Between the cells are sieve plates, structures that allow the flow of sap along the sieve tubes.

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PHLOEM

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PHLOEM The main component of phloem sap is sugar

(sucrose). This gives the sap a syrupy thickness.

A sugar source is a plant organ that produces sugar by photosynthesis. Mature leaves are the primary sugar sources.

A sugar sink is an organ that is a consumer or storage site of sugar. Growing roots, buds, stems, and fruits are sugar sinks. A storage organ, such as a tuber or a bulb, may be a source or a sink, depending on the season.

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TRANSPIRATION Gas exchange (transpiration) in plants occurs through

structures called stomata. The rate of transpiration is regulated by stomata, which are

pores in the leaves. Carbon dioxide enters through the stomata into airspaces

formed by the spongy parenchyma cells. This increases the internal surface area of the leaf by up to 30

times greater than what it appears when we look at the leaves. This increase in surface area improves the rate of

photosynthesis however it also increases water loss through the stomata.

Therefore, a plant requires a tremendous amount of water to make food by photosynthesis.

By opening and closing the stomata, guard cells balance water conservation during photosynthesis.

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Stomata and Guard Cells

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Transpiration

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TRANSPIRATION A leaf may transpire more than its

weight in water every day and water may move through the xylem at a rate which is about equal to the speed of the tip of a second hand sweeping around a clock.

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TRANSPIRATION If transpiration continues to pull sufficient water upward to the

leaves, they will not wilt. But the rate of transpiration is greatest on a day that is sunny,

warm, dry, and windy because of the increase in evaporation.

Plants adjust to these conditions by regulating the size of the stomatal openings, but some evaporation still occurs when the stomata are closed.

As cells lose water pressure, leaves begin to wilt.

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TRANSPIRATION Transpiration also results in evaporation

cooling. This prevents the leaf from reaching

temperatures that could damage enzymes involved in photosynthesis.

Cactus plants have low rates of transpiration, but have evolved to tolerate high leaf temperatures.

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NUTRIENTS Watch a large plant grow from a tiny seed, and

you cannot help wondering where all the mass comes from.

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NUTRIENTS About 90% of a plant is water which has

accumulated within their cells. However, soil, water, and air all contribute to

plant growth. Plants extract essential mineral nutrients from

the soil, especially phosphorus and nitrogen.

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NUTRIENTS They also require other minerals as well. The

symptoms of a mineral deficiency depend partly on the nutrient’s function.

For example, a deficiency of magnesium, a component of chlorophyll, causes yellowing of the leaves, known as chlorosis.

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SOIL QUALITY Along with climate, the major factors

determining whether a particular plant can grow well in a certain location are the texture and composition of the soil.

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Soil TextureTexture refers to the relative amounts of various sizes of soil particles.

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Soil Composition

Composition refers to the organic and inorganic chemical components of the soil.

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In turn, plants affect the soil, taking part in a chemical cycle that sustains the balance of terrestrial ecosystems.

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SOIL QUALITY Soil originally comes from the

weathering of solid rock. Rocks break apart over time from

several mechanisms. Water can seep into crevices, freeze,

and the expansion can fracture rocks. Acids dissolved in the water can also

break down rocks chemically. Roots that grow in fissures can also

cause fracturing.

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SOIL QUALITY

The eventual result of all this activity is topsoil (O), a mixture of rock particles, living organisms, and humus (A), the remains of partially decayed organic material.

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Texture of topsoil The texture of topsoil depends on the size of its particles,

which range from coarse sand to microscopic clay. The most fertile soils are loams, made up of equal

amounts of sand, silt (medium-size particles), and clay. The fine particles provide a large surface area for

retaining minerals and water. Coarse particles provide airspaces containing oxygen that

can be used by roots for cellular respiration. If soil does not drain adequately, roots suffocate because

the air spaces are replaced by water; the roots may also be attacked by molds that favor wet soil.

These are common hazards for houseplants that are overwatered in pots with poor drainage.

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Soil composition Soil composition includes organic components as well as

minerals. Topsoil has an astonishing number and variety of organisms.

A teaspoon of topsoil has about 5 billion bacteria along with

various fungi, algae, insects, and worms. The activities of all these organisms affect the soils

properties. Earthworms aerate the soil by their burrowing and add

mucus that holds find soil particles together. The metabolism of bacteria changes the mineral

composition of the soil. Plant roots can release organic acids, changing the soil pH. Plant roots also reinforce the soil against erosion.

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Soil composition Humus consists of decomposing organic material

formed by the action of bacteria and fungi on dead organisms, feces, fallen leaves, etc.

Humus prevents clay from packing together and builds a crumbly soil that retains water but is still porous enough for adequate aeration of roots.

It is also a reservoir of mineral nutrients that are returned gradually to the soil as microorganisms decompose the organic matter.

During heavy rain or irrigation, nitrogen and phosphate is leached away from the soil and drained into the groundwater deeper down, making them less available for uptake by roots.

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Soil conservation Soil conservation is essential. It may take centuries for a soil to become

fertile through the breakdown of rock and the accumulation of organic material, but human management can destroy that fertility within a few years.

Before the arrival of farmers, the Great Plains of the United States was covered by hardy grasses that held the soil in place despite of the long recurrent droughts and torrential rains characteristic of that region.

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Great Plains became dustbowl due to farming the same crops each year.

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Soil conservation In the late 1800s, many homesteaders settled in the

region, planting wheat and raising cattle. These land uses left the topsoil exposed to erosion

by winds that often swept over the area. During drought seasons, much of the topsoil was

blown away rendering millions of acres of farmland into what was called the Dust Bowl.

This forced hundreds of thousands of people to abandon their homes and land, as found in the story, The Grapes of Wrath.

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Soil conservation In healthy ecosystems, mineral nutrients must be

recycled by the decomposition of dead organic material in the soil.

When farmers harvest of crop, essential elements are removed.

To grow 1000 kg of wheat, the soil gives up 20 kg of nitrogen, 4 kg of phosphorus, and 4 kg of potassium.

Each year, soil fertility diminishes unless fertilizers replace these lost minerals.

Additional irrigation is also necessary. More than 30% of the world's farmland suffers from

low productivity stemming from poor soil conditions.

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Fertilizers Fertilizers are essential. Commercially produced

fertilizers are enriched with nitrogen (N), phosphorus (P), and potassium (K).

They are labeled with a three-number code called the N-P-K ratio, indicating the content of these minerals.

A fertilizer marked as 15-10-5 indicates the percentage of each mineral.

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Fertilizers Manure, fish meal, and compost are called

organic fertilizers because they are of biological origin and contain decomposing organic material.

Before plants can use organic material, however, it must be decomposed into the inorganic nutrients that roots can absorb.

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Fertilizers Whether from organic fertilizer or a chemical

factory, the minerals a plant extracts are in the same form, but organic fertilizers release minerals gradually, whereas commercial fertilizers are immediately available but may not be retained by the soil for long.

Excess minerals not absorbed by the roots are usually wasted because they are leached from the soil by irrigation.

To make matters worse, mineral runoff may pollute groundwater, streams, and lakes.

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Fertilizers Agricultural researchers are developing ways to

maintain crop yields while reducing fertilizer use. One approach is to genetically engineer “smart”

plants that inform the grower when a nutrient deficiency is imminent, before damage has occurred.

One type of smart plant will produce a blue pigment in the leaves when phosphate is being depleted in the soil.

Therefore, the farmer can add phosphate without needing to add other minerals that would be wasted.

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Smart Plants

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Soil erosion Soil erosion is another main concern. Thousands of acres of topsoil is lost to

water and wind erosion each year in the United States alone.

Certain precautions, such as planting rows of trees as windbreaks, terracing hillside crops, and cultivating in a contour pattern, can prevent loss of topsoil.

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Windbreaks

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Terracing hillside crops

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Cultivating in a contour pattern

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Alfalfa vs. Corn Rows

Crops such as alfalfa and wheat provide good ground cover and protect the soil better then corn and other crops that are usually planted in more widely spaced rows.

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NITROGEN Nitrogen is often the mineral that has the greatest effect

on plant growth and crop yields. It is ironic that plants can suffer from nitrogen deficiency

because the atmosphere is nearly 80% nitrogen. However atmospheric nitrogen is in a gas form (N2) that

plants cannot use. For plants to absorb nitrogen, it must first be converted

to of ammonium (NH4) or nitrate (NO3). These absorbable forms of nitrogen do not come from

the breakdown of rock. They are generated by the decomposition of dead

vegetation by certain kinds of bacteria, called nitrogen-fixing bacteria.

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NITROGEN All life on Earth depends on these special

bacteria that can perform nitrogen fixation. Several species of these bacteria live freely in

the soil, while others live in plant roots in symbiotic relationships.

One of the most important crops that has this symbiotic relationship is the legume family, including peas, beans, soybeans, peanuts, alfalfa, and clover.

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NITROGEN Nitrogen-fixing bacteria live

in the nodules of these plants and generate more useful nitrogen for themselves and the soil than all industrial fertilizers.

When farmers plant the right amounts of these legumes at the right time, the soil becomes enriched at virtually no cost to the farmer.

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Crop rotation Crop rotation improves the quality of

the soil. In this practice, a non-legume such as

corn is planted one year, and the following year alfalfa or some other legume is planted to restore the concentration of nitrogen in the soil.

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PLANT BIOTECHNOLOGY Plant biotechnology refers to

innovations in the use of plants or substances obtained from plants to make products that are useful to humans.

Genetic engineering is a form of biotechnology that refers to the use of genetically modified organisms to produce beneficial results.

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PLANT BIOTECHNOLOGY

Corn is a staple crop in many developing countries, but the most common varieties are poor sources of protein, requiring that diets be supplemented with other protein sources, such as beans.

The proteins in the most popular variety of corn are very low in several essential amino acids that humans require in the diet.

Forty years ago, researchers discovered a new mutant species of corn that has much higher levels of these essential amino acids; this variety of corn is more nutritious.

Swine who are fed this variety of corn gained weight three times faster than those fed with normal corn. However, the kernels are soft and are more vulnerable to attack by pests.

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PLANT BIOTECHNOLOGY Using conventional methods, plant breeders crossbred the

soft kernel species with a more desirable type; this transition took hundreds of scientists nearly 20 years to accomplish. With modern methods of genetic engineering, one laboratory can accomplish this sort of thing in only a few years.

 Unlike traditional cross-breeding techniques, modern plant biotechnologists are not limited to transferring genes between closely related species of plants.

For instance, traditional breeding techniques could not be used to insert a desired gene from a daffodil plant into a rice plant. However, modern genetic engineering makes this possible.

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Reducing World Hunger and Malnutrition

There is much disagreement about the causes of such hunger. Some argue that there is a food shortage because the world is overpopulated. Others say that there is enough food available, but poor people cannot afford it. Whatever the cause, increasing food production is a humane objective.

800 million people on Earth suffer from nutritional deficiencies. 40,000 people die each day of malnutrition, half of them children.

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Reducing World Hunger and Malnutrition Because land and water are the most limiting

resources for food production, the best option will be to increase yields on the available land.

Based on estimates of population growth, the world's farmers will have to produce 40% more grain per acre to feed the human population in the year 2020.

Plant biotechnology can help make these crop yields possible.

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Transgenic crops Transgenic crops are those which contain genes from particular

bacteria that produce a protein that repels insect pests. When the gene from the bacteria is inserted into the plant, the

plant is now able to repel insects by itself, without the use of insecticide.

Examples of transgenic crops include cotton, corn, and potatoes.

This natural insecticide is completely harmless to humans and all other invertebrates because it is only activated by a substance found in the intestines of insects.

Researchers are also engineering plants with enhanced resistance to disease.

In one case, a transgenic papaya resistant to a ring spot virus was introduced into Hawaii, thereby saving its papaya industry.

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The Debate over Plant Biotechnology One concern about plant genetic engineering is that

certain molecules within a plant cause allergies in humans.

Some people are concerned that these allergy molecules will be transferred to a plant used for food.

However, biotechnologists remove the genes that encode for the allergenic proteins from soybeans and other crops.

So far, there is no evidence that genetically modified plants designed for human consumption have adverse effects on human health.

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The Debate over Plant Biotechnology In fact, some genetically modified foods are

potentially a healthier alternative. For example, a particular species of corn contains a

cancer-causing toxin that has been found in high concentrations in some batches of processed corn products ranging from corn flakes to beer.

This toxin is produced by a fungus that can infect corn which has been damaged by an insect.

Genetically modified corn contains 90% less of this toxin.

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The Debate over Plant Biotechnology

Nevertheless, because of health concerns, opponents lobby for the clear labeling of all foods containing products of genetically modified organisms (GMO).

Some people also argue for strict regulations against the mixing of GM foods with non-GM foods during transportation, storage, and processing.

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The Debate over Plant Biotechnology

Many ecologists are concerned that the growing of GM crops might have unforeseen effects on non-target organisms.

One study indicated that the caterpillars of Monarch butterflies died following consumption of milkweed leaves (their preferred food) which had been heavily dusted with pollen from genetically modified corn. This study has since been discredited.

As it turns out, when the original researchers showered the corn pollen onto the milkweed leaves in the laboratory experiment, other floral parts also rained onto the leaves.

Subsequent research found that it was these other floral parts, not the pollen, which contained a toxin that killed the butterflies.

Unlike pollen, these floral parts would not be carried by the wind to neighboring milkweed plants under natural field conditions.

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The Debate over Plant Biotechnology

Perhaps the most serious concern is the possibility of the introduced genes escaping from a transgenic crop into related weeds by natural cross-pollination.

The fear is that the undesirable weeds will become resistant to insects, creating a “superweed” that would be difficult to control in the field.

Because of this concern, efforts are underway to breed male sterility into transgenic crops.

These plants will still produce seeds and fruit if pollinated, but they will produce no pollen.

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The Debate over Plant Biotechnology One way to accomplish this is “Terminator

Technology” which uses “suicide genes” that disrupt critical developmental sequences, which prevent pollen development.

Plants that are genetically modified to undergo the Terminator process grow normally until the last stages of pollen maturation.

At this point, a gene expressing a particular protein becomes active and stops the pollen from forming.

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The Debate over Plant Biotechnology

On a case-by-case basis, scientists and the public must assess possible benefits of transgenic products versus the risks society is willing to take.

The best scenario is for these discussions and decisions to be based on sound scientific information and testing rather than on reflexive fear or blind optimism.

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Genetically Modified Foods: Discussion

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PLANT EVOLUTION AND DIVERSITY Plants are multicellular eukaryotes that

make organic molecules by photosynthesis.

Unlike algae, plants have growth regions called apical meristems as well as male and female gametangia (pollen and ovum) and multi-cellular, dependent embryos.

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PLANT EVOLUTION AND DIVERSITY According to the endosymbiotic theory

of the origin of chloroplasts, photosynthetic prokaryotic cells (bacteria)were ingested by larger cells in plants.

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PLANT EVOLUTION AND DIVERSITY Plants have always had chloroplasts,

even before they went from living in the oceans to living on land.

However, the key adaptations plants had to make before they could live on land are: flowers, dependent embryos, gametangia, organized vascular tissues, and seeds.

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PLANT EVOLUTION AND DIVERSITY Reproduction on land presents challenges. For algae, the surrounding water insures that

gametes and offspring stay moist and provides the means for their dispersal.

Plants, however, must keep their gametes and developing embryos from drying out in the air.

Land plants produce gametes in male and female gametangia (protective jackets around the gametes).

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PLANT EVOLUTION AND DIVERSITY The egg remains in the female gametangia and is fertilized

there. Pollen containing sperm are carried by the wind or by animals

toward the egg. The fertilized egg (zygote) develops into an embryo while

attached to and nourished by parent plant.

This is called a dependent embryo, which distinguishes plants from algae.

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PLANT EVOLUTION AND DIVERSITY Plants that produce seeds rely upon wind or

animals to disperse their offspring.

As a matter of fact, the key step in the adaptation of SEED PLANTS to dry land was the evolution of wind-dispersed pollen.

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PLANT EVOLUTION AND DIVERSITY Plant reproduction may also include the

production of spores which are encased in a protective jacket called a sporangium.

A spore is a cell that can develop into a new organism without fusing with another cell.

Plants that do not produce seeds (such as ferns) often rely on these tough-walled, resistant spores for dispersal.

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Fern Sporangium

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PLANT EVOLUTION AND DIVERSITY Among the earliest seed plants were the

gymnosperms, which are “naked seeds” because they are not enclosed in any chamber.

The largest group of gymnosperms is the conifers, consisting mainly of cone bearing trees such as pine, spruce, and fir.

Later on, flowering plants evolved, known as angiosperms.

The dominant types of seed plants today are the conifers and angiosperms.

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PARTS OF A FLOWER The anther is the male organ in which pollen

grains develop. A pollen grain is called a male gametophyte. Pollen grains develop in the anther (male

reproductive segment) and the pollen is transferred to the stigma (female reproductive segment).

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PARTS OF A FLOWER Sepals are green leaves which enclose the

flower before the flower opens. Petals are usually the most striking part of

a flower, and they function to attract hummingbirds and insects.

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PARTS OF A FLOWER Plants dependent on nocturnal pollinators

typically have flowers that are highly scented.

When the insect comes to collect the nectar, it picks up some pollen grains and carries them to the stigma of another flower.

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PARTS OF A FLOWER Fertilization in angiosperms usually

occurs immediately after pollination. The carpel consists of a stalk with the

stigma at the top (which catches the pollen) and an ovary (or ovule) at the base.

The ovary is a protective chamber where the eggs develop.

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PARTS OF A FLOWER The ripened ovary of a flower, which is adapted

to disperse seeds, is called a fruit. Fruits protect and help disperse seeds. Seeds develop within fruits, and the fruits

develop at the base of flowers.

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PARTS OF A FLOWER The structure of a fruit reflects its function in

seed dispersal. Some angiosperms depend on wind for seed

dispersal.

For example, the fruit of a maple tree acts like a propeller, spinning a seed away from the parent tree on wind currents.

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PARTS OF A FLOWER Some fruits hitch a ride on animals. The barbs of cockleburs hook to the fur of animals. These fruits may be carried for miles before they

open and release their seeds.

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Angiosperms Many angiosperms produce

fleshy, edible fruits that are attractive to animals as food.

When a mouse eats a berry, it digests the fleshy part of the fruit, but most of the tough seeds pass unharmed through its digestive tract.

The mouse may then deposit the seeds, along with a supply of natural fertilizer, some distance away from where it ate the fruit.

The dispersal of seeds in fruits is one of the main reasons angiosperms are so widespread and successful.

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Angiosperms Angiosperms often have mutually

dependent relationships with animals. They disperse their seeds by producing

fleshy, edible fruits that are consumed by animals which defecate the seeds; seeds sometimes attach to animals, or the seeds may catch the wind.

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Angiosperms

Most angiosperms depend on insects, birds, or mammals for pollination and seed dispersal and most land animals depend on angiosperms for food.

These mutual dependencies tend to improve the reproductive success of both the plants and animals.

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Angiosperms

Many angiosperms produce flowers that attract pollinators that rely entirely on the flower’s nectar and pollen for food.

Nectar is a high energy fluid that is of use to the plant only for attracting pollinators.

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Angiosperms The color and fragrance of a flower are usually

keyed to a particular type of animal or insect. Many flowers also have markings that attract

pollinators, leading them past pollen bearing organs on the way to the nectar.

For example, flowers that are pollinated by bees often have markings that reflect ultraviolet light.

Such markings are invisible to us, but vivid to bees.

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Angiosperms Many flowers pollinated by birds are red or

pink, colors to which bird eyes are especially sensitive.

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Angiosperms The shape of the flower may also be important. Flowers that depend largely on hummingbirds, for

example, typically have their nectar located deep in a floral tube, where only the long, thin beak and tongue of a hummingbird are likely to reach.

155Angiosperms Insects and birds are active mainly during the

day. Some flowering plants, however, depend on

nocturnal pollinators, such as bats. These plants typically have large, light

colored, highly scented flowers that can easily be found at night.

156Angiosperms An example of this is a cactus. As the bats eat part of the flower, its body

becomes dusted with pollen which it passes on to other flowers.

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Angiosperms Human agriculture is based almost entirely on

angiosperms. Whereas gymnosperms supply most of our lumber

and paper, flowering plants provide nearly all our food.

Corn, rice, wheat, and other grains are dried fruits, the main food source for most of the world’s population and their domestic animals.

Many food crops are fleshy fruits, such as strawberries, apples, cherries, oranges, tomatoes, squash, and cucumbers.

Others are modified roots, such as carrots and sweet potatoes, or modified stems, such as onions and potatoes.

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Angiosperms We also grow angiosperms for spices,

fiber, medications, perfumes, and decoration.

Hardwoods, such as oak, cherry, and walnut, are flowering plants.

Two of the world's most popular beverages come from coffee beans and tea leaves, and cocoa and chocolate also come from angiosperms.

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Videos Non-vascular plants 3 mins Seed plants 4 mins Parasitic Plants 3 mins Prehistoric Plants 2 mins Tour of a Plant Cell 2 mins

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