SEED VASCULAR PLANTS Division Cycadophyta palms naked seeds

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SEED VASCULAR PLANTS • Division Cycadophyta: palms (naked seeds) • Division Ginkgophyta: ginkgo (naked

SEED VASCULAR PLANTS • Division Cycadophyta: palms (naked seeds) • Division Ginkgophyta: ginkgo (naked seeds) • Division Gnetophyta: only few species (naked seeds) • Division Coniferophyta: cone plants: pine, spruce, fir, larch, yew (naked seeds) • Division Anthophyta: flowering plants (seeds in fruit)

DIVISION ANTHOPHYTA • • Flowering plants Sex organs in flowers Seeds in ovary that

DIVISION ANTHOPHYTA • • Flowering plants Sex organs in flowers Seeds in ovary that ripens into fruit Two classes: – Monocotyledoneae (Monocot) – Dicotyledoneae (Dicot)

Roots, Stems, and Leaves • Cacti leaves are modified into thin, sharp spines •

Roots, Stems, and Leaves • Cacti leaves are modified into thin, sharp spines • The reduced-leaf surface area prevents excess water loss

Roots, Stems, and Leaves

Roots, Stems, and Leaves

Specialized Tissues in Plants • If you look deep inside a living plant, that

Specialized Tissues in Plants • If you look deep inside a living plant, that first impression of inactivity vanishes • Instead, you will find a busy and complex organism packed with specialized systems and subsystems • Materials move throughout the plant, and growth and repair take place continuously • Plants may act at a pace that seems slow to us, but their cells work together in remarkably effective ways to ensure the plant's survival

Seed Plant Structure • The cells of a seed plant are organized into different

Seed Plant Structure • The cells of a seed plant are organized into different tissues and organs, as shown in the figure • Three of the principal organs of seed plants are roots, stems, and leaves • These organs are linked together by systems and subsystems that run the length of the plant, performing functions such as transport and protection and coordinating plant activities

Tissues in a Vascular Plant • Vascular plants consist of roots, stems, and leaves

Tissues in a Vascular Plant • Vascular plants consist of roots, stems, and leaves • Each of these organs contains dermal tissue, vascular tissue, and ground tissue, as shown by the cross sections of the leaf, stem, and root

Tissues in a Vascular Plant

Tissues in a Vascular Plant

Roots • The root system of a plant absorbs water and dissolved nutrients •

Roots • The root system of a plant absorbs water and dissolved nutrients • Roots anchor plants in the ground, holding soil in place and preventing erosion • Root systems also protect the plant from harmful soil bacteria and fungi, transport water and nutrients to the rest of the plant, and hold plants upright against forces such as wind and rain

Stems • A stem has a support system for the plant body, a transport

Stems • A stem has a support system for the plant body, a transport system that carries nutrients, and a defense system that protects the plant against predators and disease • Stems can be as short as a few millimeters or as tall as 100 meters • Whatever its size, the support system of a stem must be strong enough to hold up its leaves and branches • Similarly, the stem's transport system must contain subsystems that can lift water from roots up to the leaves and carry the products of photosynthesis from the leaves back down to the roots

Leaves • Leaves are the plant's main photosynthetic systems • The broad, flat surfaces

Leaves • Leaves are the plant's main photosynthetic systems • The broad, flat surfaces of many leaves help increase the amount of sunlight plants absorb • Leaves also expose a great deal of tissue to the dryness of the air and, therefore, must contain subsystems to protect against water loss • Adjustable pores in leaves help conserve water while letting oxygen and carbon dioxide enter and exit the leaf

Plant Tissue Systems • Within the roots, stems, and leaves of plants are specialized

Plant Tissue Systems • Within the roots, stems, and leaves of plants are specialized tissue systems • Plants consist of three main tissue systems: – Dermal tissue: is the skin of a plant in that it is the outmost layer of cells – Vascular tissue: is the plant's bloodstream, transporting water and nutrients throughout the plant – Ground tissue: and ground tissue is everything else

Dermal Tissue • The outer covering of a plant consists of dermal tissue, which

Dermal Tissue • The outer covering of a plant consists of dermal tissue, which typically consists of a single layer of epidermal cells • The outer surfaces of these are often covered with a thick waxy layer that protects against water loss and injury – The thick waxy coating of the epidermal cells is known as the cuticle • Some epidermal cells have tiny projections known as trichomes, which help protect the leaf and also give it a fuzzy trichomes appearance • In roots, dermal tissue includes root hair cells that provide a large amount of surface area and aid in water absorption • On the underside of leaves, dermal tissue contains guard cells, guard cells which regulate water loss and gas exchange

Vascular Tissue • • Vascular tissue forms a transport system that moves water and

Vascular Tissue • • Vascular tissue forms a transport system that moves water and nutrients throughout the plant The principal subsystems in vascular tissue are xylem, a waterxylem conducting tissue, and phloem, a phloem food-conducting tissue Vascular tissue contains several types of specialized cells: – Xylem consists of tracheids and Xylem vessel elements – Phloem consists of sieve tube Phloem elements and companion cells As you can see in the figure, both xylem and phloem are made up of networks of hollow connected cells that carry fluids throughout the plant

Vascular Tissue in a Stem • Vascular tissue is made up of several different

Vascular Tissue in a Stem • Vascular tissue is made up of several different types of cells • Xylem consists of tracheids and vessel elements • Phloem consists of sieve tube elements and companion cells • Xylem tissue (left) conducts water from the roots to the rest of the plant • Phloem tissue (right) conducts a variety of materials, mostly carbohydrates, throughout a plant

Vascular Tissue in a Stem

Vascular Tissue in a Stem

Xylem • All seed plants have a type of xylem cell called a tracheid

Xylem • All seed plants have a type of xylem cell called a tracheid • Recall that tracheids are long, narrow cells with walls that are impermeable to water – These walls, however, are pierced by openings that connect neighboring cells to one another – When tracheids mature, they die, and their cytoplasm disintegrates

Xylem • Angiosperms have another kind of xylem cell that is called a vessel

Xylem • Angiosperms have another kind of xylem cell that is called a vessel element – Vessel elements are much wider than tracheids – Like tracheids, they mature and die before they conduct water • Vessel elements are arranged end to end on top of one another like a stack of tin cans • The cell walls at both ends are lost when the cells die, transforming the stack of vessel elements into a continuous tube through which water can move freely

Phloem • The main phloem cells are sieve tube elements – These cells are

Phloem • The main phloem cells are sieve tube elements – These cells are arranged end to end, like vessel elements, to form sieve tubes – The end walls of sieve tube elements have many small holes in them • Materials can move through these holes from one adjacent cell to another • As sieve tube elements mature, they lose their nuclei and most of the other organelles in their cytoplasm – The remaining organelles hug the inside of the cell wall • The rest of the space is a pipeline through which sugars and other foods are carried in a watery stream

Phloem • Companion cells are phloem cells that Companion cells surround sieve tube elements

Phloem • Companion cells are phloem cells that Companion cells surround sieve tube elements – Companion cells keep their nuclei and other organelles through their lifetime • Companion cells support the phloem cells and aid in the movement of substances in and out of the phloem stream

Ground Tissue • The cells that lie between dermal and vascular tissues make up

Ground Tissue • The cells that lie between dermal and vascular tissues make up the ground tissues, shown in figure • In most plants, ground tissue consists mainly of parenchyma – Parenchyma cells have thin cell walls and large central vacuoles cells surround by a thin layer of cytoplasm • In leaves, these cells are packed with chloroplasts and are the site of most of a plant's photosynthesis • Ground tissue may also contain two types of cells with thicker cell walls: – Collenchyma cells have strong, flexible cell walls that help support Collenchyma larger plants • Collenchyma cells make up the familiar “strings” of a stalk of celery – Sclerenchyma cells have extremely thick, rigid cell walls that make Sclerenchyma ground tissue tough and strong

Ground Tissues • • • Ground tissue is made of cells whose cell walls

Ground Tissues • • • Ground tissue is made of cells whose cell walls have different thicknesses Parenchyma cells have thin walls and function mainly in storage and photosynthesis The root cells shown are filled with purple-staining starch grains Collenchyma and sclerenchyma cells both function in support Collenchyma cells have irregularly shaped walls, but the walls of sclerenchyma cells are much thicker and harder

Ground Tissues

Ground Tissues

Plant Growth and Meristematic Tissue • Most plants have a method of development that

Plant Growth and Meristematic Tissue • Most plants have a method of development that involves an open, or indeterminate, type of growth – Indeterminate growth means that they grow and produce new cells at the tips of their roots and stems for as long as they live • These cells are produced in meristems, clusters of meristems tissue that are responsible for continuing growth throughout a plant's lifetime • The new cells produced in meristematic tissue are undifferentiated—that is, they have not yet become specialized for specific functions, such as transport

Plant Growth and Meristematic Tissue • Near the end, or tip, of each growing

Plant Growth and Meristematic Tissue • Near the end, or tip, of each growing stem and root is an apical meristem • An apical meristem is a group of undifferentiated cells that divide to produce increased length of stems and roots • The figure at right shows examples of root and shoot apical meristems • Meristematic tissue is the only plant tissue that produces new cells by mitosis

Plant Growth and Meristematic Tissue • Meristematic tissue produces new cells by mitosis •

Plant Growth and Meristematic Tissue • Meristematic tissue produces new cells by mitosis • Apical meristems, which consist of many actively dividing cells, are located at the tips of shoots tips (left) and roots (right) • The apical meristem of a root is surrounded by a root cap that protects the root as it grows through the soil

Plant Growth and Meristematic Tissue

Plant Growth and Meristematic Tissue

Plant Growth and Meristematic Tissue • At first, the cells that originate in meristems

Plant Growth and Meristematic Tissue • At first, the cells that originate in meristems look very much alike: They divide rapidly and have thin cell walls • Gradually, these cells develop into mature cells with specialized structures and functions, a process called differentiation • As these cells differentiate, they produce each of the tissue systems of the plant, including dermal, ground, and vascular tissue

Plant Growth and Meristematic Tissue • The highly specialized cells found in flowers, which

Plant Growth and Meristematic Tissue • The highly specialized cells found in flowers, which make up the reproductive systems of flowering plants, are also produced in meristems – Flower development begins when certain genes are turned on in a shoot apical meristem • The actions of these genes transform the apical meristem into a floral meristem, producing the modified leaves that become the flower's colorful petals, as well as the reproductive tissues of the flower • Many plants also grow in width as a result of meristematic tissue that lines the stems and roots of a plant

Roots • As soon as a seed begins to grow, it puts out its

Roots • As soon as a seed begins to grow, it puts out its first root to draw water and nutrients from the soil • Other roots soon branch out from this first root, adding length and surface area to the root system • The overall size of a plant's root system can be astonishing: The total surface area of the root system of a rye plant was measured at more than 600 square meters— 130 times greater than the combined surface areas of both the stems and leaves

ROOT • Absorbs water and dissolved mineral for the plant • Anchors the plant

ROOT • Absorbs water and dissolved mineral for the plant • Anchors the plant in the soil • Types: – Fibrous: • Greatly branched – Tap: • Single large root

Types of Roots • • The two main types of roots are: – Taproots,

Types of Roots • • The two main types of roots are: – Taproots, which are found mainly in dicots – Fibrous roots, which are found mainly in monocots In some plants, the primary root grows long and thick while the secondary roots remain small – This type of primary root is called a taproot, shown in figure – Taproots of oak and hickory trees grow so long that they can reach water far below Earth's surface – Carrots, dandelions, beets, and radishes have short, thick taproots that store sugars or starches

Types of Roots • Plants have taproots, fibrous roots, or both • Taproots have

Types of Roots • Plants have taproots, fibrous roots, or both • Taproots have a central primary root and generally grow deep into the soil • Fibrous roots are usually shallow and consist of many thin roots.

Types of Roots

Types of Roots

Types of Roots • In other plants, such as grasses, fibrous roots branch to

Types of Roots • In other plants, such as grasses, fibrous roots branch to such an extent that no single root grows larger than the rest • The extensive fibrous root systems produced by many plants help prevent topsoil from being washed away by heavy rain

Root Structure and Growth • • • Roots contain cells from the three tissue

Root Structure and Growth • • • Roots contain cells from the three tissue systems —dermal, vascular, and ground tissue A mature root has an outside layer, the epidermis, and a central cylinder of vascular tissue Between these two tissues lies a large area of ground tissue The root system plays a key role in water and mineral transport Its cells and tissues, as shown in the figure, contain a number of subsystems that carry out these functions The root's epidermal subsystem performs the dual functions of protection and absorption Its surface is covered with tiny cellular projections called root hairs These hairs penetrate the spaces between soil particles and produce a large surface area through which water can enter the plant Just inside the epidermis is a spongy layer of ground tissue called the cortex This layer extends to another layer of cells, the endodermis – The endodermis completely encloses the root's vascular subsystem in a region called the vascular cylinder

Root Structure and Growth • • A root consists of a central vascular cylinder

Root Structure and Growth • • A root consists of a central vascular cylinder surrounded by ground tissue and the epidermis Compare how cells in different regions of the root are structurally specialized for different functions Root hairs along the surface of the root aid in water absorption Only the cells in the root tip divide In the area just behind the root tip, the newly divided cells increase in length, pushing the root tip farther into the soil The root cap, located just ahead of the The root cap root tip, protects the dividing cells as they are pushed forward Dicot roots, such as the one shown in the cross section, have a central column of xylem cells arranged in a radiating pattern How are root hairs structurally specialized?

Root Structure and Growth

Root Structure and Growth

Root Structure and Growth • Roots grow in length as their apical meristem produces

Root Structure and Growth • Roots grow in length as their apical meristem produces new cells near the root tip – These fragile new cells are covered by a tough root cap that protects the root as it forces its way through the soil – As the root grows, the root cap secretes a slippery substance that lubricates the progress of the root through the soil • Cells at the very tip of the root cap are constantly being scraped away, and new root cap cells are continually added by the meristem • Most of the increase in root length occurs immediately behind the meristem, where cells are growing longer • At a later stage, these cells mature and take on specialized functions • The process by which unspecialized cells change to become specialized in structure and function is known as cell differentiation

ROOT GROWTH • • Meristem: – Located just behind the root cap – Zone

ROOT GROWTH • • Meristem: – Located just behind the root cap – Zone of rapidly dividing cells (become cells in the zone of elongation or root cap) Root cap: – Tip of root – Protects meristem – As root grows through the soil, cells are rubbed off and replaced by meristematic cells Zone of elongation: – Cells cease to divide but enlarge – Enlargement of cells pushes the root deeper into the soil Zone of maturation: – Cells differentiate

Root Functions • Roots anchor a plant in the ground absorb water and dissolved

Root Functions • Roots anchor a plant in the ground absorb water and dissolved nutrients from the soil • How does a root go about the job of absorbing water and minerals from the soil? – Although it might seem to, water does not just “soak” into the root from soil • It takes energy on the part of the plant to absorb water • Our explanation of this process begins with a description of soil and plant nutrients

ROOT FUNCTION • • Anchors the plant in the soil Absorption of water and

ROOT FUNCTION • • Anchors the plant in the soil Absorption of water and dissolved mineral – Root hair • Fingerlike extension of a single epidermal cell • Greatly increase surface area of the root enabling the root to absorb water and dissolved mineral from the soil • Contains many dissolved substances such as minerals, sugars, and amino acids – Soil contains fewer dissolved substances – Concentration gradient develops between the soil and root hair – Water moves into the root hair by osmosis Absorbs macronutrients (Nitrogen/Potassium) in large amount and micronutrients (Manganese) in small amounts by active transport Food storage

Uptake of Plant Nutrients • An understanding of soil helps explain how plants function

Uptake of Plant Nutrients • An understanding of soil helps explain how plants function • Soil is a complex mixture of sand, silt, clay, air, and bits of decaying animal and plant tissue • Soil in different places and at different depths contains varying amounts of these ingredients • Sandy soil, for example, is made of large particles that retain few nutrients, whereas the finely textured silt and clay soils of the Midwest and southeastern United States are high in nutrients • The ingredients define the soil and determine, to a large extent, the kinds of plants that can grow in it

Uptake of Plant Nutrients • • • To grow, flower, and produce seeds, plants

Uptake of Plant Nutrients • • • To grow, flower, and produce seeds, plants require a variety of inorganic nutrients in addition to carbon dioxide and water – The most important of these nutrients are nitrogen, phosphorus, potassium, magnesium, and calcium The functions of these essential nutrients within a plant are described in the table These nutrients are located in varying amounts in the soil and are drawn up by the roots of a plant In addition to these essential nutrients, trace elements are required in small quantities to maintain proper plant growth Trace elements include sulfur, iron, zinc, molybdenum, boron, copper, manganese, and chlorine Large amounts of trace elements in the soil can be poisonous

Essential Plant Nutrients • Soil contains several nutrients that are essential for plant growth

Essential Plant Nutrients • Soil contains several nutrients that are essential for plant growth • Each nutrient plays a different role in plant functioning and development, and produces distinct effects when deficient in the soil • If you notice that a plant is becoming paler and more yellow, what nutrient might need to be added?

Essential Plant Nutrients

Essential Plant Nutrients

Active Transport of Minerals • The cell membranes of root hairs and other cells

Active Transport of Minerals • The cell membranes of root hairs and other cells in the root epidermis contain active transport proteins – These proteins use ATP (an energy source) to pump mineral ions from the soil into the plant • The high concentration of mineral ions in the plant cells causes water molecules to move into the plant by osmosis, as shown in the figure at right

Active Transport and Osmosis in a Root • • • Roots absorb water and

Active Transport and Osmosis in a Root • • • Roots absorb water and dissolved nutrients from the soil Most water and minerals enter a plant through the tiny root hairs Water moves into the cortex, through the cells of the endodermis, and into the vascular cylinder Finally, water reaches the xylem, where it is transported throughout the plant Cells in the endodermis are made waterproof by the Casparian strip The Casparian strip is another example of how cells are specialized to perform a particular function—in this case, preventing the backflow of water out of the vascular cylinder into the root cortex

Active Transport and Osmosis in a Root

Active Transport and Osmosis in a Root

Active Transport of Minerals • • You may recall that osmosis is the movement

Active Transport of Minerals • • You may recall that osmosis is the movement of water across a membrane toward an area where the concentration of dissolved material is higher – By using active transport to accumulate ions from the soil, cells of the root epidermis create conditions under which osmosis causes water to “follow” those ions and flow into the root Note that the root does not actually pump water – But by pumping dissolved minerals into its own cells, the end result is almost the same— the water moves from the epidermis through the cortex into the vascular cylinder

Movement Into the Vascular Cylinder • Both osmosis and active transport cause water and

Movement Into the Vascular Cylinder • Both osmosis and active transport cause water and minerals to move from the root epidermis into the cortex – From there, the water and dissolved minerals pass the inner boundary of the cortex and enter the endodermis • This process is shown in the figure Active Transport and Osmosis in a Root

Movement Into the Vascular Cylinder • • The endodermis encloses the vascular cylinder and

Movement Into the Vascular Cylinder • • The endodermis encloses the vascular cylinder and stretches up and down the entire length of the root, like a cylinder – It is composed of many individual cells, each shaped a bit like a brick – Each of these cells is surrounded on four sides by a waterproof strip called a Casparian strip To imagine what the Casparian strip looks like, think of a brick with a thick rubber band stretched around it The rubber bands stick together like mortar between the bricks Imagine many of these bricks placed edge to build a cylinder – When a root is viewed in cross section, the endodermis forms a circle

Movement Into the Vascular Cylinder • Recall that water moves into the vascular cylinder

Movement Into the Vascular Cylinder • Recall that water moves into the vascular cylinder by osmosis • Because water and minerals cannot pass through the waxy Casparian strip, once they pass through the endodermis, they are trapped in the vascular cylinder • As a result, there is a oneway passage of materials into the vascular cylinder in plant roots

Root Pressure • Why do plants “need” a system that ensures the one-way movement

Root Pressure • Why do plants “need” a system that ensures the one-way movement of water and minerals? – That system is how the plant generates enough pressure to move water out of the soil and up into the body of the plant • As minerals are pumped into the vascular cylinder, more and more water follows by osmosis, producing a strong pressure – If the pressure were not contained, roots would expand as they filled with water

Root Pressure • Instead, contained within the Casparian strip, the water has just one

Root Pressure • Instead, contained within the Casparian strip, the water has just one place to go—up • Root pressure, produced within the cylinder by active transport, Root pressure forces water through the vascular cylinder and into the xylem • As more water moves from the cortex into the vascular cylinder, more water in the xylem is forced upward through the root into the stem • In the figure, you can see a demonstration of root pressure in a carrot root • Root pressure is the starting point for the movement of water through the vascular system of the entire plant • But it is just the beginning • Once you have learned about stems and leaves, you will see how water and other materials are transported within an entire plant

Demonstration of Root Pressure • As a carrot root absorbs water, root pressure forces

Demonstration of Root Pressure • As a carrot root absorbs water, root pressure forces water upward into the glass tube, which takes the place of the stem and leaves of the carrot in this demonstration

Demonstration of Root Pressure

Demonstration of Root Pressure

Stems • What do a barrel cactus, a tree trunk, a dandelion stem, and

Stems • What do a barrel cactus, a tree trunk, a dandelion stem, and a potato have in common? – They are all types of stems • Stems vary in size, shape, and method of development – Some grow entirely underground; others reach high into the air • Stems also vary in structure and internal arrangement of cells

Stem Structure and Function • In general, stems have three important functions: – They

Stem Structure and Function • In general, stems have three important functions: – They produce leaves, branches, and flowers – They hold leaves up to the sunlight – They transport substances between roots and leaves • Stems make up an essential part of the water and mineral transport systems of the plant • The vascular tissue in stems conducts water, nutrients, and other compounds throughout the plant • Xylem and phloem, the major subsystems of the transport system, form continuous tubes from the roots through the stems to the leaves – These vascular tissues link all parts of the plant, allowing water and nutrients to be carried throughout the plant • In many plants, stems also function as storage systems and in the process of photosynthesis

Stem Structure and Function • Like the rest of the plant, the stem is

Stem Structure and Function • Like the rest of the plant, the stem is composed of three tissue systems: dermal, vascular, and ground tissue • Stems are surrounded by a layer of epidermal cells that have thick cell walls and a waxy protective coating

STEM FUNCTION • Three functions: – Support – Transport – Storage

STEM FUNCTION • Three functions: – Support – Transport – Storage

Stem Structure and Function • • • In most plants, stems contain distinct nodes,

Stem Structure and Function • • • In most plants, stems contain distinct nodes, where leaves are attached, and internode regions between the nodes, as shown in figure – Small buds are found where leaves attach to the nodes Buds contain undeveloped tissue that can produce new stems and leaves In larger plants, stems develop woody tissue that helps support leaves and flowers

Buds, Nodes, and Internodes • Stems produce leaves and branches and hold leaves up

Buds, Nodes, and Internodes • Stems produce leaves and branches and hold leaves up to the sunlight, where they carry out photosynthesis • Leaves are attached to a stem at structures called nodes • These nodes are separated by regions of the stem called internodes

Buds, Nodes, and Internodes

Buds, Nodes, and Internodes

Stems Adapted for Storage and Dormancy • For example, many kinds of plants have

Stems Adapted for Storage and Dormancy • For example, many kinds of plants have modified stems that store food • Tubers, rhizomes, bulbs, and corms can remain dormant during cold or dry periods until favorable conditions for growth return • Examples of these are shown in the figure at right

Stems Adapted for Storage and Dormancy • Many kinds of plants have modified stems

Stems Adapted for Storage and Dormancy • Many kinds of plants have modified stems that store food • Tubers, rhizomes, bulbs, and corms can remain dormant during cold or dry periods until favorable conditions for growth return

Stems Adapted for Storage and Dormancy

Stems Adapted for Storage and Dormancy

STEM TYPES • Different types: – – – Upright: herbaceous (green) / woody Stolon:

STEM TYPES • Different types: – – – Upright: herbaceous (green) / woody Stolon: grow along ground surface (strawberry) Tuber: food storage (potato) Fleshy: water storage and photosynthesis (cactus) Corms: food storage (gladiolus/crocus/some begonias) – Bulb: food storage (tulip/onion/daffodil) – Rhizome: grow horizontally underground (iris/water hyacinth/some types of blueberries)

Monocot and Dicot Stems • • • The arrangement of tissues in a stem

Monocot and Dicot Stems • • • The arrangement of tissues in a stem differs among seed plants In monocots, vascular bundles are scattered throughout the stem In dicots and most gymnosperms, vascular bundles are arranged in a cylinder Recall that monocots and dicots are two types of flowering plants, or angiosperms For a comparison of monocot and dicot stems, look at the figure

Monocot and Dicot Stems • The arrangement of vascular bundles in the stem of

Monocot and Dicot Stems • The arrangement of vascular bundles in the stem of a monocot differs from that in the stem of a dicot • In a monocot, vascular bundles are scattered throughout the stem • In a dicot, vascular bundles are arranged in a ring

Monocot and Dicot Stems

Monocot and Dicot Stems

Monocot Stems • The cross section of a young monocot stem shows all three

Monocot Stems • The cross section of a young monocot stem shows all three tissue systems clearly • The stem has a distinct epidermis, which encloses a series of vascular bundles, each of which contains xylem and phloem tissue – Phloem faces the outside of the stem, and xylem faces the center • In monocots, these bundles are scattered throughout the ground tissue • The ground tissue is fairly uniform, consisting mainly of parenchyma cells

Dicot Stems • Young dicot stems have vascular bundles, but they are generally arranged

Dicot Stems • Young dicot stems have vascular bundles, but they are generally arranged in an organized, ringlike pattern • The parenchyma cells inside the ring of vascular tissue are known as pith, while those outside form the cortex of the stem • In dicots, these relatively simple tissue patterns become more complex as the plant grows larger and the stem increases in diameter

STEM STRUCTURE • Terminal bud: – Tip of a twig (develops into leaves or

STEM STRUCTURE • Terminal bud: – Tip of a twig (develops into leaves or flowers) – Contains meristem tissue which adds to the length of the stem • Lateral bud: – Above leaf scars (scars left by vascular bundle of dead leaves) – Adds lateral branches • Lenticels: raised areas (blisters) that allow the exchange of gases between the atmosphere and the inner tissues of the stem • Vascular bundles: – Monocot: scattered – Dicot: arranged in a ring

Primary Growth of Stems • • • Plants grow in ways that are distinctly

Primary Growth of Stems • • • Plants grow in ways that are distinctly different from other organisms – For their entire life, new cells are produced at the tips of roots and shoots This method of growth, occurring only at the ends of a plant, is called primary growth The increase in length produced by primary growth from year to year is shown in the figure Primary growth of stems is produced by cell divisions in the apical meristem It takes place in all seed plants

Primary Growth in a Stem • All seed plants undergo primary growth, which is

Primary Growth in a Stem • All seed plants undergo primary growth, which is an increase in length • Every year, apical meristems, shown in red, divide to produce new growth • The primary growth for one season consists of a stem and several leaves

Primary Growth in a Stem

Primary Growth in a Stem

STEM GROWTH • Vertical growth (height)only at the tip • Vertical/horizontal growth at buds

STEM GROWTH • Vertical growth (height)only at the tip • Vertical/horizontal growth at buds • Horizontal growth (circumference) at the lateral meristem – Vascular Cambium: • Between the xylem and phloem • Zone of cell division adding cells to the phloem (outside) and xylem (inside) – Spring: cells wide and thin – Summer: cells small and thick » Density difference creates annual rings

Secondary Growth of Stems • If a plant is to grow larger year after

Secondary Growth of Stems • If a plant is to grow larger year after year, its stems must increase in thickness as well as in length – They have more mass to support and more fluid to move through their vascular tissues • Yet, only meristematic tissue can produce new cells for growth • Some monocots, such as palm trees, produce thick stems from a meristem that becomes wider as the plant grows • However, most monocots, such as grasses, produce only fleshy growth and do not grow very tall • Many dicots grow extremely tall and also grow in width to support this extra weight – This growth occurs as a result of meristems other than the apical meristem

Secondary Growth of Stems • • The method of growth in which stems increase

Secondary Growth of Stems • • The method of growth in which stems increase in width is called secondary growth In the figure at right, you can see the pattern of secondary growth in a dicot stem In conifers and dicots, secondary growth takes place in lateral meristematic tissues called the vascular cambium and cork cambium The type of lateral meristematic tissue called vascular cambium produces vascular tissues and increases the thickness of stems over time Cork cambium produces the outer covering of stems Another kind of cambium enables roots to grow thicker and branch The addition of new tissue in these cambium layers increases the thickness of the stem

Secondary Growth in a Dicot Stem • Dicots produce secondary growth from meristematic tissue

Secondary Growth in a Dicot Stem • Dicots produce secondary growth from meristematic tissue called vascular cambium • This tissue forms between the xylem and phloem of the individual vascular bundles, as shown in A • Once the tissue forms, as shown in B, it divides to produce xylem cells toward the center of the stem and phloem cells toward the outside • These different tissues form the bark and wood of a mature stem, shown in C

Secondary Growth in a Dicot Stem

Secondary Growth in a Dicot Stem

Formation of the Vascular Cambium • In a young dicot stem produced by primary

Formation of the Vascular Cambium • In a young dicot stem produced by primary growth, bundles of xylem and phloem are arranged in a ring • Once secondary growth begins, the vascular cambium appears as a thin layer situated between clusters of vascular tissue • This new meristematic tissue forms between the xylem and phloem of each vascular bundle • Divisions in the vascular cambium give rise to new layers of xylem and phloem – As a result, the stem becomes wider – The cambium continues to produce new layers of vascular tissue, causing the stem to become thicker and thicker

Formation of Wood • • Most of what we call “wood” is actually layers

Formation of Wood • • Most of what we call “wood” is actually layers of xylem – These cells build up year after year, layer on layer As woody stems grow thicker, the older xylem near the center of the stem no longer conducts water and instead becomes what is known as heartwood – Heartwood usually darkens with age because it accumulates impurities that cannot be removed Heartwood is surrounded by sapwood, sapwood which is active in fluid transport and therefore usually lighter in color Both heartwood and sapwood are shown in the figure at left

Layers in a Mature Tree • In a mature tree that has undergone several

Layers in a Mature Tree • In a mature tree that has undergone several years of secondary growth, the vascular cambium lies vascular cambium between layers of xylem to the inside, and layers of phloem to the outside • The youngest xylem, called sapwood, transports water and minerals • Which layer contains meristematic cells?

Layers in a Mature Tree

Layers in a Mature Tree

Formation of Wood • • • In most of the temperate zone, tree growth

Formation of Wood • • • In most of the temperate zone, tree growth is seasonal When growth begins in the spring, the vascular cambium begins to grow rapidly, producing large, lightcolored xylem cells with thin cell walls – The result is a light-colored layer of wood called early wood As the growing season continues, the cells become smaller and have thicker cell walls, forming a layer of dark wood – This darker wood is called late wood

Formation of Wood • This alternation of dark and light wood produces what we

Formation of Wood • This alternation of dark and light wood produces what we commonly call tree rings – Each ring is composed of a band of light wood and a band of dark wood – Thus, a ring corresponds to a year of growth – By counting the rings in a cross section of a tree, you can estimate its age • The size of the rings may even provide information about weather conditions, such as wet or dry years – Thick rings indicate that weather conditions were favorable for tree growth, whereas thin rings indicate less favorable conditions

Formation of Bark • • • On most trees, bark includes all of the

Formation of Bark • • • On most trees, bark includes all of the tissues outside the vascular cambium, as shown in the figure at right – These tissues include phloem, the cork cambium, and cork How does bark form? Picture a tree as new xylem is being laid down It is expanding in width, or girth Recall that the phloem tissue lies to the outside of this xylem Phloem must grow to accommodate the larger size of the tree As the vascular cambium increases in diameter, it forces the phloem tissue outward This expansion causes the oldest tissues to split and fragment as they are stretched by the expanding stem Were this expansion left unchecked, the outer covering of the stem might eventually split and break

Formation of Bark • Another layer of growing tissue, the cork cambium, solves this

Formation of Bark • Another layer of growing tissue, the cork cambium, solves this potential problem • The cork cambium surrounds the cortex and produces a thick protective layer of cork • Cork consists of cells that have thick walls and usually contain fats, oils, or waxes • These waterproof substances help prevent the loss of water from the stem • The outermost cork cells are usually dead • As the stem increases in size, this dead bark often cracks and flakes off in strips or patches

Leaves • The leaves of a plant are its main organs of photosynthesis –

Leaves • The leaves of a plant are its main organs of photosynthesis – In a sense, plant leaves are the world's most important manufacturers of food • Sugars, starches, and oils manufactured by plants in their leaves are sources of food for virtually all land animals

Leaves • Recall from Chapter 8 that photosynthesis uses carbon dioxide and water to

Leaves • Recall from Chapter 8 that photosynthesis uses carbon dioxide and water to produce sugars and oxygen • Leaves, therefore, must have a way of obtaining the materials needed for photosynthesis as well as distributing its end products • Much of the internal structure of leaves can be understood in terms of their functions in carrying out photosynthesis

LEAF • Main organ of photosynthesis – Uses carbon dioxide and water in the

LEAF • Main organ of photosynthesis – Uses carbon dioxide and water in the presence of sunlight, enzymes, and chlorophyll to produce sugar and oxygen

Leaf Structure • • The structure of a leaf is optimized for absorbing light

Leaf Structure • • The structure of a leaf is optimized for absorbing light and carrying out photosynthesis As you can see in the figure below, leaves may differ greatly in shape, yet share certain structural features To collect sunlight, most leaves have thin, flattened sections called blades The blade is attached to the stem by a thin stalk called a petiole Like roots and stems, leaves have an outer covering of dermal tissue and inner regions of ground and vascular tissues As shown in the figure at right, leaves are covered on the top and bottom by epidermis made of a layer of tough, irregularly shaped cells The epidermis of many leaves is also covered by the cuticle Together, the cuticle and epidermal cells form a waterproof barrier that protects tissues and limits the loss of water through evaporation

Tissues in a Leaf • Leaves absorb light and carry out most of the

Tissues in a Leaf • Leaves absorb light and carry out most of the photosynthesis in a plant • Some of the most important manufacturing sites on Earth are found in the leaves of plants • The cells in plant leaves are able to use light energy to make carbohydrates • Compare the structure of the different kinds of cells in a leaf

Tissues in a Leaf

Tissues in a Leaf

LEAF PARTS • Node: region of the stem where the leaf is attached •

LEAF PARTS • Node: region of the stem where the leaf is attached • Leaf: – Two parts: • Blade: flattened portion • Petiole: stemlike portion that connects the leaf to the stem • Bud: at the base of each petiole • Veins: vascular bundles (xylem/phloem) – Monocot: parallel – Dicot: net

Leaf Shapes • Most of a leaf consists of a blade attached to the

Leaf Shapes • Most of a leaf consists of a blade attached to the stem by a petiole. The blade of a simple leaf (left) can be different shapes • In a compound leaf (right), the blade is divided into many separate leaflets

Leaf Shapes

Leaf Shapes

Leaf Shapes • The vascular tissues of leaves are connected directly to the vascular

Leaf Shapes • The vascular tissues of leaves are connected directly to the vascular tissues of stems, making them part of the plant's transport system • In leaves, xylem and phloem tissues are gathered together into bundles that run from the stem into the petiole • Once they are in the leaf blade, the vascular bundles are surrounded by parenchyma and sclerenchyma cells

LEAF SHAPE • Toothed • Smooth • Lobed

LEAF SHAPE • Toothed • Smooth • Lobed

LEAF ARRANGEMENT • Opposite • Whorled • Alternate

LEAF ARRANGEMENT • Opposite • Whorled • Alternate

SIMPLE/COMPOUND LEAF • • Simple Pinnately compound Palmately compound Bipinnately compound

SIMPLE/COMPOUND LEAF • • Simple Pinnately compound Palmately compound Bipinnately compound

Leaf Functions • A leaf can be considered a system specialized for photosynthesis •

Leaf Functions • A leaf can be considered a system specialized for photosynthesis • Subsystems of the leaf include tissues that bring gases, water, and nutrients to the cells that carry out photosynthesis

LEAF CROSS SECTION • Epidermis: – Protects the leaf from injury and desiccation –

LEAF CROSS SECTION • Epidermis: – Protects the leaf from injury and desiccation – Most cells do not contain chloroplast – Waxy covering (cuticle) prevents water evaporation • Amount varies from species to species – Some contain epidermal hairs (extension of the cell) which slow the rate of water evaporation – Upper: thicker cuticle layer – Lower: contain guard cells and stoma

Photosynthesis • The bulk of most leaves consists of a specialized ground tissue known

Photosynthesis • The bulk of most leaves consists of a specialized ground tissue known as mesophyll, shown in the figure at right • Photosynthesis in most plants occurs in the mesophyll • The carbohydrates produced move into phloem vessels of the transport system, which carry them to the rest of the plant

Tissues in a Leaf

Tissues in a Leaf

Photosynthesis • • • A leaf has specialized cells that enable it to carry

Photosynthesis • • • A leaf has specialized cells that enable it to carry out photosynthesis Just under the epidermis is a layer of mesophyll cells called the palisade mesophyll These closely packed cells absorb light that enters the leaf Beneath the palisade layer is the spongy mesophyll, a loose tissue with many air spaces between its cells These air spaces connect with the exterior through stomata (singular: stoma), porelike openings in the underside of the leaf that allow carbon dioxide and oxygen to diffuse into and out of the leaf Each stoma consists of two guard cells, the specialized cells in the epidermis that control the opening and closing of stomata by responding to changes in water pressure

LEAF CROSS SECTION • Lower epidermis: – Pair of guard cells ( containing chloroplast)

LEAF CROSS SECTION • Lower epidermis: – Pair of guard cells ( containing chloroplast) surrounds a pore (stoma) that opens and closes • Size of opening depends on the shape of the guard cells (when open gases move in and out) • Guard Cell: – Cell wall next to stoma is thick and inflexible – Cell wall next to epidermis is thin and elastic • During photosynthesis (sunny day) the guard cells become swollen with water (turgid) pushing the thinner wall into the neighboring epidermal cell and pulling the thicker walls away from each other thus opening the stoma – Carbon dioxide enters – Oxygen and water vapor exit • When it is dark (no photosynthesis), the guard cells are less turgid (loss water) and the stoma closes – No oxygen nor carbon dioxide exchange – Water is conserved

LEAF CROSS SECTION • Palisade Mesophyll: – Cells upright and parallel • Maximum exposure

LEAF CROSS SECTION • Palisade Mesophyll: – Cells upright and parallel • Maximum exposure to sunlight – Main region of photosynthesis • Spongy Mesophyll: – Loosely packed • Air pockets – Gases exchange

Transpiration • The surfaces of spongy mesophyll cells are kept moist so that gases

Transpiration • The surfaces of spongy mesophyll cells are kept moist so that gases can enter and leave the cells easily • This also means that water evaporates from these surfaces and is lost to the atmosphere • Transpiration is the loss of water through its leaves • This lost water is replaced by water drawn into the leaf through xylem vessels in the vascular tissue

Gas Exchange • Leaves take in carbon dioxide and give off oxygen during photosynthesis

Gas Exchange • Leaves take in carbon dioxide and give off oxygen during photosynthesis • When plant cells use the food they make, the cells respire, taking in oxygen and giving off carbon dioxide (just as animals do) • Plant leaves allow gas exchange between air spaces in the spongy mesophyll and the exterior by opening their stomata

Gas Exchange • • It might seem that stomata should be open all the

Gas Exchange • • It might seem that stomata should be open all the time, allowing gas exchange to take place and photosynthesis to occur at top speed This is not what happens! – If stomata were kept open all the time, water loss due to transpiration would be so great that few plants would be able to take in enough water to survive So, plants maintain a kind of balance Plants keep their stomata open just enough to allow photosynthesis to take place but not so much that they lose an excessive amount of water

Opening and Closing of Stomata • Plants regulate the opening and closing of their

Opening and Closing of Stomata • Plants regulate the opening and closing of their stomata to balance water loss with rates of photosynthesis • A stoma opens or closes in response to the changes in pressure within the guard cells that surround the opening • When the guard cells are swollen with water (left), the stoma is open • When the guard cells lose water (right), the opening closes, limiting further water loss from the leaf

Opening and Closing of Stomata

Opening and Closing of Stomata

Gas Exchange • • • Guard cells are epidermal cells found on the undersides

Gas Exchange • • • Guard cells are epidermal cells found on the undersides of leaves They are structurally specialized to control stomata and thus regulate the movement of gases, especially water vapor, into and out of leaf tissues The stomata open and close in response to changes in water pressure within the guard cells, as shown in the figure at right – When water pressure within the guard cells is high, the thin outer walls of the cells are forced into a curved shape • This pulls the thick inner walls of the guard cells away from one another, opening the stoma – When water pressure within the guard cells decreases, the inner walls pull together and the stoma closes Guard cells respond to conditions in the environment, such as wind and temperature, helping to maintain homeostasis within a leaf Notice how the structure of guard cells, which is quite different from the structure of other epidermal cells, helps them to carry out this task

Gas Exchange • In general, stomata are open during the daytime, when photosynthesis is

Gas Exchange • In general, stomata are open during the daytime, when photosynthesis is active, and closed at night, when open stomata would only lead to water loss • However, stomata may be closed even in bright sunlight under hot, dry conditions in which water conservation is a matter of life and death

CHEMICAL ACTIVITY • PHOTOSYNTHESIS – Only during sunlight • RESPIRATION – 24 hours a

CHEMICAL ACTIVITY • PHOTOSYNTHESIS – Only during sunlight • RESPIRATION – 24 hours a day

Transport in Plants • The pressure created by water entering the tissues of a

Transport in Plants • The pressure created by water entering the tissues of a root can push water upward in a plant stem • This creates more than enough pressure to force water into the vascular system and out of the root • However, root pressure does not exert enough pressure to lift water up into trees, such as the topmost needles of a redwood tree 90 meters above the ground • To draw water to such great heights, plants take advantage of some of water's most interesting physical properties

Water Transport • Recall that xylem tissue forms a continuous set of tubes that

Water Transport • Recall that xylem tissue forms a continuous set of tubes that stretch from roots through stems and out into the spongy mesophyll of leaves • This set of tubes forms a complex transport system within a plant • The transport is carried out by a subsystem of cells and tissues • Active transport and root pressure cause water to move from soil into plant roots • Root pressure alone, however, cannot account for the movement of water and dissolved materials throughout an entire plant – Obviously, other forces are at work – These include capillary action and transpiration • The combination of root pressure, capillary action, and transpiration provides enough force to move water through the xylem tissue of even the tallest plant • As you will learn, transpiration is the most powerful of these forces

STEM TRANSPORT • Transpiration (xylem: dead cells at maturity) • Transpiration-cohesion theory (xylem: dead

STEM TRANSPORT • Transpiration (xylem: dead cells at maturity) • Transpiration-cohesion theory (xylem: dead cells at maturity) • Translocation: Pressure-flow hypothesis (phloem: living cells)

Capillary Action • • Water molecules are attracted to one another by a force

Capillary Action • • Water molecules are attracted to one another by a force called cohesion Recall from Chapter 2 that cohesion is the attraction of molecules of the same substance to each other – Because of cohesion, water molecules have a tendency to form hydrogen bonds with each other Water molecules can also form hydrogen bonds with other substances – This results from a force called adhesion, which is attraction adhesion between unlike molecules Place empty glass tubes of various widths into a dish of water, as shown below, and you will see both forces at work The tendency of water to rise in a thin tube is called capillary action Water is attracted to the walls of the tube, and water molecules are attracted to one another The thinner the tube, the higher the water will rise inside it

Capillary Action • Capillary action—the result of water molecules' ability to stick to one

Capillary Action • Capillary action—the result of water molecules' ability to stick to one another and to the walls of a tube— contributes to the contributes movement of water up the cells of xylem tissue • As shown here, capillary action causes water to move much higher in a narrow tube than in a wide tube

Capillary Action

Capillary Action

Capillary Action • What does capillary action have to do with water movement through

Capillary Action • What does capillary action have to do with water movement through xylem? • Recall that there are two main types of xylem tissue in flowering plants: tracheids and vessel element • Both tracheids and vessel elements form hollow connected tubes similar to a thin, glass capillary tube • Capillary action in the tubelike structures formed by both types of cells causes water to rise well above the level of the ground

Transpiration • • For trees and other tall plants, the combination of root pressure

Transpiration • • For trees and other tall plants, the combination of root pressure and capillary action does not provide enough force to lift water to the topmost branches and leaves The major force in water transport is provided by the evaporation of water provided by the evaporation from leaves during transpiration – When water is lost through transpiration, osmotic pressure moves water out of the vascular tissue of the leaf, as shown in the figure Then, like a locomotive pulling a train with hundreds of cars, the movement of water out of the leaf “pulls” water upward through the vascular system all the way from the roots This process is known as transpirational pull

Movement of Water Through Plant • Root pressure, capillary action, and transpiration contribute to

Movement of Water Through Plant • Root pressure, capillary action, and transpiration contribute to the movement of water within a plant • Transpiration is the movement of water molecules out of leaves • The faster water evaporates from a plant, shown in A, the stronger the pull of water upward from the roots, shown in B

Movement of Water Through Plant

Movement of Water Through Plant

Transpiration • How important is transpirational pull? • On a hot day, even a

Transpiration • How important is transpirational pull? • On a hot day, even a small tree may lose as much as 100 liters of water to transpiration • The hotter and drier the air, and the windier the day, the greater the amount of water lost • As a result of this water loss, the plant draws up even more water from the roots

TRANSPIRATION • • Water eventually enters the leaf from the stem, where it is

TRANSPIRATION • • Water eventually enters the leaf from the stem, where it is used in photosynthesis and cell metabolism Water evaporates through the stomata openings of the lower epidermis of the leaf: – As water evaporates from the stoma, cells close to the stoma absorb water from neighboring cells by osmosis. Those cells, in turn, absorb water from more distant cells, until eventually water is absorbed from the xylem in the veins (vascular bundles) of the leaf – Water is pulled up the xylem – Evaporated water molecules pull other water molecules after them – Molecules of water move upward through the xylem – Water column extends from the root to the stoma • Column is held together by adhesion and cohesion – Water molecules in the soil replace those that leave the plant through its leaves (xylem vessels are never empty)

TRANSPIRATION-COHESION THEORY • • Water is pulled up the xylem in a continuous column

TRANSPIRATION-COHESION THEORY • • Water is pulled up the xylem in a continuous column that stretches from the roots to the leaves The forces of adhesion and cohesion make thin column of water in the xylem behave like a dense, tightly-pulled wire Cohesion: attraction between molecules of the same kind – Water molecules are polar – Positive (H) of one water molecule attracts the negative (O) of the neighboring water molecule and so on – A long chain of water molecules is formed – Water molecules throughout the entire xylem system are held together by these cohesion forces Adhesion: attraction of unlike molecule – Water molecules adhere to the surface of the xylem vessels (like water adhering to the glass/plastic of a graduated cylinder)

Controlling Transpiration • The leaf's gas exchange subsystem helps to maintain homeostasis by keeping

Controlling Transpiration • The leaf's gas exchange subsystem helps to maintain homeostasis by keeping the water content of the leaf relatively constant – For example: • When water is abundant, it flows into the leaf, raising water pressure in the guard cells, which then open the stomata – Excess water is then lost through the open stomata by transpiration • When water is scarce, the opposite occurs • Water pressure in the leaf falls, and the guard cells respond by closing the stomata • This reduces further water loss by limiting transpiration

Transpiration and Wilting • Osmotic pressure keeps a plant's leaves and stems rigid, or

Transpiration and Wilting • Osmotic pressure keeps a plant's leaves and stems rigid, or stiff • High transpiration rates can lead to wilting • Wilting results from the loss of water—and therefore of the pressure in a plant's cells • Without this internal pressure to support them, the plant's cell walls bend inward, and the plant's leaves and stems wilt • When a leaf wilts, its stomata close – As a result, transpiration slows down significantly – Thus, wilting helps a plant to conserve water

Nutrient Transport Functions of Phloem • Many plants pump sugars into their fruits •

Nutrient Transport Functions of Phloem • Many plants pump sugars into their fruits • This action often requires moving sugars out of leaves or roots into stems, and then through stems to the fruits – All of this movement takes place in the phloem • In cold climates, many plants pump food down into their roots for winter storage – This stored food must be moved back into the trunk and branches of the plant before growth begins again in the spring • Phloem carries out this seasonal movement of sugars within a plant

Nutrient Transport Movement From Source to Sink • A process of phloem transport moves

Nutrient Transport Movement From Source to Sink • A process of phloem transport moves sugars through a plant from a source to a sink • The source can be any cell in which sugars are produced by photosynthesis • The sink is a cell where the sugars are used or stored • How does phloem transport take place?

Nutrient Transport Movement From Source to Sink • • • One idea put forward

Nutrient Transport Movement From Source to Sink • • • One idea put forward by many plant scientists is called the pressure-flow hypothesis As you can see in the figure at right, sugars are pumped into the phloem at one point, called the source For example, sugars produced by photosynthesis may move from a leaf As concentrations of sugar increase in the phloem, water from the xylem moves in by osmosis This movement causes an increase in pressure at that point, forcing nutrient-rich fluid to move through the phloem away from nutrientproducing regions and toward a region that uses these nutrients, called the sink

The Pressure-Flow Hypothesis • The diagram shows the movement of sugars and water throughout

The Pressure-Flow Hypothesis • The diagram shows the movement of sugars and water throughout the phloem and xylem as explained by the pressure-flow hypothesis • Materials move from a source cell, where photosynthesis produces a high concentration of sugars, to a sink cell, where sugars are lower in concentration • What is the source of the water that forces nutrients through phloem tissue?

The Pressure-Flow Hypothesis

The Pressure-Flow Hypothesis

TRANSLOCATION • Movement of food (dissolved sugar) from one part of a plant to

TRANSLOCATION • Movement of food (dissolved sugar) from one part of a plant to another • Pressure-flow hypothesis: – Food is transported through the phloem (sieve tubes/sieve tube elements/companion cells) as a result of differences in pressure – When sugar molecules produced in a leaf enter a particular sieve element, the concentration of water in that element is lowered • Water enters from neighboring cells by osmosis increasing the pressure in the cell pushing the contents of the element into the next element • Energy is required

Nutrient Transport Movement From Source to Sink • Conversely, if part of a plant

Nutrient Transport Movement From Source to Sink • Conversely, if part of a plant actively absorbs nutrients from the phloem, osmosis causes water to follow • This movement of water decreases pressure and causes a movement of fluid in the phloem toward the sink • When nutrients are pumped into or removed from the phloem system, the change in concentration causes a movement of fluid in that same direction • As a result, phloem is able to move nutrients in either direction to meet the nutritional needs of the plant