Topic 9 Plant Biology

Topic 9 Plant Biology

Topic 9 Plant Biology Part One 9.1 Transport in the xylem of plants Nature of science: Use models as representations of the real worldmechanisms involved in water transport in the xylem can be investigated using apparatus and materials that show similarities in structure to plant tissues. (1.10) Understandings Transpiration is the inevitable consequence of gas exchange in the leaf Plant transport water from the roots to the leave to replace losses from transpiration

The cohesive property of water and the structure of the xylem vessels allow transport under tension The adhesive property of water and evaporation generates tension forces in leaf cell walls Active uptake of mineral ions in the roots causes absorption of water by osmosis 9.1 Transport in the xylem of plants Applications and Skills Application: Adaptations of plants in deserts and in saline soils of water conservation Application: Models of water transport in xylem using simple apparatus

including blotting or filter paper, porous pots, and capillary tubing Skill: Drawing the structure of primary xylem vessels in sections of stems based on microscope images Skill: Measurement of transpiration rates using potometer Skill: Design of an experiment to test hypothesis about the effect of temperature or humidity on transpiration rate Retro Topic 2.2 Water Understandings: Water molecules are polar and hydrogen bonds form between them

Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water Substances can be hydrophilic or hydrophobic Applications and skills: Application: Comparison of the thermal properties of water with those of methane Application: Use of water as a coolant in sweat Application: Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in relation to their solubility in water 9.2 Transport in the phloem of

plants Nature of science: Developments in scientific research follow improvements in apparatus experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available. (1.8) Understandings Plants transport organic compound from sources to sinks Incompressibility of water allows transport along hydrostatic pressure gradients Active transport is used to load organic compounds into phloem sieve tubes at the source High concentrations of solutes in the phloem at the source lead to water uptake by osmosis

Raised hydrostatic pressure causes the contents of the phloem to flow toward sinks 9.2 Transport in the phloem of plants Applications and skills Application: Structure-function relationships of phloem sieve tubes Skill: Identification of xylem and phloem in microscope images of stem and root Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labeled carbon dioxide

Flash Cards For Plants Part I do flash cards for 9.1 Transport in the xylem of plants 9.2 Transport in the phloem of plants Angiosperms What is the difference between an angiosperm and a gymnosperm? Angiosperms have flowers or fruit Gymnosperms are the naked seeds, cone-bearers

There are two classifications of angiosperms Commonly recognized and are named for the number of cotyledons, or seed leaves, present on the embryo of the plant 1. One group are the monocots which include orchids, bamboos, palms, lilies, and yuccas as well as the grasses such as wheat, corn, and rice 2. The other group is the dicots which include roses, beans, sunflowers, and oaks Monocots 1. One cotyledon

2. Veins, which carry vascular tissue, usually parallel 3. Vascular bundles usually complexly arranged spread throughout/random 4. Fibrous adventitious root system. (Unbranched roots grow from stems) 5. Floral organ parts usually in multiples of 3 (stamens, petals) 6. Pollen grain with one opening

Monocots Dicots 1. Two cotyledons 2. Veins usually netlike 3. Vascular bundles usually arranged in ring 4. Taproot with lateral branches usually present

5. Floral parts usually in multiples of four or five 6. Pollen grains with 3 openings Dicots Flowering Plant Plants have three basic organs: 1. Root

2. Stems 3. Leaves These organs are composed of different tissues and these tissues are teams of different types of cells Tissue Types Dermal tissue or epidermis- single layer of tightly packed cells that covers and protects all young parts of the plant

The epidermis of leaves and most stems secrete a waxy coating called the cuticle that helps the aerial parts of the plant retain water Root hairs are epidermal extensions found near the tips of roots and are important in the absorption of water and minerals Increase surface area Tissue Types Vascular tissue: continuous throughout the plant, is involved in the transport of materials between roots and shoots Two types of vascular tissue

1. Xylem which conveys water and dissolved minerals upward from roots into the shoots 2. Phloem which transports food made in mature leaves to the roots and to non-photosynthetic parts of the shoot system, such as developing leaves and fruit. More generally, transport of sugars from sugar source to sugar sink Tissue Types Ground tissue: tissue that is neither dermal tissue nor vascular tissue In dicot stems, ground tissue is divided into pith, internal to the

vascular tissue, and cortex, external to the vascular tissue Among the functions of pith are photosynthesis, storage and support Types of cells There are three types of cells we will discuss: parenchyma, collenchyma, and sclerenchyma Parenchyma cells: have primary walls that are relatively thin and flexible. They have a large central vacuole, are the least specialized cells

They perform most of the metabolic functions of the plant. For example, photosynthesis occurs within chloroplasts of parenchyma cells in the leaf Examples: Palisade mesophyll and spongy mesophyll Collenchyma cells- have thicker primary walls than parenchyma cells Grouped in strands or cylinders,

collenchyma cells help support young parts of the plant shoot i.e. strings of a celery stalk Functioning collenchyma cells are living and flexible and elongate with the stems and leaves they support Normally located under the epidermis Sclerenchyma cells- have thick secondary walls usually

strengthened by lignin, much more rigid then collenchyma cells Mature sclerenchyma cells cannot elongate and they occur in regions of the plant that have stopped growing in length Many are dead at functional maturity The water-conducting vessel elements and tracheids in the xylem are sclerenchyma cells called fibers and sclereids that specialize entirely in support i.e. fibers: hemp fibers used for making rope and flax fibers for weaving into linen

i.e. sclereids (shorter): impart hardness to nutshells and seed coats and the gritty texture to pear fruits. Structure of Leaves The function of leaves is to produce food for the plant by photosynthesis The leaf is adapted by its structure to carry out photosynthesis efficiently The main part of the leaf is the leaf blade or lamina. It has a large

surface area to absorb sunlight but is very thin (0.3mm) It is composed of 4 thin tissue layers with veins at intervals Tissue layers Upper epidermis: continuous layer of cells covered by a thick cuticle. It prevents water loss from the upper surface even when heated by sunlight Lower epidermis: is in a cooler position and has a thinner waxy cuticle Both upper and lower epidermis: First line of defense against

physical damage and pathogenic organisms Barrier is interrupted only by stomata and guard cells Tissue layers- Palisade mesophyll Palisade mesophylldensely packed cylindrical cells with many chloroplasts Main photosynthetic tissue and is positioned near the upper surface where the light intensity is highest

Parenchyma cells Tissue layers- Spongy Mesophyll Loosely packed rounded cells with few chloroplasts. Labyrinth of air spaces through which gases circulate up to the palisade region. Air spaces are large near the stomata Provides the main gas exchange surface so must be near the stomata in the lower epidermis. Photosynthesis depends on gas exchange over a moist surface Spongy mesophyll cell walls provide this surface. Water often evaporates

from the surface and is lost in a process called transpiration Transpiration is the loss of water vapor from the leaves and stems of plants. There are adaptations that minimize the amount of transpiration. Transpiration Transpiration is the loss of water vapor from the leaves and stems of plants Stomata are openings on the underside of the leaf that allow for gas exchange during photosynthesis

Stomata allow for oxygen and water vapor to exit and carbon dioxide to enter Due to this exchange transpiration is the inevitable consequence Plants try to minimize this through guard cells that open and close the stomata Stomata Vascular Tissue- Phloem

In phloem sucrose, other organic compounds, and some mineral ions are transported through sieve tubes formed by chains of cells called sieve-tube members which remain alive at functional maturity These sieve-tube members are connected to nucleated companion cells which are connected to phloem via plasmodesmata All that remains of sieve tubes are plasma membranes Also have porous end walls We will continue discussing phloem later Vascular Tissue- Xylem

So how do plants replenish this lost water? Plants transport water from the roots to the leaves through xylem The structure of xylem vessels allows for the efficient transport of water Xylem is composed of tracheids and vessel elements These are elongated cells that are dead at functional maturity So at maturity xylem is non-living

Primary Xylem Xylem vessels are long continuous tubes Primary xylem is helical or ring-shaped thickenings Thickenings made of the cellulose cell wall are impregnated with lignin. This makes them hard to resist inward pressures. Pores in the outer cellulose cell wall

conduct water out of the xylem vessel and into cell walls of adjacent cells Transport of materials Transport in plants occurs on three levels: 1. Uptake and loss of water and solutes by individual cells such as the absorption of water and minerals from the soil by cells of a root 2. Short-distance transport of substances from cell to cell at the level of tissues and organs, such as loading of sugar from photosynthetic cells of a mature leaf into the sieve tubes of phloem

3. Long-distance transport of sap within xylem and phloem at the level of the whole plant Root absorption of mineral ions from the soil: Active Transport Active transport is the pumping of solutes across membranes against their electrochemical gradients The cell must expend metabolic energy ATP to transport a solute uphill. Active transport in root cells is involved in the absorption of

potassium, phosphate, nitrate, and other mineral ions from the soil The concentration of these ions in the soil is usually much lower than inside root cells Proton pump is the major pump in plant cells Used to generate a hydrogen ion gradient and membrane potential (voltage) Inside of the cell is negative while the outside of the cell is positive. This is a form of stored energy that can be harnessed to perform

cellular work Plants use this energy storage in the proton gradient and membrane potential to drive the transport of many different solutes This mechanism is called co-transport because the transport protein couples the downhill passage of one solute (H+ ions) to the uphill passage of another (e.g. nitrite) Substitute K+

or Na+ or Cl- or any Ion or mineral Transport of water Differences in water potential drive water transport in plant cells: The net uptake or loss of water by a cell occurs by osmosis, the

passive transport of water across a membrane There are 2 factors that influence the direction of water movement in plant cells 1) Water will move from a hypotonic (lower solute concentration) to a hypertonic (higher solute concentration) area. 2) The cell wall adds a second factor affecting osmosis: physical pressure. Physical pressure causes water to move. If a solution is separated from pure water by a selectively permeable membrane, external pressure on the solution can counter its tendency to take up water due to the presence of solutes

Transport of water When plant tissue is placed in pure water, the cell begins to swell and push against the cell wall, producing a turgor pressure The partially elastic wall pushes back against the pressurized cell. When this wall pressure is great enough to offset the tendency for water to enter because of the solutes in the cell a dynamic equilibrium will be reached and the cell will be

turgid Healthy plant cells are turgid most of the time. Their turgor contributes to support in non-woody parts of the plant Transport of water As of 1990, scientists have suggested that even water transport is mediated by selective channels. They have since found these channels in both plant and animal cells The specific channels for passive traffic of water are transport

proteins called aquaporins They do not affect the gradient or the direction of water flow, but rather the rate at which water diffuses down its gradient So water is transported by osmosis (passive transport) Nutrients (mineral ions) are actively co-transported from soil to root Absorption of Water and Mineral by Roots Route: enter through epidermis of roots, cross the root cortex, pass into the stele, flow up xylem vessels to the shoot system

Root tips: epidermis is permeable to water. Much of the absorption of water and minerals occurs near root tips Root hairs are extensions of epidermal cells, and account for much of the surface area of roots Lateral Transport Water and solutes move from one location to another within plant tissues and organs This occurs when water and

minerals are absorbed by a root from the outer cells and moved to the inner cells of the root Endodermis In order for water and minerals to pass from the soil and the root cortex to the rest of the plant, they must enter the xylem of the stele The endodermis surrounds the stele and functions as a last checkpoint for the selective passage of minerals from the cortex into the vascular tissue

Minerals in the cytoplasm continue through the endodermal cells via plasmodesmata and into the stele Minerals traveling via the cortex encounter a dead end that blocks their passage into the stele Casperian strip In the wall of each endodermal cell is the Casperian strip, a belt of suberin, a waxy material that is impervious to water and dissolved minerals Material must cross the plasma membrane of the endodermal cell

and enter the stele via the cytoplasm This assures that all solutes must pass through at least one cell membrane before entering the xylem Water then passes into the tracheids and vessel elements of the xylem using both diffusion and active transport Apoplastic vs Symplastic Apoplastic route is through the cell walls (and intercellular spaces)

Symplastic route is through the cytoplasm (and plasmodesmata) Water is pulled to the xylem due to a pulling force called the transpiration pull Summary of movement of water from roots to xylem Roots and root hairs increase surface area to absorb water in soil Water is absorbed by osmosis

Solute concentration is higher inside the root than in the soil so water moves in by osmosis Ions move in by active transport Both apoplastic and symplastic transport across the root Apoplastic route is through the cell walls and intercellular spaces Symplastic route in through the cytoplasm and plasmodesmata Water moves from epidermis to cortex to endodermis Water moves through the endodermis and is blocked by casparian strips

Water moves into the xylem due to pulling force called transpiration pull Cohesion of water molecules makes this possible Long Distance Transport Bulk flow functions in long-distance transport Diffusion is efficient for transport over distances defined by cellular dimensions but is too slow for long-distance transport e.g. from roots to leaves Water and solutes move through xylem vessels and sieve tubes by bulk flow the movement of a fluid driven by pressure

In xylem, it is actually tension (negative pressure) that drives longdistance transport. Transpiration and evaporation of water from a leaf, reduces pressure in the leaf xylem. This creates a tension that pulls xylem sap upward from the roots In phloem, for example, hydrostatic pressure is generated at one end of a sieve tube, forcing sap to the opposite end of the tube Transport of Xylem Sap Tracheids are spindle-shaped, elongated cells with pits through which water flows from cell to

cell They are dead at functional maturity When the living interior of a tracheid or vessel element disintegrates, the cells thickened cell walls remain behind forming a nonliving conduit through which water can flow Water moves from cell to cell without having to cross thick secondary walls The secondary walls of xylem are strengthened with

lignin so tracheids function in support as well as water transport Vessel elements are generally wider, shorter, thinner walled, and less tapered than tracheids. They are aligned end to end forming long micropipes, the xylem vessels The end walls of vessel elements are perforated, enabling water to flow freely through xylem vessels Water streams from cell to cell through perforated end walls and can also migrate laterally between

neighboring vessels through pits Other features of xylem There are no plasma membranes in xylem vessels, so water can move in and out freely. The lumen of the xylem vessel is filled with sap because the cytoplasm and nuclei have disintegrated. There are pores in the outer cellulose cell wall to conduct water out of the xylem vessel and into cell walls of adjacent leaf cells. Cellulose rings with lignin make xylem hard so that they can resist

inward pressures. Accent of Xylem Sap Depends mainly on transpiration and the physical properties of water. Must rise against gravity. At night, when transpiration is very low or zero (stomata are closed, temperatures are lower), root is still using energy to pump mineral ions into xylem Water will flow in from root cortex, generating a positive pressure that forces fluid up the xylem. This upward push of xylem sap is called root

pressure In most plants, root pressure is not the major mechanism driving the ascent of xylem sap. At most, root pressure can force water upward only a few meters For the most part, xylem sap is not pushed from below by root pressure, but pulled upward by the leaves themselves Transpirational pull: water vapor diffuses from the moist air spaces of the leaf to the drier air outside via stomata Evaporation from the water film coating the mesophyll cells maintains

the high humidity of the air spaces This loss of water causes the water film to form menisci (singular meniscus) that become more and more concave as the rate of transpiration increases The tension of water lining the air spaces of the leaf is the physical basis of transpirational pull, which draws water out of the xylem The cohesion of water due to hydrogen bonding makes it possible to pull a column of sap

from above without the water separating Also helping to fight gravity is the strong adhesion of water molecules (H bonds) to the hydrophilic walls of the xylem cells Skill: Drawing the structure of primary xylem vessels in sections of stems based on microscope images

Skill: Drawing the structure of primary xylem vessels in sections of stems based on microscope images Plan diagram of a dicot plant Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots, and capillary tubing Models allow one factor or aspect to be studied independently of other factors

Water has adhesive properties. Glass capillary tubes can be used to model adhesion between water and xylem vessel walls. Water adheres to glass so rises up the capillary tube. A substance like mercury does not adhere so does not rise. Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots, and capillary tubing Porous pot can be used to model flow in a xylem vessel due to transpiration from the leaf

Porous pot is similar to leaf cell walls as water adheres to it and there are many narrow pores Water evaporates from the surface of the pot so more water is drawn into the pot to replace losses Water rises up in the tube Application: Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots, and capillary tubing Blotting paper can be used to model capillary attraction or adhesion

Strip of blotting, filter, or chromatography paper is suspended by a rubber stopper from the top of a test tube into a small amount of water at the bottom of the test tube Paper is made of cellulose (like cell walls) so water rises up through it against gravity in pores in the paper Skill: Measurement of transpiration rates using potometer Potometer

Factors that effect transpiration 1. Light Light leads to photosynthesis, the need for gas exchange, and increases in transpiration. Light stimulates stomatal opening Guard cells close the stomata in darkness 2. Temperature Transpiration assists the plant in evaporative cooling and prevents the leaf from reaching temperatures that could denature various enzymes involved in photosynthesis and other metabolic processes

So increases in temperature lead to increases in transpiration Heat is needed for evaporation of water from the surface of spongy mesophyll cells, so as temperature increases, evaporation increases, and transpiration (water loss) increases. temperature evaporation transpiration

Factors that effect transpiration 3.) Wind Wind leads to increase in evaporation Wind blows the saturated air away and leads to increases in the rate of transpiration which leads to an increased movement of water through the xylem and an increase in transpiration High wind velocities can cause stomata to close Still air, reduces the rate of transpiration. 4.) Humidity

Water diffuses out of the leaf when there is a concentration gradient between the air spaces inside the leaf and the air outside The air spaces inside are always nearly saturated The lower the humidity outside the leaf, the steeper the gradient and therefore the faster the rate of transpiration The higher the humidity outside, the less steep the gradient and transpiration rate is decreased 5.) Number, size, and distribution of stomata 6.) Surface area of leaf

7.) Carbon dioxide levels in air Skill: Design of an experiment to test hypothesis about the effect of temperature or humidity on transpiration rate Keep everything else constant except the independent variable If examining temperature you can use 2 plants with 2 lights, one putting out light and heat and the other one putting out just light at the same wattage (compact fluorescent bulbs do not put out heat) If examining humidity you can also use 2 plants with and without use

of a mister and a plastic bag. Can monitor humidity using kestrel. Control leaf exposed to air of known humidity. Experimental leaf bagged with plastic bag, misted with 5 ml of water and sealed In both cases describe the potometer set up and/or the gas pressure probe set up Application: Adaptations of plants in deserts and in saline soils of water conservation Xerophytes are plants adapted to arid climates. They have various leaf modifications that reduce the rate of transpiration

Cereus giganteus, the Saguaro or giant cactus that grows in deserts in Mexico and Arizona Adaptations of Xerophytes 1. They have small, thick leaves, an adaptation that limits water loss by reducing surface area relative to leaf volume 2. A thick cuticle gives some of these leaves a leathery consistency and reduces water loss 3. Stomata are concentrated on the lower (shady) leaf surface, and they are often located in depressions that shelter the pores from the dry wind.

These pits are surrounded by hairs. 4. During the driest months, some desert plants shed their leaves. 5. Other plants such as cacti, subsist on water the plant stores in its fleshy stem during the rainy seasons (these modified stems are the photosynthetic organs of cacti; the spines are modified leaves). CAM Plants CAM stands for crassulacean acid metabolism. Succulents are CAM plants. These plants have a metabolic adaptation that allow them to incorporate carbon dioxide into organic acids during the night

During the daytime, the organic acids are broken down to release carbon dioxide in the same cells, and sugars are synthesized by the conventional C3 pathway The leaf takes in its carbon dioxide at night, the stomata can close during the day, when transpiration is most severe Application: Adaptations of plants in deserts and in saline soils of water conservation Halophytes are plants that live in saline soils

They are adapted to grow in water with high salinity. They are a promising biofuel because they do not compete with food crops for resources Only 2% of all plant species Come in contact with saline water through its roots or by salt spray, such as in saline semi-deserts, mangrove swamps, marshes, and sloughs and seashores

Adaptations of halophytes Leaves are reduced to small scaly structures or spines They shed their leaves when water is scarce The stem becomes green and takes over roll of photosynthesis Water is stored in the leaves They have thick cuticle and multiple layered epidermis Stomata are sunken They have long roots that can grasp water They have structures for removing salt buildup

9.2 Transport in the phloem of plants 9.2 Transport in the phloem of plants Nature of science: Developments in scientific research follow improvements in apparatus experimental methods for measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were only possible when radioisotopes became available. (1.8)

Understandings Plants transport organic compound from sources to sinks Incompressibility of water allows transport along hydrostatic pressure gradients Active transport is used to load organic compounds into phloem sieve tubes at the source High concentrations of solutes in the phloem at the source lead to water uptake by osmosis Raised hydrostatic pressure causes the contents of the phloem to flow toward sinks 9.2 Transport in the phloem of plants Applications and skills

Application: Structure-function relationships of phloem sieve tubes Skill: Identification of xylem and phloem in microscope images of stem and root Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labeled carbon dioxide Application: Structure-function relationships of phloem sieve tubes Phloem cells are sieve tube members which are arranged end to end to form long sieve tubes

Sieve tube members have no nuclei or ribosomes Between the cells are sieve-plates porous cross-walls that allow the flow of sap along the sieve tube Alongside each sieve-tube member is a nucleated companion cell This cell is a non-conducting cell which is connected to the sieve tube member by numerous channels called the plasmodesmata Translocation is the transport of any biochemical in phloem whether produced by the plant or not This includes plant hormones and small RNA molecules

Plasmodesmata Plasmodesmata are channels of cytoplasm that pass through cell walls Phloem Sieve Tube Companion Cells Adjacent sieve-tube members have no nucleus or ribosomes

Companion cells help these sieve-tube members In some plants, companion cells in leaves also help load sugar produced in the leaf into the sieve-tube members The phloem then transports the sugar to other parts of the plant Direction of phloem sap

Direction of phloem sap is variable but always moves from a sugar source to a sugar sink Sugar source- sugar is produced by photosynthesis or through the breakdown of starch. Mature leaves are major sugar source Sugar sink- an organ that is a net consumer or storer of sugar. Growing roots, shoot tips, stems, and fruit are sugar sinks supplied by phloem Modified shoots A storage organ, such as a tuber (potato) or bulb (e.g. onion), may be

either a source or a sink depending on the season Other solutes, such as minerals, may be transported to sinks along with sugar and later transported to developing fruit Phloem Loading and Unloading Sieve tubes have no nuclei and ribosomes but have plasma membrane and transport proteins Sugar must move into sieve tube members before it can be exported to sugar sinks Loading

1. Sugar can move through the cytoplasm (plasmodesmata) through diffusion (Symplastic) 2. Sugar can move through a combination of the cytoplasm and cell wall pathways (Apoplastic) Sucrose can be concentrated 2-3 times higher than concentrations in mesophyll Phloem loading requires active transport Proton pumps do the work which enables the cells to accumulate sucrose Phloem Loading- Short Distance Transport

Sieve Tube Pressure Concentration of free sugar in the sink is lower than that in the sieve tube As the result of this gradient, sugar molecules diffuse from the phloem into the sink tissues, and water follows by osmosis Bulk flow or pressure flow is the

mechanism of translocation Pressure Flow- Long Distance Transport 1. Step 1: Phloem loading results in a high solute concentration at the source end of a sieve tube which lowers the water potential and causes water to flow into the tube 2. Step 2: Hydrostatic pressure develops within the sieve tube and the pressure is greatest at the source end of the tube 3. Step 3: At the sink end, pressure is relieved by the loss of water

which follows the exodus of sucrose 4. Step 4: Water is recycled back from sink to source by xylem vessels Summary of Sugar Movement in Plants Cellular level- active transport across plasma membranes in phloem cells Short distance level of lateral transport within organs- sucrose migration from mesophyll to phloem via the cytoplasm through plasmodesmata or through cell walls

Long distance level of transport between organs- bulk flow/pressure flow within sieve tubes Skill: Identification of xylem and phloem in microscope images of stem and root Stem Root Roots- X marks the spot!

Skill: Analysis of data from experiments measuring phloem transport rates using aphid stylets and radioactively-labeled carbon dioxide Phloem sap is an aqueous solution in which the prevalent solute is sugar primarily sucrose It may be as high as 30% by weight Phloem sap may also contain minerals, amino acids, and hormones in

transit from one part of the plant to another Skill: Aphids Using aphids is a method of obtaining samples of phloem sap from single sieve tubes Aphids have long piercing mouth parts called stylets High pressure inside sieve tubes pushes phloem sap out through the stylet and into

gut of aphid The aphid is cut off from the stylet while it is still feeding The stylet is left and sap continues to flow In 1940s began using 14CO2 in leaf for photosynthesis Labeled sucrose produced

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