Biology and Health Sciences

Key unit competency

To be able to explain the process of uptake and transport of mineral and organic saps, transpiration and translocation, and their roles in plants.

Learning objectives

After studying this unit, I should be able to:

• Identify xylem and phloem tissues from transverse sections and state their functions.

• Explain the mechanism by which water moves upwards in the xylem.

• Explain the adaptations of plant leaves to controlling water loss.

• Describe transpiration and its effects.

• Explain the adaptations of desert plants.

• Explain how translocation takes place.

• Use a potometer to measure the rate of water uptake in a given plant.

• Appreciate the importance of absorption and transport of water in plants.

Introduction

Plants manufacture food in the leaves. They absorb water and mineral salts which they use as raw materials from the soil. How then do these important plant requirements reach other parts of the plant from their source? Look at the tree alongside. What is going on in the tree?

The tree is big and complex. How will food manufactured in the leaves reach its stem and roots? By what process will water absorbed in the soil reach the stem and leaves? What does this tell you about what you will learn in this unit?

In unicellular organisms, diffusion is enough to transport materials into and out of the body of the organisms. However, plants are complex multicellular organisms. Diffusion alone is therefore, not enough to transport materials within it. This is because they have a lower surface area to volume ratio. Their cells are far away from the outside environment where these materials are located.

This necessitates a transport system to enable movement of materials in plants.

8.1  Transport system in plants 

All living organisms are made up of cells. In order to stay alive, these cells take up useful substances from their environment. They also produce and release waste substances.

Water and mineral salts are transported from the soil through roots to the leaves and other parts of the plant. Manufactured food is also transported from the leaves to other parts of the plant.

A transport system in living organisms is made up of specialised tissues and organ systems. In plants, the transport system is made up of specialised tissues called vascular bundles. The vascular bundles contain two types of tissues: xylem and phloem.

Substances that need to be transported in plants are:

    •  Water required in photosynthesis.

    •  Mineral salts used in various plant processes.

    •  Organic substances manufactured mainly by photosynthesis taking place in leaves.

Discussion corner

1. Discuss the following questions with a friend.

2.  Share your work with the rest of the class.

Phloem and xylem tissues are found in the root, the stem and the leaves. Phloem tissues transport food substances such as glucose and amino acids from the leaves to other plant tissues where they are used or stored. The xylem tissues transport water and mineral salts absorbed by the roots to different parts of the plant.

Water and mineral uptake

Water and mineral salts are absorbed from the soil through the root hair cells found in the roots. In order to understand how this takes place, we will first look at root hairs and the internal structure of the root.

Activity 8.1: Observing permanent slides of dicotyledonous and monocotyledonous roots

Requirements 

•    Microscopes

•    Permanent slides of transverse sections of:    

•  Dicotyledonous roots     

•  Monocotyledonous roots

Procedure

1.  Place a prepared slide of a dicotyledon root under the microscope. Observe under low power and high power objective lenses.

2.  Note the different tissues present and their location.

3. Draw a plan diagram to show the position and layout of different layers of tissue. Do not draw any cells or shade.

4.  Compare your diagram with that of the plan diagram of the dicotyledonous root section in fig 8.2 (a) and use it to identify the tissues.

5.  Repeat this procedure for the monocotyledon root and compare your drawing with that in fig 8.2 (b).

Study questions

(a) Describe the pattern in the arrangement of xylem in relation to phloem in the 

(b) What is the difference in distribution of tissues between the dicot root and the monocot root?

The distribution of tissues in a transverse section of the dicotyledonous root is not the same as that in the monocotyledonous root. In both the monocot and dicot roots the vascular bundle occupies a central position.

In the dicotyledonous root, the xylem occupies the centre where it forms a star shape. The phloem is found in between the two rays of the star. The vascular bundles are arranged in a ring.

In the monocotyledonous root, the xylem and phloem are arranged to form a ring in which xylem tissue alternates with the phloem tissue.  

The root hair

Discussion corner

You will be provide with a chart showing the structure of root hair. Study it and do the following.

1.  Discuss the following questions with a friend.

     (a) Describe the appearance of the root hair cells.

     (b) What do you think is the function of the root hair cell?  

     (c) What is the significance of

             (i)  The number of the root hair cells found on each root?

             (ii)  The size of the root hair cells? 

     (d) What do you think is the importance of the structure of the root hair?

2. Share your findings with other class members.

Root hair cells are found growing from the outer surface of the root towards the end but not at the tips. Their function is to absorb water and mineral salts.

Adaptations of the root hair cells

(i)  They are numerous so as to increase the surface area over which absorption of water and mineral salts occurs.

(ii) They are thin and fine so that they can penetrate the spaces in between the soil particles where water is found.

Absorption of water

The soil particles are usually surrounded by a film of water except when there is drought. Root hair cells absorb water from the soil by osmosis.

The cell sap in the vacuole of the root hair cell has a high concentration of salts and sugars. It is therefore hypertonic to the water found between the soil particles. Due to this concentration gradient, water molecules move by osmosis from the soil through the semi-permeable membrane of root hair cells into the cell sap. Root hair cells will take up water as long as their concentration of salts is higher than that in the soil and the cell wall pressure is not large enough to prevent osmosis.

Within the roots, water moves from cell to cell by the process of osmosis. This is because adjacent cells contain different water quantities.

Different theories have been put up to try to explain the movement of water in plants. They include capillarity, root pressure, transpiration pull and cohesion tension theory.

a. Capillarity

In plants, xylem form narrow tubes through which water moves. Water rises in the xylem because of strong forces of attraction between the water molecules and the cell walls of the tubes or xylem vessels hence capillarity.

b. Root pressure

Root pressure is responsible for the rising of water up the stem. When water finally finds its way into the xylem vessels of the stem, two other processes take over the  transportation process. Root pressure is demonstrated when a tree trunk is cut and water oozes out as shown in Fig 8.5 alongside.

c. Transpiration pull and cohesion tension theory

Transpiration pull is as a result of water being lost at the leaves. Leaves contain small pores called stomata. Stomata continuously lose water vapour. Since water molecules stick together, a pull is created known as transpiration pull. It allows lost water to be compensated for.

Since the xylem vessels are dead and narrow, the adhesive forces between water molecules and the walls of the xylem vessels allows water to climb up. Cohesive forces allow a stream of water which is unbroken to ascend up the plant. The continuous movement of water in columns in the root xylem, stem xylem and leaf xylem, to the air spaces in the leaf is referred to as the transpiration stream.

Once water and mineral salts are absorbed into the root hair cells, they need to reach the xylem tissue which is located at the centre of the root. It is at the xylem where water and mineral salts are distributed to other parts of the plant. Once in the root xylem, water moves up into the stem xylem and eventually reaches the leaf xylem.

Activity 8.2: Demonstrating movement of dyed water up the xylem

Requirements

Procedure

1.  Set up the apparatus as shown below. Colour the solution with red ink or eosin stain.

2.  Leave the set up for three or four hours then remove the plant and cut sections of the root, stem and leaf.

3.  Examine the sections under low power of the microscope or with a hand lens.

      • What do you observe in the cut section? 

Study questions

(a) Which tissues are stained red by the dye?

(b) Suggest a control for this experiment.

Active uptake of mineral salts

Soil water contains dissolved mineral ions such as potassium, magnesium, nitrates and phosphates. This means that a root hair must take up these mineral ions against their concentration gradient. Mineral salts are taken up by plants in ion form, for example, calcium as Ca²⁺ and magnesium as Mg²⁺.

Roots have higher concentrations of ions as compared to the soil, yet they continue taking in the mineral salts. They do this through active transport which requires the expenditure of energy. The energy is obtained from the process of respiration.

Mineral salts find their way into the xylem vessels, where they move together with water in form of a solution up the plant.

Transport of water takes place within the xylem vessels. Xylem is composed of a system of interconnected tubes which run all the way from the roots to leaves. In leaves they are in form of veins. Xylems contain two types of modified cells namely tracheids and vessel elements. Xylems are hollow tubes which act like pipes allowing water and dissolved minerals to flow through them. The cell walls in xylem vessels contain a substance called lignin. Lignin strengthens the cells and gives them structural support.

Activity 8.3: Observing permanent slides of dicotyledonous and monocotyledonous stems

Requirements 

Permanent slides of dicotyledonous and monocotyledonous stems

Procedure

1. Place a prepared slide of a dicotyledonous stem on the stage of a microscope. Observe under low power and high power objective lenses.

2. Note the different tissues present and their location.

3. Draw a plan diagram to show the position and layout of different layers of tissues. Do not draw any cells.

 4. Compare your diagram with that of the plan diagram of the dicotyledonous stem section in a plant in Fig. 8.9 and use it to identify the tissues.

5. Repeat this procedure for the monocotyledonous stem and compare your drawing with that in Fig. 8.10.

Study questions

a) Is the distribution of tissues in the cross (transverse) section of a dicotyledonous stem the same as that in a monocotyledonous stem?

b) Describe the pattern in the arrangement of xylem and phloem.

The distribution of tissues in a transverse section of the dicotyledonous (dicot) stem is not the same as that in the monocotyledonous (monocot) stem. In the dicotyledonous stem the vascular bundles which contain both xylem and phloem are arranged to form a ring as shown in Fig. 8.9. In the monocotyledon stem the vascular bundles appear scattered in the stem as shown in Fig. 8.10

Most of the tissues in the root and stem are similar. This is because these tissues are continuous from the root into the stem. The stem has additional tissue known as pith. Pith is the central part of the roots of monocots and dicots and the stem of dicots. It is lacking in the stem of monocots. It is made up of parenchyma cells.

Self-evaluation Test 8.1

1.  Why should plants have an elaborate transport system?

2. Differentiate between monocotyledonous and dicotyledonous roots.

3. Come up with a diagram to show how mineral salts move from the roots to the leaves.

8.2 Transpiration

Transpiration is the evaporation of water from the plant surface mainly through the leaf. Much of the water that plants take up through their roots is lost to the atmosphere by evaporation. Through transpiration, plants are able to maintain a steady supply of water since the lost water has to be compensated for. If plants lose a lot of water than they can gain, they wilt. If this continues for a long period of time, they may die. Thus transpiration is always referred to as a necessary evil.

Leaves contain small pores called stomata on their surfaces, which open and close. This allows the exchange of respiratory gases as well as the loss of water in form of vapour. This is known as stomatal transpiration. Most water in plants is lost through this way.

Other than the stomata, there are other ways through which plants can lose water. They include;

Lenticular transpiration: This is the loss of water through the lenticels found in woody stems. Because of the limited distribution of lenticels, this type of transpiration accounts for less than 1% of the total loss of water by a plant.

Cuticular transpiration: This is the loss of water through the cuticle in herbaceous stems. Leaf surfaces and stems are normally covered with a waxy substance called cuticle. It accounts for up to 10% of the total water loss by the plant.

Activity 8.4: To demonstrate the process of transpiration

Requirements

• Potted plants (one with leaves, the other with its leaves removed)

• Two polythene bags

• Strings

Procedure

1.  Set up the potted plants and cover each with a polythene bag. Tie the polythene bag round the stem as shown below.

2. Leave the two set-ups in a sunny place.

3. Collect and test any liquid which collects in the plastic bag with anhydrous copper (II) sulphate or anhydrous cobalt chloride paper.

Study questions

(a) What observation was made in set-ups A and B after several hours?

(b) What conclusion can be made from the above observations?

(c) Which is the control experiment and why?

(d) What changes are observed on the anhydrous cobalt chloride paper?

Factors that affect the rate of transpiration

A plant is always absorbing water from the soil and losing it into the atmosphere by the process of transpiration. If the water is not replaced as fast as it is lost, wilting of the plant takes place. If this situation continues for a long time then the plant can die due to environmental or structural factors. The environmental factors affecting transpiration are:

Temperature

This is the factor that affects transpiration the most. The temperature surrounding a plant indicates the amount of heat present around the plant. On a hot day, the temperature is high because there is a lot of heat in the atmosphere.

This causes faster evaporation of water from the leaf and therefore the rate of transpiration is high. On cold days the temperature is low because there is very little heat in the environment. As a result of this, little evaporation of water from the leaf occurs. This causes the rate of transpiration to decrease.

Humidity

Humidity is the amount of water vapour in the air. If this amount of water is a lot then humidity is high. If it is little, then the humidity is described as low. When humidity is very high, air around the plant becomes saturated with water vapour. Under these conditions the water vapour gradient is low. This means that transpiration reduces or even stops. However, when the air is dry that is humidity is very low, there is a high water vapour gradient between the inside of the leaves and the surrounding environment. Therefore the rate of transpiration is high.

Wind

Wind is moving air. Wind carries with it moisture that has evaporated from the leaf surface creating room for more to occupy. This prevents the air surrounding the stomata from becoming saturated with water vapour from the leaf. The faster the air moves, the faster the rate of transpiration. Plants will lose a lot of water during windy conditions as compared to calm conditions.

Light intensity

Light intensity affects transpiration because it has an effect on the opening and closing of the stomata. The rate of transpiration is high when there is high light intensity because the stomata open
more. When the intensity of light is low, the rate of transpiration is reduced because stomata opens less. Stomata close in darkness, therefore at night very minimal amounts of water is lost.

All these factors discussed above are called external factors since they affect transpiration from the outside. Other factors affect the rate of transpiration from within the plant. These are called internal factors that affect transpiration. They include; stomatal distribution, leaf surface area, presence of a cuticle and the number of stomata on a leaf.

Potometer

Transpiration is measured using an instrument known as a potometer. This works on the principle that the amount of water lost is equal to the amount of water taken up by the plant.

Vaseline is applied around the rubber bungs to ensure an airtight seal, thus the only water loss from the apparatus is via transpiration. The function of the reservoir is to allow the air bubble to travel back to the start of the measuring scale on repeating the experiment. As water moves up through the plant the air bubble moves along the scale giving a measure of water absorbed by the plant over time and hence the  transpiration rate.

Activity 8.5: Measuring the rate of transpiration

Requirements

• Leafy plant freshly uprooted or a freshly cut leafy branch e.g. a tomato plant, bean or any other suitable plant

• Basin of water

• Scalpel

• Means of timing, for example, stop watch or wristwatch

• Potometer

• Polythene bag

Procedure

1.  Immerse the potometer in a basin of water making sure it is completely filled with water.

2.  Put the plant into the water and cut through the stem under water.

     • Why do you think the leafy stem is cut under water?

3.  Attach the freshly cut end of the stem into the potometer still under water.

4.  Remove the plant and potometer from the water and mount them in a fixed position. The end of the capillary tube should rest in a beaker of water.

5.  Setting up the potometer as shown below.

6. Carry out the following activities:

    (a) (i)  Place the potometer with the shoot in a windy place outside the classroom. 

         (ii)  Introduce an air bubble into the capillary tube by removing the beaker of water at the end of the tube     

              •  What happens to the bubble? 

        (iii) Measure the distances moved by the bubble for five minutes and record your results in the table as shown below.

(b) Repeat the above procedure but place the plant where air is still. Record the distance covered by the bubble.  Calculate the rate of water uptake by the same shoot.

(c) (i) Place the set-up outside in the hot sun and again in a cool place.

     (ii) Calculate the rate of water uptake in each case using the procedure described in (a) above.

(d) (i)  Put the plant in a humid  environment. By covering the leaves with a polythene bag and leaving it without the polythene bag to compare.

      (ii) Calculate the rate of water uptake in each case.

Study questions

1.  What does the potometer measure:

           (i) Directly 

           (ii) Indirectly? Explain?

2. What conclusion can you draw from your results when the following environmental conditions were investigated?

             (i)  Wind 

             (ii)  Humidity

             (iii)  Temperature?

3.  Evaporation alone cannot account for the movement of water through a plant? What other forces might be involved?

The potometer measures directly the rate of uptake of water. It also indirectly measures the rate of transpiration. Evaporation of water from the leaf leads to the replacement of this water by its uptake.

Self-evaluation Test 8.2

1.  Explain why plants growing in an enclosed environment (greenhouse) have a lower rate of transpiration than plants growing in the open field?

2.  Explain the following observation. A freshly cut stump of a tree will continue releasing water for some time.

8.3  Adaptations of plants to different environmental conditions 

Characteristics that enable a plant to survive in its specified habitat are called adaptations. 

Plants that grow in dry areas are called xerophytes, whereas others that grow in or near water are called hydrophytes.

Plants that grow in areas that are neither too dry nor too wet are called mesophytes, whereas plants that grow in saline habitats, for example, sea or ocean are called halophytes. 

Activity 8.6: To examine the adaptive features of different plants

You will be provided with samples, pictures and photographs of different plants. 

Procedure 

1.  Study each of the plants provided and note the following for each plant.

                        (a) Their leaves, stems and roots 

                        (b) Where they are found 

                        (c)  Their sizes 

2. Why do you think the plants have the features above? Find out from textbooks or the internet. 

3.   Share your findings with other class members.

Plants are structurally adapted to reduce the rate of transpiration. This depends on the environment they live in. The following are some of the ways through which plants growing in different habitats reduce transpiration rate. 

a. Xerophytes 

Xerophytes are plants that have characteristics suited to areas with very little water and very high temperatures. These plants grow in dry areas such as the arid and semi-arid areas. 

Adaptations of xerophytes to their habitat 

1.   They have a high ability to absorb water from the soil. Their roots are usually more developed and grow deep into the soil and extend over a wide area. 

2.   They have water storage tissues. Many xerophytes have swollen stems or leaves which contain special water storage tissues.  Such plants are called succulent plants.    

Examples of succulent plants are Bryophyllum, sisal, cactus, Euphorbia, and Aloe vera. A baobab tree has a thick stem for water storage. 

3.    Xerophytes reduce water loss through transpiration in many ways: 

(a) They have a thick waxy cuticle which prevents excessive water loss through the leaf by evaporation. 

(b) They have hairs that trap damp air near the leaf surface causing saturation.   

(c) In some xerophytes for example Nerium oleander, the stomata are found sunken in pits. The rate of transpiration is reduced because there is a space containing moist air between the stomata and the atmosphere.  

(d) Many xerophytes have small leaves to reduce surface area for transpiration. 

(e) Some xerophytes have very few stomata which are located on the lower epidermis away from the direct heat of the sun. Others have reversed stomata rhythm. Their stomata opens during the night and close during the day unlike ordinary plants.  

4.  Some xerophytes have life cycles that enable them to evade dry seasons, for example, some shed their leaves during the dry season. 

b. Mesophytes 

Mesophytes are plants that grow under average conditions of water supply and temperature. These plants grow very well on land and develop into forests and grasslands.

       Adaptations of mesophytes to their habitats 

1.  They have thin leaves, which ensure rapid diffusion of gasses from the stomata to the photosynthetic cells. 

2.  They have broad and flat leaf blades that provide a large surface area for absorption of light and carbon dioxide. 

3.  Mosaic arrangement of leaves on the plant to make sure that each leaf receives maximum amount of sunlight. 

4.  Presence of stomata on the upper and lower leaf epidermis to allow for efficient gaseous exchange and also for transpiration. 

5.  Internal structures of their leaves have air spaces that allow free circulation of gases. 

6.  Their leaves have cells with chlorophyll so that photosynthesis takes place. 

7.  They have thick transparent cuticles to prevent water loss. 

8.  They have a well developed root system with long tap or fibrous roots. 

c. Hydrophytes 

Hydrophytes are plants that live in water or in very wet places. 

Examples are Nymphaea and water hyacinth. There are three types of water plants: 

(i)  Emergent plants – they have roots and part of stem under water. While their leaves are above water. They have a problem of taking in excess water, for example, reeds. 

(ii) Floating plants – they float on the water surface with roots in water. Water lilies is an example.

(iii) Submerged plants - these are found completely under water, for example, spirogyra.

(a) Their cuticle is thin or lacking. This permits the plant to absorb water, minerals and carbon dioxide over its whole surface. 

(b)  Since some hydrophytes absorb water over their whole body surface, their roots are not well developed. The roots may be used for anchorage, for example, in water lily or used for absorption of nutrients. 

(c)  The presence of many air spaces in the stem and leaf tissue; a special tissue called aerenchyma which makes the plants buoyant for support and for gaseous exchange.  

(d) They contain little xylem and support tissue. They are supported by aerenchyma and the buoyancy of the water. 

(e) Submerged leaves do not have stomata and floating types have many on their upper surface.

d. Halophytes 

These are plants that grow in salty places such as rocky shores, seas and sand dunes which occur along coastal regions.  Some halophytes such as Atriplex and mangroves grow near sea water. 

Therefore they have a problem of taking up water from their salty surroundings. They have cells that absorb salt. As a result, they create a higher osmotic pressure which enables the plant to absorb water. Because of taking much salt, they excrete excess salts using salt glands. 

The salt is washed from the plant surface by rain.  Some halophytes absorb salt from their habitats and remove it by shedding leaves that have accumulated salt. 

Self-evaluation Test 8.3 

1.  What is the significance of modified leaves in xerophytes? 

2.  Halophytes are found in salty environments. How do they avoid having too much salt in their tissues? 

3.  Mutuyimana found a plant that did not have stomata. Where do you think she got the plant from?

Translocation 

Discussion corner

1.  Discuss with a classmate the meaning of the following terms  

               •  Source 

               • Sink  

               • Photosynthates  

               • Translocation 

2.  What structures are responsible for translocation in a plant?.

• The products of photosynthesis are called photosynthates.  They are usually in the form of simple sugars, such as sucrose. Photosynthates are produced by sources and are translocated to sinks. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks. Seeds, tubers and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season. 

• The movement of organic products of photosynthesis from leaves to other parts of the plant is called translocation. 

• Sugars are produced in the leaves (source) but other nonphotosynthetic parts of the plant like roots and stems also need part of this food.  For this reason, organic food products are translocated form the source to the sink. 

• Some important sinks are roots, flowers, fruits, stems and developing leaves. Leaves are particularly interesting in this regard because they are sinks when they are young and become sources later, when they are about half grown. 

• Organic products of photosynthesis (photosynthates) are translocated through the phloem tissue.

• Photosynthates are directed primarily to the roots during early development, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. The products from the source are usually translocated to the nearest sink through the phloem. 

The high percentage of sugar in phloem sap causes water to move from the xylem into the phloem. This increases water pressure inside the phloem, causing the sap to move from source to sink. 

Activity 8.7:  The ringing experiment  

Requirements 

Procedure 

1. Remove completely a ring of bark with its phloem from two branches. The xylem tissue which makes up the bulk of the stem is left intact. 

2.  The setup is left undisturbed for four weeks. 

Study question 

(a) What observations were made in the stem after four weeks? 

      Explain these observations. 

(b) Discuss with your friends how the bark of medicinal trees can be harvested without killing these important trees.

When the ring of bark is removed, the phloem beneath it is also removed. After several weeks, swelling above the cut ring is noted. 

The swelling is due to the accumulation of food substances. They were being transported from the leaves but could not get across the debarked part of the stem. That is why there is no swelling on the lower part of the ring. 

Self-evaluation Test 8.4 

1.  Differentiate between a source and a sink? 

2.  The movement of organic products of photosynthesis from________ to other parts of the plant is called_____________.

Unit summary 

• Transport in plants involves movement of materials in and out of the cells. It also involves materials being taken away from the cells to outside of the organism. 

• Transport is necessary because all cells in living organisms need food substances and oxygen from their surroundings. They produce waste substances that need to be eliminated from their bodies.  

• Plants have a transport system for the moving substances. They use two different types of transport tissue. 

• Xylem transports water and solutes from the roots to the leaves. 

• Phloem transports food from the leaves to the rest of the plant. 

• Water and mineral salts move through the xylem by the forces of cohesion, adhesion, capillarity, root pressure and transpiration pull. 

• Transpiration is the process by which a plant loses water in form of water vapour. 

• Transpiration is important because it cools the plant on hot days. It also assists the movement of water and mineral salts up the plant. 

• Environmental factors that affect transpiration are humidity, temperature, wind and light intensity. 

• Structural factors that affect transpiration include size of leaf and number of stomata. 

• Organisms have different adaptations that enable them survive in their habitats. Some plants live in or around wet places (hydrophytes), others live in dry areas (xerophytes) and others live in areas which are neither too wet nor too dry (mesophytes). There are also some that live in salty places (halophytes). 

• Translocation is the movement of materials from leaves to other tissues throughout the plant.