2.1.1 - Anatomy of dicotyledenous plants1.p… · This diagram of a cross section shows the...

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2.1.1 - Anatomy of dicotyledenous

plants*

Siyavula

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1 Anatomy of Dicotyledonous Plants

Plant structure:http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookplantanat.htmlPlants are made up of roots, stems, leaves and �owers. The function of the root is to hold the plant

�rmly in the ground as well as to absorb water from the soil. The function of the stem is to transport thefood made by the leaf to the rest of the plant as well as to hold the plant upright. The main function of theleaves is to photosynthesize (make food).

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Figure 1

1.1 Di�erences between monocotyledonous and dicotyledonous plants

Traditionally the �owering plants (angiosperms) are divided into two groups, monocotyledons (monocots)and dicotyledons (dicots). Monocots are the grass and grass-like �owering plants (e.g. maize), while dicotsinclude the rest of the �owering plants (e.g. bean). The embryos of monocots have only a single (mono-)cotyledon (the �rst leaf) while the embryos of dicots have paired (di-) cotyledons. Other di�erences betweenmonocots and dicots are shown in the table below. Monocots have long narrow leaves with parallel veinswhile dicots have broad leaves with net-like veins. In monocots the �ower parts are in multiples of three whilein dicots they are in multiples of four or �ve. In monocots the vascular bundles of the stem are scatteredwhile in dicots there is a ring of vascular bundles. Monocots grains have one furrow or pore while dicotgrains have three furrows or pores. Monocots have adventitious roots while in dicots the roots develop froma radicle.



Figure 2

1.2 Plant structure

Most plants are stationary which means that they cannot move from place to place. Some plants growreally tall in order to obtain sunlight. They need to stand tall and erect and therefore need to supportthemselves. They have tissues present in almost all parts of their body e. g. roots, stems, branches, leaves.These supporting tissues keep the stem �rm and other parts such as leaves in a favourable position forphotosynthesis to occur as e�ciently as possible.

Refer to Unit 1.4 for functions of the di�erent tissues found in roots, stems and leaves.

1.3 Dicotyledonous Root

External structure of the dicot root



Figure 3

This diagram shows the external structure of a dicot root.

• Root cap protects the tip of the root and it is slimy to facilitate movement through the soil as the rootgrows.

• Above the root cap is the meristematic region where cells divide continuously by mitosis to producenew cells.

• Cells enlarge in size in the region of elongation. This results in the root growing in length.• Thousands of tiny root hairs are found in the root hair region. The function of this region is to absorb

water and dissolved mineral salts from the soil.• The root grows thicker and may produce lateral roots in the mature region.



1.3.1 Internal structure of the dicot root

Figure 4

• No waterproof cuticle in the root as this would hinder the absorption of water.• The epidermis is a single layer of cells on the outside that protects the inner tissues. Some epidermal

cells are specialized to form root hair cells. These absorb water and dissolved mineral salts.• The cortex consists of parenchyma cells. These cells are large to store water and food. They also

facilitate the movement of water from the root hair cells on the outside to the xylem on the inside.• The endodermis is lined with Casparian strips, distinctive bands made of a water-impermeable, waxy

substance called suberin, that prevents water and minerals from passively seeping between the cellsand thus forces water to enter through the cell membranes of the endodermal cells in order to enterthe stele (vascular cylinder).

• The stele consists of the:

· Pericycle (responsible for forming lateral roots)· Xylem (responsible for transporting water and mineral salts to the stem)· Phloem (responsible for transporting food from the leaves to the roots)



1.3.2 Movement of water through the dicotyledonous root

Figure 5

This diagram shows the movement of water through the root

• Water is found in the spaces between the soil particles. Water enters through the cell wall and cellmembrane of the root hair cell by osmosis. Water �lls the vacuole of the root hair cell.

• Water can now move across the parenchyma cells of the cortex in two ways:• Most of the water passes along the cell walls of the parenchyma cells by di�usion. Movement of

water and solutes between the intercellular spaces without crossing the plasma membrane is known asapoplastic movement.

• Some of the water passes from the vacuole of one parenchyma cell to the vacuole of the next cell byosmosis. Movement of water and solutes through the cells is known as symplastic movement.

• The water must pass through the endodermis to enter the xylem.• Once water is in the xylem of the root, it will pass up the xylem of the stem.

Transpiration and movement of water: http://www.phschool.com/science/biology_place/l abbench /lab9/xylem.html

This website shows a diagram of how water moves up through the plant.http://www.neok12.com/Plants.htm



This video shows plant transport and provides some interactive quiz games.

1.3.3 Investigation: Water uptake by roots

Aim : To measure the uptake of water by rootsApparatus

• Plastic 2 litre Coke bottle• water• soil• scissors• measuring scale• tree or plant cuttings• ruler

Method

1. Remove the label from the Coke bottle. Cut the top of the bottle o� 20cm from the bottom of thebottle. Poke holes in the bottom of the bottle for drainage. Hold the plant cutting in the containerwhile you �ll the container with soil. Leave one or two leaf buds about 5cm above the soil.

2. Weigh the container to get the total weight of the bottle, soil and plant.3. Water the plant with enough water so that it starts to run out of the bottom of the bottle through the

holes.4. Weigh the container again after the water has stopped running out and subtract the total weight in

step 2 from the weight in step 4 to get the weight of the water. 1 litre of water is equal to 1 kilogramof water, therefore you can work out exactly how much water is in the container.

5. Set the containers by the window where they will receive enough sunlight. Wait for the leaves to startgrowing (1-3 weeks).

6. After the leaves are growing, weigh the containers every 1-3 days for 3 weeks. Subtract the new weightfrom the weight calculated in step 4. The new number is the amount of water that the plant is using.Water the plant as necessary (when the soil becomes dry), but remember to reweigh the containerwhen you add more water so that you can still tell how much water the plant is taking up.

7. Draw a graph of the amount of water the plant is using. The X-axis should show the number of dayssince the beginning of the experiment and the Y-axis should show the amount of water that the planthas used.

Question on investigation � Surely as the plant grows it forms new leaves etc which have mass? How do wesubtract the mass of the new leaves to prevent

this making the mass of the entire container inaccurate in terms of how much mass isjust water usage? - I also wondered this but obtained the investigation elsewhere so don' t know. I

thought perhaps the plant does not put on that much mass compared to the mass of the water so it becomesinsigni�cant

1.4 Dicotyledonous stem

• Leaves develop from the nodes.• The sections of stem between the nodes are called internodes.• An axillary bud is often found at the node. These forms lateral branches.• A terminal bud is found at the tip of the stem and allows the stem to increase in length.



Figure 6



1.4.1 Internal structure of the dicotyledonous stem

Figure 7

This diagram of a cross section shows the internal structure of a young dicot stem

• A waterproof cuticle is found on the outside of the epidermis to prevent water loss.• The epidermis consists of a single layer of cells to protect the underlying tissue.• The cortex is made up of parenchyma cells that stores water and food.• The vascular bundles are arranged in a ring in the medulla and are surrounded by non-living scle-

renchyma cells for strengthening and support.• Each vascular bundle contains the following:

· Cambium (contains meristematic cells that divide to widen the stem)· Phloem (transports food from leaves to the roots)· Xylem (transports water from the roots to the stem)

http://bcs.whfreeman.com/thelifewire/content/ch p36/36020.htmlThis is a link to an online tutorial about phloem, xylem and pressure �ow.

1.4.2 Movement of water up the stem

• Water moves up the xylem from the roots to the leaves.• Adaptations of xylem for transporting water:

· Long, elongated tubes joined end-to-end without any cross-walls, forming good conducting tubes.· The cell walls are thickened with lignin for support (annual or spiral thickening) so that they do

not collapse due to the upward pull of water



· Pitted vessels and tracheids allow for lateral movement of water into neighbouring xylem vessels.· Cells are dead, so there is no obstruction to water transport

Figure 8

Diagram of xylemThree forces are responsible for the movement of water up the xylem � capillarity, root pressure and

transpiration suction force.

• Capillarity involves forces of cohesion (forces of attraction between water molecules) and adhesion(forces of attraction between water molecules and the sides of the xylem vessels). Because the xylem'slumen (opening) is so tiny, water will move up by capillary. However, this force is weak and its role inmoving water up the stem is small.

• Root pressure is a force that pushes water up the xylem. As water enters the root by osmosis, it pushesthe water that is already in the xylem of the stem upwards.

• Transpiration suction force is a very important force that pulls water up the xylem of the stem. Aswater evaporates from the stomata of the leaves during transpiration, it creates a sucking force thatwill pull the water up the xylem.



1.4.3 Investigation: plant tissue anatomy (root and stem)

Aim: To examine the structure of the root and stemApparatus

• Scalpel or knife• Celery stalk (stem)• Carrot (root)• Glass slide• Iodine solution (Stain) or water• Cover slip• Dissecting needle or tweezers• Paper and pencil

Method1. Cut a very thin slice (cross section) from the middle of the celery stem or the carrot root.2. Place this section on a glass slide.3. Cover the specimen with iodine solution in order to stain it. This makes it more visible under the

microscope. The specimen can also be placed on a drop of water if iodine is not available.4. Cover the specimen by carefully lowering the cover slip onto it with a dissecting needle or tweezers.

Take care not to trap any air bubbles.This link gives information about making a wet mount microscope slide and shows an instructional video.http://www.microbehunter.com/2010/08/13/making-a-wet-mount-microscope-slide/5. Call your teacher.6. Switch on the microscope making sure the lowest objective is in position (the 4x objective).7. Place your slide on the stage.8. Focus the image under the 4x objective (lowest objective) and view the structure of the celery stem.

Switch to the 10x objective to look a little more closely. To see amazing details of the structure of planttissue, use the 40x objective and the slide, carefully observing all of the parts and di�erent cells.

9. Once you are able to see cells,10. Call your teacher.11. Make a biological drawing of your specimen as viewed under the microscope. Take note of the

magni�cation and draw a scale bar. Label your diagram according to the tissues you have learnt about.Variation: Be creative and try using your favourite vegetables! Which vegetables are roots, stems and

leaves?To prepare a slide:



Figure 9

Place the sample in the centre of the slide. Add a drop of iodine or water on top of the sample. Placethe cover slip next to the droplet as shown in the diagram.

Lower the coverslip into place with tweezers. As you lower the coverslip downwards, the drop will spreadoutward and suspend the sample between the slide and the coverslip.

(Diagrams from http://www.ehow.com/how_5164819_prepare-wet-mount-slide.html )

1.4.4 Investigation: water uptake by stem

Aim: To examine the uptake of water by the stemApparatus:

• Water• Food colouring dye (available at supermarket)• White �ower on a stem, e.g. Impatients, carnation or chrysanthemum• Scissors• Two jars, cups or measuring cylinders• Plastic tray• Sticky tape

Method:



Before starting this experiment, try to guess how the dye might move up the stem into the �ower.1. Fill one jar with plain water, and one with water containing several drops of food colouring dye.2. Take the �ower and carefully cut the stem lengthwise, either part way up the stem or right up to the

base of the �ower (try both � the results will be di�erent!)3. Put one half of the stem into the jar containing plain water and one half of the stem into the jar

containing food colouring dye. To make it easier to insert the stalks without breaking them, it helps towedge paper underneath the jars so that you can tilt them towards each other. Tape the jars or cylindersdown onto a tray so that they do not fall over.

4. Observe the �owers after a few hours and the next day, and note where the dye ends up in the�owerhead. You can leave the �owers up to a week but be sure to make sure that they have enough water.

Variation: Instead of using one cylinder with water and one with food dye, use two di�erent colourfood dyes (e.g. blue and red). At �rst the �ower will show two separate colours, but as time goes by thewhole �ower will show both dyes. This is because water can move sideways between xylem vessels throughopenings along their length. The ability of water to move sideways between vessels is useful for when airbecomes trapped in a vessel, causing a blockage. If you cut the stem right up to the base of the �ower, thiswill limit movement between the xylem vessels.

Variation: Try using celery stalks with leaves. Cut open the celery stalk (cross-section) and you willsee that the little holes inside are coloured � these are the vessels.

An example of this experiment with photographs can be found at: http://www.practicalbiology.org/areas/intermediate/cells-to-systems/transport-in-plants/investigating-transport-systems-in-a-�owering-plant,70,EXP.html

1.5 Secondary growth

1.5.1 Meristematic Tissue

• Meristematic tissue consists of small cells that are unspecialized. These cells divide by mitosis to formnew cells that can di�erentiate (undergo changes in their structure) and can become specialized tissue(e.g. xylem, phloem, epidermal cells)

• Primary meristematic tissue is found in the tips of roots, stems and buds. When it divides new cellsare produced which causes the plant to grow longer. This is referred to as primary growth.

• Secondary meristematic tissue originates from permanent tissue, usually parenchyma tissue whichdivides by mitosis. Cambium is secondary meristematic tissue that is found in roots and stems. Whenthese cells divide by mitosis it results in the plant becoming wider. This is called secondary growth.

• Every growing season the stem of a plant increases in width � this is known as secondary thickening.• Towards the end of the �rst year of growth, the parenchyma cells between the vascular bundles become

meristematic and link up with the cambium tissue to form a cambium ring.• The cells in the cambium ring start dividing to form secondary phloem (on the outside) and secondary

xylem (on the inside).• Each year another ring of secondary phloem and secondary xylem is formed, making the stem grow

wider.• It is not possible to see the layers of secondary phloem but the secondary xylem are visible. These

form rings called annular rings which can be used to work out the age of a plant.• As new rings are formed each year, the older rings are pushed inward and the xylem vessels collapse

due to the pressure. The wood in the centre becomes denser and harder than the wood at the surfaceand is called heartwood.

• The youngest annual rings found on the outside serve its function of transporting water. This wood isnot as dense and is called sapwood.

• The light-coloured rings are called spring wood. They are formed during spring and summer when thegrowing conditions are favourable. The rings are therefore relatively thick and light in colour as thexylem cell walls are thin.

• The dark-coloured rings are called autumn wood. They are formed during autumn and winter whenthe growing conditions are unfavourable. The rings are therefore relatively thin and dark in colour as



the xylem cell walls are thick.

http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookplantanat.htmlThis website provides information on plant structure and support.

Figure 10

This diagram shows the process of secondary thickening in stems



Figure 11

This diagrams shows the annual rings of a tree trunk

1.5.2 Investigation - Tree rings and climate change

Every year a tree forms a new layer of xylem around the trunk. This forms tree rings, which are visible ascircles in a cross section of a tree that has been cut down. Each tree ring, or wood layer, consists of twocolours of wood; light wood that grows in spring and summer and dark wood that grows in autumn andwinter. Tree rings can be counted to give you a rough estimation of the age of a tree. Occasionally a treewill form many rings in one year or miss forming rings in a year. The width of tree rings is greater in yearswhere good growing conditions occur. In years with droughts or low temperatures, the trees will producesmaller rings. Therefore, by looking at the tree rings you can get an idea of the weather a�ecting a tree ina particular year. Scientists can use this information to help determine the weather patterns of the past aswell as events such as forest �res, earthquakes and volcanic eruptions. The study of past events using thegrowth rings of trees is known as dendrochronology (�dendros� = tree, �chronos� = time).

Aim: to observe annual tree rings to assess age and climatic conditions1. Find a cut or fallen tree, and count the tree rings, starting with the innermost ring. Measure the

width of each ring using a ruler, or make a note of whether a ring is narrow or wide. Make a note of anyscars caused by events such as �res or pests.



2. Draw a bar graph showing the width of your tree rings for every year of the tree's life.3. How old is your tree? What can you say about the climatic conditions throughout the life of your

tree?http://www.classzone.com/books/earth_science/terc/content/investigations/es2905/ es2905page01.cfm

This is a link to an online tutorial about counting tree rings.http://www.arborday.org/kids/carly/lifeofatree/This is a link to a great cartoon video about the di�erent tissue layers in trees (xylem, phloem, etc) and

the formation of tree rings.http://www.worsleyschool.net/science/�les/tree/rings.htmlThis is a link to a good website about tissue layers in trees.

1.6 Economic importance of plant support tissues

Plant support tissue supplies with two important resources namely wood and �bre . Xylem is a sourceof wood and the sclerenchyma is a source of �bre.

Of course in order to obtain wood and �bre we need to cut down numerous trees. This is called defor-estation . Deforestation has escalated in the recent years due to the growing need for wood.

ActivityCollect data showing the area covered by forests in the years 1990 and 2010. Find this data for the

following countries: South Africa, Europe, Asia, North and Central America and South America.Divide the class into two teams. One group will argue the need for us to cut down trees and the other

will be responsible for convincing us that deforestation must be reduced dramatically.

1.7 INDIGENOUS KNOWLEDGE

Making paper, �ax, cotton, sisal. Traditional use of �brous plants by san bushman using sansevaria to makerope etc. thatching. Can we please get help with this?

1.8 Dicotyledonous leaf

1.8.1 Internal structure of the dicotyledonous leaf

Refer to chapter 1 to remind yourselves of the internal structure of a dicotyledonous leaf.



Figure 12

This diagram shows the movement of water through a dicot leaf.


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