CBSE Class 10 Science Practical Skills – Stomata

CBSE Class 10 Science Practical Skills – Stomata

BASIC BUILDING CONCEPTS
Stomata are tiny pores present on both upper and lower surfaces of the leaves but cannot be observed by an unaided eye. A compound microscope is required to study the minutest details of a stoma(singular).
We should know how to use a compound microscope in order to observe minute things while conducting experiments in the Biology labs.

Compound Microscope
A compound microscope is an instrument which is used to observe the enlarged and magnified image of the small objects.
A compound microscope consists of two systems of lenses called the objective lens and ocular lens (eyepiece) to obtain higher magnification.

Parts of a Compound Microscope

I. Mechanical Parts

1. Base
   It is a horse shoe-shaped structure that provides sta-bility and support to the microscope.
2. Pillar
   It is a vertical pro-jection fixed with hinge to the foot (base).
3. Arm
   It is curved and sup¬ports body tube and is used for handling the microscope.
4. Inclination joint
   It is the point where arm is attached to pillar and this permits tilting of microscope to adjust to eye level.
5. Stage
   Stage is rectangular flat metallic plate fixed to the lower end of arm. It has a hole in centre for the light to reach the object. It is used for keeping the object to be observed.
6. Clips
   Two clips on stage help in holding the slide.
7. Diaphragm
   It is present below the stage and regulates the entry of light on to the slide. Diaphragm may be disc diaphragm and iris diaphragm.
8. Body tube
   It is hollow tube which is attached to upper part of the arm. It can move up and down with the help of screws.
9. Nosepiece
   It is the circular metallic piece attached to lower end of body tube. It is fitted with three objective lens having different magnifications.
10. Coarse adjustment screw
    This can move body tube up and down for focussing
11. Fine adjustment screw
    This is small-size screw needed for fine adjustments.

II. Optical Parts

1. Mirror
   It is used for reflecting light into the microscope and is located at lower end of the arm, below the stage. It is concave on one side and plane on the other.
2. Eyepiece
   It is lens fitted at the top of body tube. It can be of 10 X or 15 X magnification.
3. Objective lenses
   These are attached to nosepiece. Usually, objectives with magnification lOx (low power) and 45 X (high power) are available to the students.

How to use Microscope

1. Take a compound microscope and clean it with the help of cotton cloth.
2. Move the objective lens (low power) and set it right above the central hole in the stage.
3. Adjust the mirror, while looking in the microscope to set the light. Adequate light should be available to illuminate the microscope.
4. Take a clean prepared slide and clip it on to the stage.
5. Move the coarse adjustment screw of microscope to focus object on the slide.
6. Now, once slide is focussed and observed under low power (10 X), then move the nosepiece and adjust the high power (45 X) without moving the coarse adjustment.
7. Focussing at high power (45 X) can be made sharp by use of fine adjustment screw.

Mounting a slide

1. Take a clean glass slide and wipe it with cotton cloth to remove any dust ).
2. To locate the centre, place the slide on a paper and draw its outline. Remove the slide and draw the diagonals to mark the centre in the outline of slide. Now, place the slide on this paper over the outline drawn and hence centre point can be observed.
3. In a watch glass, take some stain and place the material in it for staining. After a minute, wash the material with water so that excess stain get removed.
4. Now, place a drop of glycerine on the centre of slide and transfer the material on to drop with the help of brush.
5. Place the coverslip on the slide in such a position that its one side touches the liquid on the slide.
   Hold the other side edge with the help of needle, obliquely placed. Now, gently lower down the needle till the coverslip touches the slide and covers the material. In this way air bubbles will be eliminated.
   

AIM
To prepare a temporary mount of a leaf peel to show stomata.

MATERIALS REQUIRED
Leaf of Tradescantia or periwinkle or balsam, slide, coverslip, forceps, needles, cotton cloth, brush, blotting paper, watch glass, blade, dropper, glycerine, safranin and compound microscope.

THEORY
Stomata (singular: stoma) are tiny pores present on the surface of the leaves. Though they are found on both upper and lower epidermis of leaf but they are more in number on the lower epidermis. Each stoma is bounded by two kidney-shaped guard cells. These guard cells possess a nucleus and a number of chloroplasts. The walls of guard cells are differentially thickened and elastic, i.e. they are thickened on inner side and thinner and more elastic on outer side.
The opening and closing of stomata is operated by the change in the turgidity of guard cells. The stomata help in exchange of oxygen, carbon diodide and water vapour between atmosphere and plant.

PROCEDURE

1. Take a freshly plucked leaf, and remove the peel from its lower surface by tearing it.
2. Put the leaf peel in a watch glass containing water so that the peel does not dry.
3. To the watch glass containing leaf peel, add 1-2 drops of safranin to stain the peel.
4. With the help of a brush, place the leaf peel in the centre of a clean slide.
5. Put a drop of glycerine on the slide over the peel.
6. Now, with the help of a needle, gently place a coverslip over the peel.
7. Remove the excess glycerine with a blotting paper.
8. Observe the slide, first, under the lower magnification (i.e., 10 X) of a compound microscope and then, under the higher magnification (i.e., 45 X).

OBSERVATIONS

1. Epidermis or leaf peel consists of a number of cells which are irregular in outline and are arranged in single layer with no intercellular spaces.
2. Tiny pores called stomata are seen in the epidermal cells.
3. Each stoma consists of two kidney-shaped guard cells.
4. Each guard cell has a nucleus and many chloroplasts.
   

RESULT
Minute apertures called stomata are seen in the temporary mount of leaf peel. Each stoma is enclosed by two kidney-shaped guard cells. These guard cells differ from other epidermal cells in having chloroplast.

PRECAUTIONS

1. Peel should be taken from freshly plucked leaf.
2. Peel should not be allowed to dry.
3. Leaf peel should not be over stained.
4. The slide should not be dirty. –
5. Use a brush to transfer the leaf peel from watch glass to slide.
6. Peel should be placed in centre of slide.
7. Curling of peel should be avoided while placing it on slide.
8. The epidermal peel should be small in size.
9. Place the coverslip gently to avoid entry of air bubbles.
10. Excess stain and glycerine should be removed with blotting paper.

INTERACTIVE SESSION

Question 1.
Why is it that we most commonly use safranin as a stain for staining sections of plants?
Answer:
Safranin (red coloured stain) stains the lignin and suberin and other plant materials easily.

Question 2.
Why should the leaf peel be mounted in glycerine?
Answer:
The leaf peel should be mounted in glycerine so that the cells of peel do not dry up.

Question 3.
Why is it preferred to take an epidermal peel from lower surface of leaf?
Answer:
In monocots, the distribution of stomata on lower and upper surface of leaf is almost similar. But in case of dicots, the number of stomata are more on the lower surface than upper surface. Hence, it is preferred to take epidermal peel from lower surface of leaf.

Question 4.
What are guard cells?
Answer:
Guard cells are the cells surrounding the stomatal pore. Each guard cell consists of a nucleus and many chloroplasts.

Question 5.
What is the shape of guard cells in

1. Dicots
2. Monocots?

Answer:
The shape of guard cells in

1. Dicots — Kidney-shaped
2. Monocots — Dumb-bell-shaped.

Question 6.
Name the instrument used for knowing the relative size of stomata.
Answer:
Porometer.

Question 7.
Where can you find chloroplasts in the stomata?
Answer:
Chloroplasts are present in the guard cells.

Question 8.
Give an example of a plant where stomata is absent.
Answer:
Hydrilla.

Question 9.
How are guard cells different from the rest of the epidermal cells?
Answer:
Guard cells are nucleated and have peculiar shape (either kidney-shaped as in dicots or dumb¬bell-shaped as in monocots) and possess many chloroplasts. Guard cell walls show differential thickenings. Epidermal cells on the other hand lack chloroplast and are irregular in shape.

Question 10.
What is the most important function of stomata?
Answer:
Functions of stomata include gaseous exchange and transpiration.

Question 11.
What is transpiration?
Answer:
The loss of water in the form of vapour from leaves of plant is called transpiration.

Question 12.
State the type of modification in xerophytic plants with respect to structure of stomata.
Answer:
They possess sunken stomata.

Question 13.
Name the plant hormone that helps in opening of stomata.
Answer:
Cytokinin.

Question 14.
Name the plant hormone which causes stomatal closure.
Answer:
Abscisic acid (ABA).

Question 15.
Name the inorganic ions which help in opening and closing of stomata.
Answer:
K+ ions (potassium ions).

Question 16.
Why do guard cells bend, when they swell up?
Answer:
The guard cells bend because of the differential thickenings of its inner and outer walls. The inner wall of guard cell is thicker and less stretchable as compared to the outer wall.

Question 17.
State the most important functions of leaf.
Answer:
Important functions of leaf include

• Photosynthesis
• Respiration
• Transpiration.

Question 18.
State the most important significance of transpiration.
Answer:
Transpiration helps in upward movement of water (against gravity) in plants.

NCERT LAB MANUAL QUESTIONS

Question 1.
What is the function of guard cells in stomata?
Answer:
Guard cells control the opening and closing of stomata.

Question 2.
Why is the number of stomata greater on the lower surface of a leaf?
Answer:
If more stomata are present on upper surface of leaf, then there would be more transpiration. Hence, to avoid more transpiration the number of stomata is greater on the lower surface of leaf.

Question 3.
Why are stomata absent in roots?
Answer:
Stomata are present on aerial parts of plant for transpiration to occur. Roots are present in soil which would block them.

Question 4.
What is the shape of guard cells in stoma of grass leaf?
Answer:
Dumb-bell shaped.

Question 5.
Do guard cells have rigid or elastic walls? Justify your answer.
Answer:
When the guard cells become turgid, their thin walls get extended and thick walls become concave. This results in opening of stomata. On the other hand, when guard cells lose water and become flaccid, the thin and thick walls revert back to original position resulting in the closure of stomatal pore. This shows that guard cells have elastic walls.

PRACTICAL BASED QUESTIONS

Multiple Choice Questions/VSA (1 Mark)

Question 1.
While preparing a temporary mount of stomata, four students used different stains as given below: [CBSE Sample Paper 2009]

The correct stain was used by the student
(a) A
(b) B
(c) C
(d) D

Question 2.
To prepare a good temporary mount of the petunia leaf peel showing many stomata, the student has to get the peel from the [Foreign 2009]
(a) tip of the leaf
(b) upper surface of the leaf
(c) lower surface of the leaf
(d) point of attachment of the leaf to its petiole

Question 3.
Why do we place the leaf peels in water ?
(а) Guard cells become turgid
(б) Peels remain green
(c) Peels don’t dry up
(d) To wash the leaf peels.

Question 4.
Precautions to be taken by students while preparing a temporary mount of a leaf peel to show its stomata is/are
(a) the slide should not be dirty
(b) peel should be taken from freshly plucked leaf
(c) leaf peel should not be over stained
(d) all of these

Question 5.
While preparing a temporary stained mount of a leaf epidermal peel, the extra glycerine is removed by [CBSE 2012, 2011, Delhi 2007]
(a) washing with water
(b) washing with calcium chloride solution
(c) soaking with blotting paper
(d) absorbing with cotton wool

Question 6.
In order to complete the diagram of stomatal
apparatus given below, nuclei should be drawn in the parts marked. [Delhi 2007C]

(a) A and B
(b) A and C
(c) B and C
(d) A, B and C

Question 7.
In the following diagram of the stomatal apparatus, which parts are incorrectly labelled? [AI 2007C]

(a) I and IV
(b) II and III
(c) IV and V
(d) I and V

Question 8.
A well stained leaf peel preparation when focussed under high power of the microscope would show [AI 2007]
(a) epidermal cells, guard cells each with one nucleus and many chloroplasts and stomata.
(b) epidermal cells, stomata, guard cells with many nuclei and one chloroplast each
(c) stomata and guard cells without nuclei or chloroplasts
(d) stomata but no guard cells or epidermal cells

Question 9.
A well stained leaf peel mount when observed under the high power of a microscope shows nuclei in [CBSE 2012, AI 2008]
(a) only epidermal cells
(b) only guard cells
(c) guard cells and epidermal cells
(d) guard cells, epidermal cells and stoma

Question 10.
In the slide of an epidermal peel, the parts which appear pink coloured after staining with safranin are [Foreign 2008]
(a) stomata only
(b) nuclei only
(c) cell membrane and cytoplasm
(d) all parts in the peel

Question 11.
In the diagram of stomata shown below, the labelling by four students was tabulated by the teacher in the table given below. Whose labelling was correct?

(a) A
(b) B
(c) C
(d) D

Question 12.
Students observed the epidermal peel of a leaf under the high power of a microscope. The following are the sketches made by them.

The correct sketch is
(a) A
(b) B
(c) C
(d) D

Question 13.
Epidermal cells are different from guard cells with respect to
(a) thickenings of wall
(b) shape of cell
(c) cytoplasmic contents
(d) all of these

Question 14.
The steps involved in making a slide of epidermal peel of leaf are given as follows:
I. Pull out a thin peel from the lower surface of the leaf.
II. Place a drop of glycerine on the slide.
III. Stain the peel in safranin.
IV. Place the stained peel on the glycerine.
V. Remove the extra stain by washing with water.
VI. Place the cover slip over the peel.
Which one is the correct sequence of steps to be followed?
(a) I, II, III, IV, V, VI
(b) I, III, V, II, IV, VI
(c) I, III, IV, II, V, VI
(d) I, II, IV, III, V, VI

ANSWER KEY

1. (c)
2. (c)
3. (c)
4. (d)
5. (c)
6. (c)
7. (b)
8. (a)
9. (c)
10. (d)
11. (c)
12. (c)
13. (d)
14. (b)

Short Answer Questions (2 Marks)

Question 1.
Tradescantia leaf is usually taken for preparing temporary mount of leaf peel to observe stomata. Why? Name any two other leaves that can be used for this experiment.
Answer:
Tradescantia provides good leaf peel. Therefore, it is commonly used for this experiment. Lily leaf and Bryophyllum leaf can also be used for preparing temporary mount of leaf peel to observe stomata.

Question 2.
How can you take out peel from lily leaf? Why is it necessary to keep the peel in water?
Answer:
Take a fresh lily leaf and simply fold it in the centre. Remove the peel from the lower surface. Leaf peel should be put in a watch glass containing water so that the peel does not dry. If peel gets dried up, shape of the cell boil becomes distorted.

Question 3.
Name the cells that form stoma. What is their shape?
Answer:
Two guard cells constitute a stoma. In dicotyledons they are kidney-shaped but in monocotyledons they are dumbbell shaped.

Question 4.
List the various steps of observing a slide under the microscope.
Answer:

• Take a clean prepared slide and clip it on
• Look in the microscope and adjust the mirror to illuminate the slide.
• Move the coarse adjustment screw of microscope to focus the slide (object) under low power.
• Move the nosepiece and adjust the high power objective lens.
• Focus the slide (object) at high power by using fine adjustment screw. the stage.

Question 5.
When a student observes a temporary mount of leaf peel under a microscope, he observes two different types of cells in leaf peel. Name these two different types of cells. On what basis can a student differentiate between these two cells.
Answer:
Two different types of cells are epidermal cells and guard cells.
Epidermal cells lack chloroplast and are irregular in shape while guard cells have chloroplast. The guard cells can be kidney-shaped or dumb-bell shaped.

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