NCERT Class 9 Science Lab The World of Living

The World of Living

AIM

To prepare a stained, temporary mount of onion peel and to study its cells.

THEORY

All living organisms are composed of cells. New cells arise by the division of pre-existing cells. Cell is the structural and functional unit of life. In plants, cells have an outermost rigid cell wall beneath which is a cell membrane. The cell membrane encloses cytoplasm, cell organelles, and a nucleus.

MATERIALS REQUIRED

An onion bulb, slides, cover slips, two watch glasses, needle, brush, forceps, razor blade, compound microscope, blotting paper, methylene blue (or safranin) solution, glycerine, and water.

PROCEDURE

1. Take one fleshy scale leaf of an onion. Break it into two and using a forcep pull out a thin membranous peel adhering to the inner surface of the leaf. This is the epidermal peel.

2. Place the peel in a watch glass containing water and cut it into small rectangular pieces.

3. Mix 1 or 2 drops of methylene blue or safrarin in a small quantity of water taken in another watch glass and transfer the peels into it. Leave the peels for about 3 minutes. Dip the peel in water to remove excess stain.

4. Take a clean slide with a drop of glycerine in the middle and using a brush transfer the washed and stained peel on to it.

5. Place a cover slip over it by slowly lowering it with a needle. Avoid entry of air bubbles.

6. Remove excess glycerine from the edges of cover slip with the help of a piece of blotting paper.

7. Observe the slide under the microscope, first in low power and then in high power.

8. Draw a labelled diagram of the cells as seen under microscope.

9. Note the features listed in the observation table.

RESULTS AND DISCUSSION

The cells that form the peel are rectangular in shape, compactly arranged and without any intercellular spaces. Each cell has a distinct cell wall, a prominent nucleus and a vacuole. The cells form the outer layer of the leaf known as epidermis.

PRECAUTIONS

• Staining of the peel must be appropriate. Excess stain can be removed by rinsing the peel with water taken in a watch glass.

• Use a brush to transfer the peel on to the slide.

• While placing the cover slip care should be taken to avoid air bubbles.

Please refer to attached file for NCERT Class 9 Science Lab The World of Living