To prepare stained temporary mounts of onion peel - Lab Work

To prepare stained temporary mounts of onion peel and to record observations and draw labelled diagrams - Class 9th Science Lab Work

Experiment -3

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 solution (or safranin), glycerine, and water.

Procedure
Step 1: One fleshy scale leaf of an onion is taken. It is broken into two and a thin membranous peel is pulled out using a forcep adhering to the inner surface of the leaf. This is the epidermal peel.
Step 2: The peel is placed in a watch glass containing water and is cut into small rectangular pieces.
Step 3: 1 or 2 drops of methylene blue or safrarin is mixed in a small quantity of water taken in another watch glass. The peel is transfered into it. Peels are leave for about 3 minutes. The peel is dipped in water to remove excess stain.
Step 4: Clean slide is taken with a drop of glycerine in the middle and a brush is used to transfer the washed and stained peel on to it.
Step 5: Cover slip is placed over it by slowly lowering it with a needle. Avoid entry of air bubbles.
Step 6: Excess glycerine is removed from the edges of cover slip with the help of a piece of blotting paper.
Step 7: The slide is observed under the microscope, first in low power and then in high power.
Step 8: A labelled diagram of the cells is drawn as seen under microscope.
 

Observation Table
Sl. No. Feature Observation
1 Shape of cells spherical
2 Arrangement of cells compact
3 Inter -cellular spaces absent
4 Nucleus present
5 Cell wall present
6 Stained portions of cell cytoplasm
7 Unstained portions of cell                          cell wall and  vacuole         

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
1. Staining of the peel must be appropriate. Excess stain can be removed by rinsing the peel with water taken in a watch glass.
2. Use a brush to transfer the peel on to the slide.
3. While placing the cover slip care should be taken to avoid air bubbles.

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