A LESSON PLAN ON THE BIOLOGY EXPERIMENT CARRIED OUT AT “IIS NELSON MANDELA”, CASTELNOVO NE’ MONTI, ITALY

OBSERVATION OF ONION AND VEGETAL TISSUES WITH MICROSCOPE

1. Introduction

The bulb is an underground shoot with a more or less ovate globular shape. It consists of a very short stem surrounded by modified, fleshy leaves, which have the function of storing reserve nutrients. In the lower part the roots are visible. It is typical of biennial plants such as onion.

Once the plant has sprouted, the first year of life it enlarges the bulb, while the second year it uses the accumulated substances to flower. (This is why onions are harvested in the first year). The bulb is also a vegetative (agamic) propagation organ of the plant. 

2. Goals

To observe the cells of the onion by visualizing in particular the cell the wall and the nucleus.

3. Materials

1) An onion bulb

    2) Microscope

    3) Microscope slides and coverslips

    4) Violet dye of gentian

   5) Distilled water

   6) A small beaker

   7) Dropper

   8) Scalpel

   9) Tweezers

   10) Glass rods

 11) Absorbent paper

  12) Latex gloves

4. Products

Slides on which to observe the onion cells

5. Procedure

1) Pour some water into the beaker and add a few drops of gentian violet (the solution must not be too colored) and mix

2) Cut the onion and leaf through the bulb, take a fragment of the film present on the inside of the leaves of the bulb.

3) place the sample on the slide and add a drop of solution taken from a small beaker, cover with the coverslip and press lightly with absorbent paper

4) Place the slide on the microscope slide and observe starting from the lower magnification and then moving on to the higher ones

6. Observations

 The cell wall made up of cellulose is observed. Cellulose is a complex glucose polymer with a structural function in plants.

The cell nucleus is observed: it contains the genetic material (DNA and RNA) of the species (and variety)

The nucleus can be found in the central area of the cell or near the wall.

A great part of the cell can be occupied by the vacuole which contains various organic and inorganic substances.