The observation of the onion peel is to see the arrangement of the cells that are separated by the thin layer. In onion, the peel is taken out from the scaly leaves that do not perform photosynthesis. They perform the function of storage. So, they do not show the stomata or the chloroplast. After the removal, the peel is kept in water to avoid drying of the leaves.

The material which is mounted on the slide is usually of fluid consistency. So, to avoid its spilling from sides, it is mounted in the centre. Also, if it is mounted at left or right side, then under microscope the material is not seen clearly. Hence, usually material is mounted in the centre. 

All living things are composed of cells. They are the building blocks of all life. Cells come in many different shapes and have different functions. Plant and animal cells are different, too. The main difference between plant and animal cells is that plant cells have a cell wall on the outer layer, whereas animal cells only have a cell membrane. The cell wall is a protective layer outside the cell membrane that also provides support for the cell's structure.

The onion peels were used to observe the vacuoles in the laboratory because the onion has scaly leaves that perform the function of storage of food and have a large vacuole present in their cells. 

The clear epidermal cells exist in a single layer and do not contain chloroplasts, because the onion fruiting body (bulb) is used for storing energy, not photosynthesis. Each cell has a cell wall, cell membrane, cytoplasm, nucleus and a large vacuole. The nucleus is present at the periphery of the cytoplasm. The vacuole is prominent and present at the centre of the cell, surrounded by cytoplasm. 

An onion is a multicellular (consisting of many cells) plant organism. As in all plant cells, the cell of an onion peel have all the key characteristics of the plant cells such as cell wall, nucleus towards the plasma membrane, prominent and large centrally located vacuole. The presence of a cell wall and a large vacuole are indicators that help identify plant cells, such as seen in the onion peel.

Onion cell is a plant cell with a cell wall and a large vacuole. On the other hand, human cheek cell is an animal cell with a prominent nucleus. Onion cells are brick-like in shape while human cheek cells are rounded. Human cheek cells do not have a cell wall or a large vacuole. Onion cell is an example of plant eukaryotic cell and cheek cell is an example of an animal eukaryotic cell.

Safranin stains epidermal peel of leaf pinkish red in colour. Safranin is a biological stain used in histology and cytology. Safranin is used as a counterstain in some staining protocols, colouring all cell nuclei red. It can also be used for the detection of cartilage, mucin and mast cell granules.

Bacterial cells are primitive cells hence, called as prokaryotic cells. They do not possess membrane bound organelles as mitochondria, lysosomes, etc. They also do not have well-defined membrane bound nucleus. 
Onion peel cell is an eukaryotic cell with well-defined membrane bound nucleus, membrane bound organelles as mitochondria, vacuole, etc. 

Plastids are double membrane bound organelles found inside plants and some algae, which are primarily responsible for activities related to making and storing food. Many plastids are photosynthetic but some are not.

Initially, onion peel has to be taken out.
Then, keep the onion peel in water in a petridish. This will remove the impurities sticked to onion peel. 
Then, add a few drops of safranine stain and transfer to a slide.
Then, add a drop of glycerine on onion peel. This step prevents the peel from drying out. 
Then, cover it with a cover slip for observing it under the microscope and to study the cell pattern. 

Glycerine is isotonic to the cell sap. Due to this, cells used for mount will not shrink or swell and can be seen in their natural form. Also, glycerine makes the mounted material moist and prevents it from drying out.