Why we used dyes when we prepare onion , human cheek eqithelium smears, and didn’t use it in cork and leaf epidermal cells smears! ?

Explanation:

Staining is done for onion cells to visualize the cytoplasm of yellow and white onions because they are not pigmented. The staining of human cheek cells with methylene blue is necessary to stain the nucleus to get clear image of it under microscope because methylene blue binds to negative charge nucleic acids  

While for leaf epidermal cells smears, staining is not necessary as they are visible under light microscope without staining and they also have pigmented materials inside them such as chlorophyll chloroplast.  The cork cell is a non-living and have a cell wall whose structure is visible under light microscope.  

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