What is the first step in the microscopy practical when preparing a slide with onion epidermal tissue?

The first step in preparing a slide with onion epidermal tissue involves adding a drop of water. This step is crucial as the water serves as a mounting medium, helping to keep the onion cells flat and allowing for better visibility under the microscope. The water prevents the tissue from drying out during the observation, ensuring that the cells remain in a suitable state for examination.

Before adding the onion epidermal tissue, it is also essential to have the slide prepared with a liquid medium to facilitate the observation of the transparent cells. Once the water is added, a thin layer of the onion epidermis can be placed on the slide, followed by other steps such as adding a stain and covering with a cover slip for clarity during microscopy.