Preparing biological material for viewing

Preparation is everything you do to biological material before it goes under the lens. Its whole purpose is to give the light a thin, flat, transparent piece of tissue to pass through, and to make the structures inside it show up.

The wet mount

A wet mount is a specimen held in a drop of water between a slide and a coverslip. It is the base method — every technique below ends in a wet mount.

• Start with a clean, dry slide. Grease and dust both show up at ×400 and both look like structures.
• Place one small, thin piece of material in the middle, in a single drop of water. More water than that floats the coverslip and the specimen drifts while you draw.
• Add the stain, if you are using one, as a drop at one edge of the coverslip.
• Lower the coverslip slowly, from an angle, resting one edge on the slide and letting it down on a mounting needle. Dropping it flat traps air.
• Blot the excess liquid from the opposite edge with filter paper. That also pulls stain across underneath the coverslip.

Air bubbles are the commonest thing to mistake for a cell.

• A bubble has a thick black rim and a completely empty bright centre, and it sits at a different focus from the tissue.
• It hides everything underneath it, so a bubble over your specimen means remounting.

Getting the material thin enough

Which method you choose depends on what the material is like.

• Epidermal tear — snap a leaf or a fleshy scale leaf and peel back the skin. The epidermis you pull off is one cell thick, which is exactly what you want. Onion bulb and many leaves both tear well.
• Cellulose impression — paint a thin layer of clear cellulose varnish on a leaf surface, let it dry, lift it off with clear tape and mount it. It takes a print of the surface, so you get the shape and arrangement of the epidermal cells and the stomata without damaging the leaf. Use it on a leaf too tough to tear, such as harakeke.
• Cutting sections — hold the material steady and cut across it with a sharp blade, taking several slices and mounting the thinnest one. Soft material can be supported inside a piece of carrot or pith so it does not squash. Use it for a stem, a root, or a leaf in cross section.
• Cavity slide — a slide with a hollow ground into it, used for material that is alive and moving or too deep for a flat slide. Pond water goes on a cavity slide.
• Squashing — press gently on the coverslip through a paper towel to spread a thick piece into a single layer. Press straight down, never sideways, or the cells shear apart.

Stains

Most plant tissue is nearly colourless, so a stain is what makes the detail visible.

• Iodine solution stains starch grains blue-black, and gives the cytoplasm and nucleus a yellow-brown tint. It is the stain for storage tissue such as potato.
• Methylene blue stains the nucleus dark blue against a paler cytoplasm, which is what makes a nucleus countable and drawable.
• Choose the stain for what you want to show. A stain that does not bind to the structure you are drawing has only added colour, not information.
• Use one drop. Over-staining turns the whole field one dark colour and buries the outlines you needed.
• Chloroplasts need no stain — they are already green and already distinct.

Working safely and getting a usable slide

• Cut away from your fingers, on a tile or board, never in your hand.
• Take several attempts. A section is cheap; the first one is almost never the thinnest.
• Label the slide with what it is, so three slides made in three sessions do not become three unknowns.