Nail varnish peels provide a method of studying the surface structure of glabrous or sparsely haired leaves, often without damaging the plant. The method can also be applied to stems, petals, sepals, bud or cone scales or bark etc where these are large enough, although thin petals tend to shrivel. It could perhaps also be used for cut surfaces of wood, so long as these are first shaved smooth with a sharp knife.
Thick leaves like Ivy appear to suffer no ill effects, but thinner leaves like Wood Sorrel and most petals are killed by the nail varnish solvents.
Equipment: clear nail varnish, fine forceps, microscope slides, compound microscope.
Paint a thin patch of transparent nail varnish onto the surface to be studied. Leave to dry and peel off. Don't be tempted to apply a second coat as this just pevents it lying flat on the slide. Place on a microscope slide (shiny side down) and run a drop of water under the film to hold it flat against the slide. Examine under the microscope at magnification of x40 to x400. The shapes of epidermal cells, stomata and any surface sculpture are clearly visible.
The patch should be about 10mm diam, but, with practice, smaller patches can be used if the plant part is small. For standardisation, leaf patches should be applied at a point between half and two-thirds of the distance from the end of the petiole to the leaf apex. They should start at the midrib and cover part of the adjacent lamina. But try patches elsewhere to compare morphology. Use a mature leaf, but again, compare younger and senescent leaves.
The upper and undersides of leaves are often different and should both be sampled. Be careful to distinguish the two peels. (If in doubt, the lower surface often has more stomata.)
Ideally the varnish should be applied to living material in the field. The peels should be placed in separate labelled tubes or packets. Alternatively, wait until the varnish is at least half-dry then detach the leaf and take it home. Drying is quite quick in warm weather, but at lower temperatures it may be necessary to leave the film for several hours or over night. Sometimes the peel develops a "snake-skin" appearance due to the formation of tiny pits on the outer surface as it dries. This may be due to condensation but has not been investigated. Take care to ignore such artefacts when interpreting films.
When studying picked plant material, the varnish should be applied before wilting to avoid shrinkage or changes of shape due to loss of turgor.
Leaf microstructure is often asymmetrical. Try to keep track of the orientation of the peel, although this can sometimes be inferred from the venation.
Thin peels adhere well to the slide (van der Waals forces) and are reasonably tough. Each peel can be anchored with a small piece of exhibition tape for additional security. For long term storage slides should be labelled and need little more than wrapping in aluminium foil for protection, with the name repeated on the foil.