Water-based varnish is a great affordable alternative to traditional nail varnish used to take an impression of leaves in order to measure stomata density. Simply paint onto the leaf, wait until it is dry then peel off and view under a microscope. The varnish is non-toxic meaning leaves do not have to be removed from the plant, and, being water-based, used brushes can be easily washed.
Traditional clear nail varnish is also available.