Squash leaf. Coloured scanning electron micrograph (SEM) of the underside of a squash (Curcubita maxima) leaf, showing trichomes (mushroom-shaped) and a stoma (slit, upper centre). Trichomes are tiny hair-like structures that can be glandular, secreting oils or other substances, or structural for defence. Stomata are pores that are the site of gaseous exchange in the plant. The opening and closing of the stomata is controlled by semicircular guard cells (dark), which swell and become turgid to close the openings.