Light micrograph of the surface of a leaf of the elder tree, Sambucus nigra. The small oval structures scattered over the surface are stomata (pores), which provide the plant with a means of gaseous exchange between the atmosphere & the internal tissues. The pores are held open during the day & are closed at night to conserve water. Surrounding each stoma are two curved lip-like guard cells which control the opening & closing of the pore. Magnification: x200 at 35mm size.