Yew tree leaf surface. Coloured scanning electron micrograph (SEM) of the surface of the leaf of an English yew (Taxus baccata). The little pores are stomata which regulate gas and water exchange between the leaf and the atmosphere. During the day they open to allow the exchange of gases during photosynthesis, closing at night to prevent water loss. They are opened and closed by two bordering guard cells. Yew is a long-lived poisonous tree which inhabits cool, hilly areas throughout Europe and northern Asia. Magnification x224 (x55 at 10cm wide).