Horsetail stem. Coloured scanning electron micrograph (SEM) of the stem of a horsetail (Equisetum giganteum). Gases enter and leave the stem through the stomata (pores, across centre for example) in its protective outer layer, the epidermis (grey). The pores are surrounded by guard cells (green). Photosynthesis only occurs in the stem of the plant. Horsetails (division Sphenophyta) are vascular plants with a hollow jointed stem and scale-like leaves. E. giganteum, which grows in the American tropics, can reach over 9 metres in length. Magnification: x165 at 6x7cm size. x260 at 4x5 inch size.