Horsetail stem. Coloured scanning electron micrograph (SEM) of a section through the stem of a horsetail (Equisetum giganteum). Gases enter and leave the stem through the stomata (pores) in its outer layer, or epidermis (grey, upper frame). The stomata are surrounded by guard cells (green/ brown). Photosynthesis occurs in the chloroplasts (tiny green spheres such as at bottom centre) of the stem's parenchyma cells (brown). 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: x400 at 6x6cm size. x600 at 4x5"~SQUARE"