Fig. 9.3-10. Transverse section of leaf of Brassica oleracea (this species has many varieties: broccoli, Brussels sprouts, cabbage, cauliflower, kale and kohlrabi). The secretory structure in this leaf is a hydathode, a gland that secretes water (see page 157 in Plant Anatomy (Mauseth) for details of the physiology). Hydathodes are supplied by a vascular bundle that ends just at the border of the hydathode or actually within it (as here). The bulk of the glandular tissue is a very open, aerenchymatous parenchyma with the special name of epithem. Often, there is a compact boundary layer (called a sheath) that separates the epithem from the rest of the leaf cells, but the hydathode shown here lacks a sheath. Epithem cells absorb mineral nutrients out of the xylem sap and transfer them to the leaf tissues, and excess water accumulates in the intercellular spaces; as the water accumulates, pressure builds and forces water out of wide stomata called water pores (arrows). The process of secreting excess water is called guttation.