Fig. 17.1-2a and b. Transverse section of phellem cells in oak (Quercus, above) and Clusia (below). The cork cells of geranium in the previous micrograph appeared to be empty, but these in oak and Clusia obviously are not. Before dying, they deposited large amounts of materials, probably tannins, into their central vacuoles. Tannins deter insects and other animals from feeding on bark, adding to the protective capacity of bark.