hollow
Water horsetail Equisetum fluviatile with strobili
Marsh horsetail Equisetum palustre with strobili
Field horsetail Equisetum arvense
Horsetails: Calamophyte Equisetum arvense, palustre, & fluviatile
Oak Quercus robur acorn and leaves
Oak Quercus robur tree
Oak bark Quercus robur
Oak polypore fungus piptoporus quercinus
Oak Quercus robur
Lesser Water Parsnip Berula erecta
Diagram of the cross section of a bird bone