hollow

Water horsetail Equisetum fluviatile with strobili shown emerging from fresh water.

Marsh horsetail Equisetum palustre with strobili growing in shallow water or marshland.

Field horsetail Equisetum arvense, with background.

Horsetails: Calamophytes Field horsetail Equisetum arvense, Marsh horsetail Equisetum palustre, & Water or River horsetail Equisetum fluviatile

Oak  acorn and lobed leaves with long acorn stalk

Oak tree showing typical growth habit of an old tree

Oak bark Quercus robur study

Oak polypore fungus piptoporus quercinus bracket fungus growing on the side of an oak tree

Oak Quercus robur pen and ink tree shape without leaves

Lesser Water Parsnip Berula erecta with individual flower and fruit

Bird bone cross section showing cross bracing for strength and hollow structure