Old English treo, treow "tree" (also "wood"), from Proto-Germanic *trewan (cf. Old Frisian tre, Old Saxon trio, Old Norse tre, Gothic triu), from PIE *deru- "oak" (cf. Sanskrit dru "tree, wood," daru "wood, log;" Greek drys "oak," doru "spear;" Old Church Slavonic drievo "tree, wood;" Serbian drvo "tree," drva "wood;" Russian drevo "tree, wood;" Czech drva; Polish drwa "wood;" Lithuanian derva "pine wood;" Old Irish daur, Welsh derwen "oak," Albanian drusk "oak").
Importance of the oak in mythology is reflected in the recurring use of words for "oak" to mean "tree." In Old English and Middle English, also "thing made of wood," especially the cross of the Crucifixion and a gallows (cf. Tyburn tree, gallows mentioned 12c. at Tyburn, at junction of Oxford Street and Edgware Road, place of public execution for Middlesex until 1783). Sense in family tree first attested 1706; verb meaning "to chase up a tree" is from 1700. Tree-hugger, contemptuous for "environmentalist" is attested by 1989.
Minc'd Pyes do not grow upon every tree,
But search the Ovens for them, and there they be.
["Poor Robin," Almanack, 1669]
Any of a wide variety of perennial plants typically having a single woody stem, and usually branches and leaves. Many species of both gymnosperms (notably the conifers) and angiosperms grow in the form of trees. The ancient forests of the Devonian, Mississippian, and Pennsylvanian periods of the Paleozoic Era were dominated by trees belonging to groups of seedless plants such as the lycophytes. The strength and height of trees are made possible by the supportive conductive tissue known as vascular tissue.