Throughout the major cultures of Europe the oak tree has been held in high esteem. To the Greeks, Romans, Celts, Slavs and Teutonic tribes the oak was foremost amongst venerated trees, and in each case associated with the supreme god in their pantheon, oak being sacred to Zeus, Jupiter, Dagda, Perun and Thor, respectively. Each of these gods also had dominion over rain, thunder and lightning, and it is surely no coincidence that oak trees appear to be more prone to lightning strikes than other trees, whether because of their wood's low electrical resistance or the fact that they were frequently the largest, tallest living things in the landscape.
The Druids frequently worshipped and practised their rites in oak groves (the word Druid was probably a Gaelic derivation of their word for oak, Duir, and meant men of the oaks). Mistletoe, probably the Druids' most potent and magical plant, frequently grew on oak trees and its presence was believed to indicate the hand of God having placed it there in a lightning strike.
Ancient kings presented themselves as the personifications of these gods, taking on the responsibility not only for success in battle but also the fertility of the land, which relied on rainfall. They wore crowns of oak leaves, as a symbol of the god they represented as kings on Earth. Similarly, successful Roman commanders were presented with crowns of oak leaves during their victory parades, and oak leaves have continued as decorative icons of military prowess to the present day.
Perhaps because of the oak's size and presence, much of its folklore concerns specific, individual oak trees. Many parishes used to contain what became known as the Gospel Oak, a prominent tree at which part of the Gospel was read out during the Beating of the Bounds ceremonies at Rogantide in spring. In Somerset stand the two very ancient oaks of Gog and Magog (named after the last male and female giants to roam Britain), which are reputed to be the remnants of an oak-lined processional route up to the nearby Glastonbury Tor. The Major Oak in Sherwood Forest is purported to be the tree where Robin Hood and his Merry Men hatched their plots
In Leicestershire the Topless Oaks in Bradgate Park were said to have been pollarded as a sign of mourning following the beheading, in 1554, of Lady Jane Grey who had lived at the nearby Bradgate Hall. After the battle of Worcester in 1651 King Charles II hid from the Roundheads in a large oak at Boscobel. In 1660 he instigated the 29th of May as Royal Oak Day to celebrate the restoration of the monarchy.
Children would wear oak leaves (or better still, oak apples) as part of a custom which officially lasted until 1859 but in fact continued until well into the twentieth century. Once again the symbol of oak leaves had royal connections. And so it won't be a surprise which plant was the clan badge of the Royal Clan Stewart.