One of the pieces of evidence often cited in proving the authenticity of the Prince Henry Sinclair journey to North America in 1398 (and the related Westford Knight carving) is the existence of various North American flora carved on the walls and ceiling of the iconic Rosslyn Chapel. The Chapel was built in 1456 by Henry Sinclair's grandson, William Sinclair, and the thought is that William possessed drawings of flora from his grandfather's journey.
I have often seen maize and aloe mentioned as two such North American plants. But on a recent trip to Scotland our Rosslyn Chapel tour guide pointed out a third plant, trillium, pictured here:
Here is an image of trillium from Wikipedia:
To be more particular, this is trillium cernuum. This plant is native to eastern Canada and New England--in fact, trillium is the official flower of the province of Ontario. It is not found in Europe. According our tour guide, a botanist the Chapel retained confirmed that the carving was, indeed, North American trillium.
To state the obvious, someone from Scotland must have been to North America and seen trillium prior to 1456.