http://www.ci.cambridge.ma.us/~Historic/faq.html

Q:  Did George Washington take command of the American Army under the Washington Elm?
A:  This popular legend became part of American popular culture as early as the 1830s.  The story was made famous during the centennial year of 1876, with the publication of a fictitious "eye-witness" journal,  The Diary of Dorothy Dudley.  The tree itself was indeed real and stood in the middle of Garden Street at the intersection of Mason Street. In 1923, the diseased and very fragile trunk was accidently pulled over by a city worker.  A plaque embedded in the street's pavement marks the original location of the tree. Washington did take command of the army in Cambridge on July 3, 1775, but there is no documentation to indicate that the event took place under the elm tree.  Washington established his New England headquarters at the Vassal House on Brattle Street (now better known as the Longfellow House), where he resided until April 1776.  The myth of Washington and the elm is still widely known today, and the image of the tree remains a symbol of patriotism in Cambridge.

Q:  What are the symbols on the city seal?

A:  The current city seal is a revision of the original seal, which was adopted in 1846. The seal contains an image of the Gothic Revival style building, Gore Hall, the former library building at Harvard College, and an image of the Washington Elm, the Cambridge tree made famous by the popular legend of George Washington taking command of the American Army under the tree during the Revolution. The Latin motto, which is often included around the city seal, reads: "Literis Antiquis Novis Institutis Decora."  It can be translated as: "Distinguished for Classical Learning and New Institutions."  Also written in Latin are the founding and chartering dates for the town and city, which are translated as: "Built in A.D. 1630.  Chartered a city in A.D. 1846."