If the question you are asking is where did you find the information
on the transformation of the apocryphal gospel palm tree to the English cherry tree, the place I would look is the Oxford Book of Carols (or the New Oxford Book of Carols, depending on what edition you found). That title has a load of information on the background of of hundreds of Christmas carols (famous and obscure alike).

Chris Brennan
cristoir@rochester.rr.com