In the summers of 1898 and 1899, Howard Pyle had an old mill converted into a studio for his students from the Drexel Institute. After establishing his own art school, Pyle continued the arrangement in 1900, 1901, 1902, and 1903. N. C. Wyeth spent the last summer there and later settled in the area. 
The artist Leander Fontaine did a colored pencil drawing of that unique tree and a corner of the famous mill. 
And you can't help but wonder if N.C. Wyeth based this tree from Robin Hood (Correction: Black Arrow) on that beloved sycamore.The nearby Brandywine River Museum displays the work of Pyle and the Wyeths, but that was not the location of the school.
The building, known as Turner’s Mill, has just been refurbished as town offices and meeting rooms.
More modern pictures here.
Thanks to Kev Ferrara for clueing me in to the art based on the tree.
Here's a very complete biography and bibliography about Pyle.
Addendum: I have reworded the main paragraph to make it a bit more accurate, thanks to the help of Pyle expert Ian Schoenherr, whose blog is a must-visit for Pyle aficionados.