As the new layers of wood are formed round it year by year, the pith becomes more and more compressed, and it sends out thin walls of its own substance, radiating to the outside of the stem, so as to separate the woody tissue into wedge-like blocks. Theseradiating walls of soft matter are the medullary rays. Theycarry on the work of the pith, and form channels for theflow of the sap."
"But the sap does not always rise through the medullary rays, does it, sir?" asked Willie.
