Why call this hat Xylem? The lacy eyelets and other design features reminded me of xylem under a microscope (see here, or check out the Google image search results)! It all seemed to click into place, since the color of this lovely yarn is called Sap -- enjoy!
This pattern is also included in The Knitgrrl 9 ebook.
This product is the digital download of a PDF knitting pattern.