Schurr, U., & Schulze, E. D. (1995). The concentration of xylem sap constituents
in root exudate, and in sap from intact, transpiring castor bean plants (Ricinus communis L.). Plant,
Cell and Environment, 18(4), 409-420. doi:10.1111/j.1365-3040.1995.tb00375.x.