Memoirs of The New York Botanical Garden
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of Vascular Plants of Northeastern United States and adjacent Canada
Maguire, Bassett & Wurdack, John J. 1957. The Botany of the Guiana Highland -- Part II. Mem. New York Bot. Gard. 9 (3): 235-392.
Shrubby with lateral compound inflorescence; has achieved the ultimate in capitulum reduction. All heads are uniflorous. The genus is geographically restricted to the eastern part of the Guayana Highland.