P. A.  Rydberg, ?Studies on the Rocky Mountain Flora. - 5.?, Bull. Torrey Bot. Club, vol. 28, pp. 266 - 284, 1901.

P. A.  Rydberg, ?Studies on the Rocky Mountain Flora. - 11.?, Bull. Torrey Bot. Club, vol. 31, pp. 399 - 410, 1904.