Triangulating from the cairns…
At the present time the genus Melampsorella is considered by Arthur (2) to be monotypic, with the single species M. cerastii confined in its aecial stage to Picea and Abies, on which it forms characteristic witches' brooms, and in its telial stage to Cerastium and Stellaria. The basis for Arthur's treatment is found in the footnote at the end of the species description, in which he states that the pycnia are definitely subcuticular, but have been erroneously considered to be subepidermal in the Picea form, a fact which Arthur says has tended to obscure the true relationship. Earlier authors, among them Arthur and Kern (3) have considered that the forms on Picea and Abies were distinct species largely because the pycnia were subcuticular in Abies and subepidermal in Picea. While doing research work at the Rocky Mountain Biological Laboratory at Gothic, Colo., during the summer of 1939, the writer had access to abundant infections on Picea engelmanni, and also to infections on Abies lasiocarpa. The aecial stage is of interest, not only because of the size and abundance of the witches' brooms, but also because the mycelium is systemic and perennial. Since the writer has been for some time interested in the systemic perennial rusts (5) (6) an attempt was made to collect and study this fungus at various stages during its development in order to throw some light upon the true relationship of this species. This paper represents a preliminary report on the differences that have been found in the pycnial and aecial stages on these two hosts.
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