The late Professor Oersted, of Copenhagen, was of opinion that he had demonstrated the polymorphy of the Tremelloid Uredines, and satisfied himself that the one condition known as _Podisoma_ was but another stage of _Roestelia_.[d] Some freshly gathered specimens of _Gymnosporangium_ were damped with water, and during the night following the spores germinated profusely, so that the teleutospores formed an orange-coloured powder. A little of this powder was placed on the leaves of five small sorbs, which were damped and placed under bell-glasses. In five days yellow spots were seen on the leaves, and in two days more indications of spermogonia. The spermatia were discharged, and in two months from the first sowing, the peridia of _Roestelia_ appeared, and were developed. "This trial of spores," says Oersted, "has conduced to the result expected, and proves that the teleutospores of _Gymnosporangium_, when transported upon the sorb, give rise to a totally different fungus, the _Roestelia cornuta_, that is to say, that an alternate generation comes between these fungi. They appertain in consequence to a single species, and the _Gymnosporangium_ ceased to be an independent species, and must be considered as synonymous with the first generation of _Roestelia_. The spores have been transported upon young shoots of the juniper-tree, and have now commenced to produce some mycelium in the bark. There is no doubt that in next spring it will result in _Gymnosporangium_." Subsequently the same learned professor instituted similar experiments upon other hosts, with the spores of _Podisoma_, and from thence he concluded that _Roestelia_ and _Podisoma_, in all their known species, were but forms the one of the other. Hitherto we are not aware that these results have been confirmed, or that the sowing of the spores of _Roestelia_ on juniper resulted in _Podisoma_. Such experiments should be received always with care, and not too hastily accepted in their apparent results as proven facts. Who shall say that _Roestelia_ would not have appeared on _Sorbus_ within two months without the sowing of _Podisoma_ spores?--because it is not by any means uncommon for that fungus to appear upon that plant. It is true many mycologists write and speak of _Roestelia_ and _Podisoma_ (or _Gymnosporangium_) as identical; but, as we think, without the evidence being so complete as to be beyond suspicion. It is, nevertheless, a curious fact that in Europe the number of species of _Roestelia_ and _Podisoma_ are equal, if one species be excluded, which is certainly not a good _Podisoma_, for the reception of which a new genus has been proposed.[e]