e _AEcidia_ contain, within a cellular membranous
sac, a fructifying disc, which produces necklaces of spores, which
ultimately separate from each other in the form of a granular powder.
The grains of which it is composed germinate in their turn, no
longer avoiding the stomates as before, but penetrating through
their aperture into the parenchym. The new resultant mycelium
reproduces the _Uredo_, or fifth form of fructification, and the
_Uredo_ spores fall off like those of the _AEcidium_, and in respect of
germination, and mode of penetration, present precisely the same
phenomena. The disc which has produced the _Uredo_ spores now gives
rise to the resting spores, and so the cycle is complete.[c]

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