317 
MICROSCOPIC FUNGI. 
RUSTS. 
BY M. C. COOKE. 
B EFORE entering more especially upon the subject of this 
paper, it may be advisable to attempt an explanation 
of a phenomenon of no uncommon occurrence in many groups 
of Fungi, and which is termed Di-morphism. 
In the Uredines, or uredo-like fungi, as well as other of the 
Goniomycetes (in which the spores are the principal feature), 
the same fungus appears under two distinct forms, not neces- 
sarily mere differences of age, but so distinct that they have 
been regarded (and some are so still) as different species 
belonging to different genera, often far removed from each 
other, and bearing different names. One plant, for instance, 
sprinkled over the under surface of a rose-leaf, like turmeric- 
powder, has its mycelium, or root-like threads, penetrating 
the tissue, whilst bearing above its spherical golden- coloured 
spores. Its vegetative system is complete, and, apparently, 
its reproductive also ; hence it seems to claim recognition as a 
perfect plant, and under the name of TJredo Rosce was so recog- 
nized, until microscopical investigation determined otherwise. 
Thus it has been discovered that certain dark brown spots 
which appear later in the season are produced upon the same 
mycelium, and are indeed aggregations of more perfect and 
complex fruits of the same plant. Before this point was 
satisfactorily decided, the brown spores, which are borne on 
long stalks, and are themselves septate or divided (apparently 
or really) by transverse partitions into a complex fruit, received 
the name of Puccinia Rosce. At this period, TJredo Rosce and 
Puccinia Rosce , or the yellow fungus and the dark brown 
fungus, were believed to be distinct and different plants ; 
now, on the contrary, they are believed to be different forms 
of fruit produced by the same plant ; i.e., an instance of 
di-morphism. Arcgma mucronatum , Fr., is the present scien- 
tific name of what is regarded as the perfect fungus, whilst 
the uredo form either bears the name of Lecythea Rosce, Lev., 
or by some mycologists is rejected altogether as a spurious 
species. 
