284 M.E. P. Fries on the Geographical 
hymenium of various Hymenomycetes; but they only show 
themselves in certain years peculiarly favourable to the pro- 
duction of Fungi, and then they ordinarily occur in great abun- 
dance. How and where their spores are preserved in the inter- 
vals of these recurrent appearances is a question that cannot be 
satisfactorily answered. Several species are epizoic, as, for ex- 
ample, some species of Cordiceps among the Pyrenomycetes, and 
sundry Isarie among the Gymnomycetes,—the examples of the — 
genus first named living upon the larvee of insects, whilst those 
of the second live upon perfect insects. Only one example of 
an epizoic Hymenomycetes can be adduced, viz. Agaricus (Clito- 
cybe) cerussatus, y. nauseosus, which has been met with in Russia 
on the carcase of a wolf; this habitat may, however, have been 
merely accidental. 
The Myxogastres exhibit, with reference both to their habitats 
and to their entire development, a striking contrast with other 
natural families among the Gasteromycetes. They are frequently 
destitute of a visible matrix; their spores, in the course of deve- 
lopment, form a drop of mucilage, which, in certain species, 
augments with incredible rapidity, and assumes a very great va- 
riety of shapes: of the most remarkable types among these are 
those which protrude vein-like rays, which spread themselves 
outwards over surrounding bodies, even on living plants, and by 
their ramifications produce a sort of network. These species 
often grow with such rapidity that the eye can with difficulty 
follow their increase, as is illustrated in Diachea elegans. The 
Spumaria alba, a common species of Myxogastres, hangs like a 
frothy mucoid mass to the stems of grasses*; and when fructifi- 
cation, or the actual Fungi make their appearance on this myce- 
lium, they have neither the form, nor the consistence, nor the 
colour of the primitive mucus, but the whole organism assumes 
a yellow hue, whilst, too, its spores are black. Similar trans- 
formations largely prevail among the Myxogastres: hence it is 
necessary to follow attentively the different phases of their de- 
velopment, in order to avoid the mistake of constructing several 
species out of any one of them. 
* Dr. W. Nylander, the translator of Fries’s memoir into French, appends 
the following note to these remarks :—“ From the not very correct manner 
inwhich Fries here speaks of the Myxogastres, and especially of the Spumaria 
alba, it might be suspected that his views respecting these Fungi are not 
very exact. Some readers might even suppose that an imperfect examina- 
tion had led him to confound the mucoid mass which conceals the larva 
of Cercopis spumaria with the earliest phase of development of the Reticu- 
laria alba—a Fungus certainly less common in Sweden than the homo- 
pterous insect above named: at least, the preceding remarks of Fries con- 
cerning the first stage of Spumaria apply with more truth to the singular 
product from the Cercopis, 
