, and at first consist of a
white mouldy stratum, composed of delicate mycelium, on which erect
threads are produced, which break up into subglobose joints or
conidia. The species on grass was named _Oidium monilioides_
before its relationship was known, but undoubtedly this is only the
conidia of _Erysiphe graminis_. In like manner the vine disease
(_Oidium Tuckeri_) is most probably only the conidia of a species of
_Erysiphe_, of which the perfect condition has not yet been
discovered. On roses the old _Oidium leucoconium_ is but the conidia
of _Sphaerotheca pannosa_, and so of other species. The _Erysiphe_
which ultimately appears on the same mycelium consists of globose
perithecia, externally furnished with thread-like appendages, and
internally with asci containing sporidia. In this genus there are
no less than five different forms of fruit,[E] the multiform
threads on the mycelium, already alluded to as forms of _Oidium_, the
asci contained in the sporangia, which is the proper fruit of the
_Erysiphe_, larger stylospores which are produced in other
sporangia, the smaller stylospores which are generated in the
pycnidia, and separate sporules which are sometimes formed in the
joints of the necklaces of the conidia. These forms are figured in the
"Introduction to Cryptogamic Botany" from _Sphaerotheca Castagnei_,
which is the hop mildew.[F] The vine disease, hop mildew, and rose
mildew, are the most destructive species of this group, and the
constant annoyance of cultivators.

[Illustration: FIG. 103.--_Erysiphe cichoracearum._ _a._ Receptacle; _o._
mycelium. (De Bary.)]

When first describing an allied fungus found on old paper, and named
_Ascotricha chartarum_, the Rev. M. J. Berkeley called attention to
the presence of globose conidia attached to the threads which surround
the conceptacles,[G] and this occurred as long since as 1838. In a
recent species of _Chaetomium_ found on old sacking, _Chaetomium
griseum_, Cooke,[H] we have found tufts in all respects similar
externally to the _Chaetomium_, but no perithecium was formed, naked
conidia being developed apparently at the base of the coloured
threads. In _Chaetomium funicolum_, Cooke, a black mould was also found
which may possibly prove to be its conidia, but at present there is no
direct evidence.

The brothers Tulasne have made us acquainted with a greater
number of instances amongst the _Sphaeriacei_ in which multiple
organs of reproduction prevail. Ver