years since, a quantity of dead box
leaves were collected, on which flourished at the time a mould named
_Penicillium roseum_. This mould has a roseate tint, and occurs in
patches on the dead leaves lying upon the ground; the threads are
erect and branched above, bearing chains of oblong, somewhat
spindle-shaped spores, or, perhaps more accurately, conidia. When
collected, these leaves were examined, and nothing was observed or
noted upon them except this _Penicillium_. After some time, certainly
between two and three years, during which period the box remained
undisturbed, circumstances led to the examination again of one or two
of the leaves, and afterwards of the greater number of them, when the
patches of _Penicillium_ were found to be intermixed with another
mould of a higher development, and far different character. This
mould, or rather _Mucor_, consists of erect branching threads, many of
the branches terminating in a delicate globose, glassy head, or
sporangium, containing numerous very minute subglobose sporidia. This
species was named _Mucor hyalinus_.[j] The habit is very much like
that of the _Penicillium_, but without any roseate tint. It is almost
certain that the _Mucor_ could not have been present when the
_Penicillium_ was examined, and the leaves on which it had grown were
enclosed in the tin box, but that the _Mucor_ afterwards appeared on
the same leaves, sometimes from the same patches, and, as it would
appear, from the same mycelium. The great difference in the two
species lies in the fructification. In the _Penicillium_, the spores
are naked, and in moniliform threads; whilst in _Mucor_ the spores are
enclosed within globose membraneous heads or sporangia. Scarcely can
we doubt that the _Mucor_ alluded to above, found thus intermixed,
under peculiar circumstances, with _Penicillium roseum_, is no other
than the higher and more complete form of that species, and that the
_Penicillium_ is only its conidiiferous state. The presumption in this
case is strong, and not so open to suspicion as it would be did not
analogy render it so extremely probable that such is the case, apart
from the fact of both forms springing from the same mass of mycelium.
In such minute and delicate structures it is very difficult to
manipulate the specimens so as to arrive at positive evidence. If a
filament of mycelium could be isolated successfully, and a fertile
thread, bearing the fruit of each form, could be traced from