142 
ANNALS OF THE 
as in Mucor / or into a brush-like one as in Aspergillus^ so named from a fancied 
resemblance to the brush used for sprinkling holy- water in Roman Catholic 
Churches. 
Such is, briefly, the mode of development of these minute plants under favor- 
able conditions. But there are occasional deviations to be met with, which are 
deserving of attention as throwing a clearer light upon certain forms which are to 
be mentioned presently This will be manifest when I state that from the results 
of numerous experiments I have made, the plant may be caused to remain in any 
one of its different stages of growth by supplying it with food suitable for that 
purpose. The bearing of this statement will be seen in the subsequent remarks 
upon the identity of the parasitic fungi. A familiar illustration of the power 
above mentioned is to be observed in common yeast, which presents itself as a collec- 
tion of spherical cells containing nuclei and capable of endless multiplication in two 
ways, viz; — by the formation of buds, or by the bursting of the cells and the liber- 
ation of nuclei which become cells. Yeast is derived from the aerial spores of one 
or more common species of mould. This I have proved by experiment ; and I have 
recently obtained additional proof of the correctness of this view from the examina- 
tion of spontaneous yeast of the tan-pits kindly forwarded to me from Kingston, 
Canada, by Professor Lawson. This in no way differs from brewers' yeast which 
has been long kept. The favorite food of the yeast-cell is sugar, upon which it acts 
in such a manner as to disturb the feeble combination of its chemical elements. 
This process, which is termed catalysis by chemists, causes decomposition of the 
sugar and a new arrangement of its particles, giving rise to carbonic acid and al- 
cohol. Sugar is essential to the maintenance of yeast in its integrity. As soon as 
its requirements in this respect fail to be supplied, the plant turns for its prey upon 
the new element it has evoked, the alcohol, which is at once converted, by a sim- 
ilar process, into vinegar. Here the cell becomes changed in form. It is now oval, 
and this condition, which has received the name of Torula, it may be made to retain 
indefinitely ; but under ordinary circumstances, it proceeds to convert the acid into 
other compounds, and its development goes on rapidly until it has assumed the 
form of a filamentous mycelium. In this stage, again, it can be retained at will, as 
the vinegar plant, or as it is popularly termed the ''mother''' of vinegar, which pos- 
sesses the power of at once converting saccharine matter into acid apparently with- 
out the intermediate alcoholic fermentation. If now exposed to the air it com- 
pletes its growth by producing spores which in their turn go through the same 
cycle. 
With this slight sketch of the natural history of one of these minute beings, we 
will now notice some of the more important eflects which they are reputed to pro- 
duce. And, first, let us glance at their influence on the higher forms of vegetables. 
It appears certain that before any great damage can be done by these para- 
