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MICRO-ORGANISMS IN RELATION TO MAN. 93 
With some of the members of the first of these groups 
we are all of us familiar. We see them growing upon the 
walls of damp cellars, upon articles of food which have been 
left too long exposed to air and moisture, and the housewife 
finds them flourishing upon her preserves which have been 
imperfectly secured from the access of air. ‘hey form masses 
of various colours—white, grey, black, green, yellow, .or 
brown—which throw up filaments, in many cases visible to 
the naked eye, carrying clusters of coloured spores. The 
ground mass consists of a ramification of threads, called 
mycelia, from which the perpendicular filaments are thrown 
up. The members of this group have little or no import- 
ance industrially or otherwise. Figure 1 shows the common 
green mould. 
The Yeasts, the results of whose action is so well known 
to us, are small unicellullar plants, which vary much in size 
and shape. The ordinary beer-yeast has a spherical form 
(Fig. 2, a), with a diameter of about one four-thousandth of 
an inch; the yeast of wine (Fig. 2, b), is elliptical and smaller. 
Other species have other forms, some being pear or sausage 
shaped, and one species, which always occurs in the sponta- 
neous fermentation of fruit juices, has a very curious and 
characteristic citron-shaped cell. 
But it is to the members of the third group that I chiefly 
wish to direct your attention. These constitute by far the 
largest portion of the micro-organisms at present known, and, 
with but a few exceptions, it is to their action, injurious or 
the reverse, that we can now trace so many of the pheno- 
mena in Nature. The members of this group are usually 
somewhat loosely referred to by the name ‘“ Bacteria,” which 
term should, speaking correctly, be employed for one class 
only of the group. 
Bacteria, to use the word in its more general sense, vary 
greatly in size and form, and from these features, combined 
