376 
Micro-organisms and their Action 
division becomes exhausted, and in certain cells a concentra- 
tion of the contents then takes place, thus forming yery small 
corpuscles. These are the spores, which are destined to pre- 
serve the species to which thej belong after the parent-plant has 
died ; and they are well adapted for this purpose, since their 
resistance to external influences, such as cold, heat, dryness, &c., 
is very great, indeed far beyond the resisting power of the 
developed plant. 
In consequence of their lightness, these spores easily float 
about in the air, which is, therefore, always more or less charged 
with them. When they enter a medium which offers conditions 
suitable for their development, they grow into cells. These 
cells live and multiply in the manner just described. At the 
same time the medium will undergo certain changes, or in 
other words, fermentation. This is not merely the result of the 
presence of micro-organisms, but the consequence of their 
development and physiological activity. 
The action of micro-organisms or organised ferments does 
not exclude entirely the action of chemical ferments or enzymes. 
One organized ferment, common yeast, has been proved beyond 
doubt to contain in itself an enzyme capable of converting 
unfermentable into fermentable sugar, which in its turn is then 
acted upon by the yeast and undergoes alcoholic fermentation. 
Rennet is said to have been found in micro-organisms living 
on the casein of milk, and other organisms of a similar kind are 
supposed to contain a chemical ferment capable of dissolving 
casein. 
Milk has often, and very rightly, been described as a model 
food, for it contains in the most assimilable form all those 
compounds which are necessary for the sustenance of life, viz. 
nitrogenous matter (casein, albumen), fat (butter), carbohydrates 
(milk-sugar), and mineral salts (chlorides and phosphates of 
potash, soda, lime and magnesia, besides traces of other salts). 
These are exactly the constituents of micro-organisms, and it 
is, therefore, not at all surprising that milk, containing &t the 
same time the necessary nutriment and fermentable substances, 
should become a very suitable medium for the development of a 
number of micro-organisms. 
li has been mentioned that our knowledge of micro-organisms 
and their action is, comparatively speaking, of very recent date. 
Thanks, however, to the researches of Pasteur and other investi- 
gators, this branch of science has been much advanced during 
the last two decades. 
Lactic Ferment. — That milk after having been drawn from the 
uddei and kept for a certain time, turns sour, loses its liquid 
character, and — provided it has not been disturbed — forms a 
