290 HOW CROPS FEED, 
difficult of alteration, such as stems and leaves, consist- 
ing chiefly of cellulose, with but little albuminoids, and 
both in insoluble forms. 
What is said in a former paragraph on the “ Decay of 
Vegetation,” p. 137, applies in general to eremecausis. 
Fermentation is a term commonly applied to any 
seemingly spontaneous change taking place with vegeta, 
ble or animal matters, wherein their sensible qualities 
suffer alteration, and heat becomes perceptible, or gas 1s 
rapidly evolved. Chemically speaking, fermentation is 
the breaking up of an organic body by chemical decom. 
position, which may go on in absence of oxygen, and is 
excited by a substance or an organism called a ferment. 
There are a variety of fermentations, viz., the vinous, acetic, lactic, ete. 
In vinous fermentation, the yeast-fungus, Torvula cerevisie, vegetates 
in an impure solution of sugar, and causes the latter to break up into 
alcohol and carbonic acid with small quantities of other products. In 
the acetic fermentation, the vinegar-plant, Mzycoderma vini, is believed 
to facilitate the conversion of alcohol into acetic acid, but this change 
is also accomplished by platinum sponge, which acts as a ferment. In 
the lactic fermentation, a fungus, Lenicilium glaueum, is thought to de- 
termine the conversion of sugar into lactic acid, as in the souring of milk. 
The transformation of starch into sugar has been termed the saccha- 
rous fermentation, diastase being the ferment. 
Putrefaction, or putrid fermentation, is a rapid internal 
change which proceeds in comparative absence of oxygen. 
It most readily attacks animal matters which are rich in 
albuminoids and other nitrogenous and sulphurized prin- 
ciples, as flesh, blood, and urine, or the highly nitrogenous 
parts of plants, as seeds, when they are fully saturated 
with water. Putrefying matters commonly disengage 
stinking gases. According to Pasteur putrefaction is oc- 
casioned by the growth of animalcules ( Vibrios). 
Fermentation is usually and putrefaction is always a 
reducing (deoxidizing) process, for either the ferment it- 
self or the decomposing substances, or some of the prod- 
ucts of decomposition, are highly prone to oxidation, and 
