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 j)aragraph on the " Decay of 

 Vegetation," p. I.'IT, applies in general to eremecausis. 



Fermentation is a term commonly applied to any 

 seemingly spontaneous change taking place Avith vegeta- 

 ble or animal matters, Avherein their sensible qualities 

 suffer alteration, and lieat becomes perceptible, or gas is 

 rapidly evolved. Chemically speaking, fermentation is 

 the breaking up of an organic body by chemical decom^ 

 position, A\ hich may go on in absence of oxygen, and is 

 excited by a substance or an organism called a ferment. 



There are a variety of fernaentations, viz., the vinous, acetic, lactic, etc. 

 In vinous fermentation, the yeast-fungus, Tonnda cerevisice, vegetates 

 in an impure solution of sugar, and causes the latter to breaic up into 

 alcohol and carbonic acid with small quantities of other jiroducts. In 

 the acetic fermentation, the vinegar-plant, Jlycodeiina viiii, is believed 

 to facilitate the conversion of alcohol into acetic acitl, but this change 

 is also accomplished by platiniim sponge, which acts as a ferment. In 

 the lactic fermentation, a fungus, Penicilium glaucum, 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 raydd internal 

 change which j^roceeds 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 



