278 THE LIFE OF THE PLANT 



the surrounding medium is very apparent ; whereas in 

 the majority of cases plants passively acquire the 

 temperature of the medium. But if we compare the 

 respiration of plants with that of the so-called cold- 

 blooded animals, e.g. with frogs, or even with the 

 respiration of mammals in a state of torpor (for in- 

 stance during their winter sleep), we shall see that in 

 both cases the respiration does not differ so sharply 

 from the respiration of plants, either in the quantity of 

 gaseous interchange, or in the difference in temperature 

 of the organism from the surrounding temperature. 



A question arises here ; Is respiration one of the 

 processes absolutely necessary to the organism of the 

 plant ? We have seen that motion of any kind ceases 

 in the absence of oxygen. For a long time the growth of 

 cells was also believed to be impossible without oxygen, 

 but it now appears that another chemical process can 

 be substituted for respiration, a process similar to it in 

 its results. This process is called fermentation, and 

 consists in the decomposition of sugar — glucose — into 

 alcohol and carbonic acid. This process underlies the 

 manufacture of alcohols, i.e. the manufacture of wines, 

 the distilling of spirits, the brewing of beer, etc. In all 

 these cases fermentation takes place owing to the 

 development in the fermenting liquids of a peculiar 

 microscopic organism, a fermenting fungus known as 

 yeast. The yeast cells grow and multiply without 

 oxygen. The process of fermentation differs essentially 

 from respiration in the fact that it is not accompanied 

 by the absorption of oxygen, but is similar to it in that 

 in both cases carbonic acid and heat are given off.^ 



^ During respiration carbonic acid is formed at the expense of the 

 oxygen of the air, while during fermentation it is formed at the expense 

 of oxygen contained in sugar itself. Something Uke the combustion 

 of tinder or gun-powder takes place here. We know that both these 

 substances can bum without air at the expense of the oxygen in the 

 saltpetre which enters into their composition. 



