30 PLANT PHYSIOLOGY 



which they act into available food compounds ; but it is also 

 certain that, among the enzyms produced, there are some 

 which bring about such changes in the surrounding sub- 

 stances that these become available as sources of kinetic 

 energy. For example, the diastase formed in the germinat- 

 ing seed, which dissolves the starch deposited as a reserve 

 food in the seed, converting it into sugar, makes the reserve 

 food available for at least three purposes ; first, for the con- 

 struction of nitrogenous compounds ( amides and proteids ) ; 

 second, for the formation of cell-wall (cellulose) ; and third, 

 for the liberation of energy by aerobic respiration. The pro- 

 duction and action of this enzym furnishes material for 

 respiration as well as for nutrition. The enzyms formed by 

 the lower plants are also useful in more than one way, and 

 in many cases one of these uses is undoubtedly the con- 

 version of irrespirable into respirable substances. 



FERMENTATIONS 



In the group of processes which are commonly called fer- 

 mentations ( alcoholic, lactic, butyric, etc. ) , the physiologist 

 must study both respiration and nutrition. A thorough 

 understanding of the physiology of fermentation* is impos- 

 sible without previous thorough knowledge of the chemistry 

 of fermentation. As this chemical knowledge is still incom- 

 plete, it is possible to form only general ideas concerning 

 fermentation and the allied processes of decay and disease. 

 The distinction usually made between fermentation and 

 decay is this, that in fermentation there is evolution of gas, 

 whereas, in decay, there is little or none an artificial and 

 even misleading distinction. Even the attempted distinc- 

 tion between disease on the one hand, and fermentation and 

 decay on the other, is anything but fundamental; for dis- 

 ease, so far as it is the result of the activity of foreign 

 organisms, is nothing more than decay or fermentation car- 

 ried on by those foreign organisms upon or in the living 

 body of the host. 



* On the subject of fermentation the student may well consult J. Rey- 

 nolds Green's "The Soluble Ferments and Fermentation." Cambridge, 

 England, 1901. In this book there is a very useful bibliography. 



