METABOLISM. 



25 



energy, packed away for the moment in the store- 

 houses of the plant ; and we know that quite 

 apart, however, from intermediate transformations 

 of the energy thus stored this energy reappears 

 in the kinetic state eventually, when the starch is 

 burned off, in presence of oxygen, and transformed 

 into carbon-dioxide and water. It matters not 

 how quickly or how gradually this combustion 

 occurs, or whether it is accomplished by burning 

 in a fire, or by slow and complex stages in respira- 

 tion or metabolism : the point is that the unit of 

 weight of starch yields so many units of heat 

 when its structure tumbles down to the original 

 components, carbon-dioxide and water. 



Clearly, if we know how many units of heat are 

 yielded by the combustion of one gram of starch, 

 we can obtain an estimate of the amount of 

 energy, measured in terms of heat, which the 

 foliage gains and stores up an estimate which 

 will approach the truth in proportion as our esti- 

 mate of the total assimilative activity is correct. 



A word of warning is necessary here, however, 

 for those best acquainted with physiology recog- 

 nise that however useful such calculations as the 

 above may be, and undoubtedly are, to give a 

 general idea of the fact that the energy represented 

 is large, it would be a mistake to suppose that 

 such estimates give even an approximate measure 

 of the energy of potential which may be got from 

 the carbohydrate, and still less of the amount of 

 work that may be got from its employment, 

 according to the way it is employed or presented 



