SILOS, ENSILAGE AND SILAGE. 51 



that the cells of all living tissues, whether of plants or 

 animals, in the exercise of their vital activities elaborate 

 zymases as required in the complex metabolism * of the 

 processes of nutrition. 



The first step in the fermentation of starch and cane 

 sugar, which is the work of a soluble ferment (zymase), 

 seems to be identical with the first step, of the germina- 

 tion of seeds, of the transformation of the reserve mater- 

 ials in the growth of the seed stalk in tuberous roots, and 

 of animal digestion. 



In these nutritive processes of plants and animals, 

 heat is liberated as one of the constant results of the 

 metabolism of the cells in the exercise of their vital 

 activities. The heat developed by plants, as an incident 

 of their nutritive processes, is not noticeable under ordi- 

 nary conditions, as it is obscured by the constant loss of 



* Under the old physiological theories many of the changes taking place in the 

 tissues, or nutritive materials, were erroneously attributed to a process of oxida- 

 tion. For example, respiration was assumed to be a combustive process of oxida- 

 tion, in which the carbonic acid exhaled was formed by the direct union of car- 

 bon with the inhaled oxygen. It is now known that the carbonic acid of respira- 

 tion is formed in the destructive metamorphoses of the tissues, and not by the 

 direct combination of oxygen with carbon, as in ordinary combustion. With the 

 progress of physiological knowledge, oxygen is, more and more, looked upon 

 as an essential food constituent, required in the constructive processes of the 

 tissues, and there is no evidence that destructive oxidation, in the ordinary 

 acceptation of the term, occurs, to any considerable extent, in living organisms. 

 Metabolism is the term now used to denote the assemblage of vital changes 

 involved in the processes of nutrition, whether chemical or physical, without 

 attempting to indicate their precise character, or attributing them to the more 

 than questionable process of oxidation. "We may picture to ourselves this 

 total change which we denote by the term 'metabolism,' as consisting on the one 

 hand of a downward series of changes (katabolic changes), a stair of many steps, 

 in which more complex bodies are broken down with the setting free of energy 

 into simpler and simpler waste bodies, and on the other hand, of an upward 

 series of changes (anabolic changes), also a stair of many steps, by which the 

 dead food, of varying simplicity or complexity is, with the further assumption 

 of vital energy, built up into more and more complex bodies. The summit of 

 this double stair we call 'protoplasm.' Whether we have a right to speak of it as 

 a single body, in the chemical sense of that word, or as a mixture in some way of 

 several bodies, whether we should regard it as the very summit of the double 

 stair, or as embracing, as well, the topmost steps on either side, we cannot, at 

 present, tell. Even if there be a single substance forming the summit, its exist- 

 ence is absolutely temporary ; at one instant it is made, at the next it is unmade. 

 Matter which is passing through the phase of life, rolls up the ascending steps to 

 the top, and forthwith rolls down on the other side." Foster, Art. Phys. Encycl. 

 Brit. 9th ed. XIX, p. 13. 



