TEANSACTIONS OF SECTION D. 697 



the ground, in wliich case it will be consumed in providing material for the 

 growth of a potato-plant next year. But the processes by which the insoluble 

 starch, is made available for nutrition are, I cannot doubt, closely similar in either 

 case. 



When we inquire further about these mysterious and all-important bodies, the 

 answer we can give is extremely inadequate. It is very difficult to obtain them in 

 amount sufficient for analysis or in a state of purity. We know, however, that 

 they are closely allied to albuminoids and contain nitrogen in varying proportion. 

 Papain, which is a vegetable ferment derived from the fruit of the papaw, and 

 capable of digesting most animal albuminoids, is said to have the same ultimate 

 composition as the pancreatic ferment and as peptones, bodies closely allied to pro- 

 teids ; the properties of all three bodies are, however, very diffijrent. It seems 

 clear, nevertheless, that ferments must be closely allied to proteids, and, like these 

 bodies, they are, no doubt, directly derived from protoplasm. 



I need not remind you that, unlike other constituents of plant tissues, protoplasm, 

 as a condition of its vitality, is in a constant state of molecular activity. The 

 maintenance of this activity involves the supply of energy, and this is partly- 

 derived from the waste of its own substance. This ' self-decomposition ' of the 

 protoplasm liberates energy, and in doing so gives rise to a number of more stable 

 bodies than protoplasm. Some of these are used up again in nutrition ; others are 

 thrown aside and ai-e never drawn again into the inner circle of vital processes. In 

 the animal organism, where the strictest economj^ of bulk is a paramount neces- 

 sity, they are promptly got rid of by the process of excretion. In the vegetable 

 economy these residual products usually remain. And it is for this reason, I may 

 point out, that the study of the chemistry of plant-nutrition appears to me of such 

 immense importance. The record of chemical change is so much more carefully 

 preserved ; and the probability of our being able to trace the course it has followed 

 is consequently far more likely to be attended with success. 



This preservation in the plant of the residual by-products of protoplasmic 

 activity no doubt accounts for the circumstance which otherwise is extremely 

 perplexing — the profusion of substances which we meet vtdth in the vegetable king- 

 dom to which it is hard to attribute any useful purpose. It seems probable that 

 ferments, in a great many cases, belong to the same category. I imagine that it is 

 in some degree accidental that some of them have been made use of, and thus the 

 plant has been able to temporarily lock up accumulations of food to be drawn upon 

 in future phases of its life with the certainty that they would be available. 

 Without the ferments the key of the storehouse would be lost irretrievably. 



Plants, moreover, are now known to possess ferments, and the number will 

 doubtless increase to which it is difficult to attribute any useful function. Papain, 

 to which I have already alluded, abounds in the papaw ; but it is not easy to 

 assign to it any definite function ; still less is it easy, on teleological grounds, 

 to account for the rennet ferment contained in the fruits of an Indian plant, 

 Withania coagulans, and admirably investigated by Mr. Sheridan Lea. 



Having dwelt so far on the action of ferments, we may now turn to fermentation 

 and that other kind of change in organic matter called ' putrefaction,' which is 

 known to be closely allied to fermentation. Ferments and fermentation, as I have 

 already remarked, have very little to do with one another ; and it would save 

 confusion and emphasise the fact if we ceased to speak of ferments but used some 

 of the alternative names which have been proposed for them, such as zymases or 

 enzymes. 



The classical case of fermentation, which is the root of our whole knowledge 

 of the subject, is that of the conversion of sugar into alcohol. Its discovery has 

 everywhere accompanied the first stages of civilisation in the human race. Its 

 details are now taught in our text-hooks ; and I shou d hardly hope to be excused 

 for referring to it in any detail if it were not necessary for my purpose to draw 

 your attention more particularly to one or two points connected with it. 



Let us trace what happens in a fermenting liquid. It becomes turbid, it 

 froths and effervesces, the temperature sensibly increases ; this is the first stage. 

 After this it begins to clear, the turbidity subsides as a sediment ; the sugar which 



