maybe means are provided by which the sugar can escape from the influence of the enzyme immediately it is produced. In roots such as that of the sugar beet, in which cane sugar is merely stored up as a reserve material, no enzyme is present.
The close association, in the leaves of many plants, of starch with chlorophyll, in the chloroplasts, has led to the view that it may be an all but direct product of synthetic activity and not formed from glucose. It is indeed conceivable that a directive (enzymic) mechanism may exist which can induce both the production of glucose from formaldehydrol and the simultaneous assemblage and union of the glucose units into starch.
The enzymes are agents comparable with the acids in their hydrolytic activity, but selective and directive. Unlike the acids they are catalysts participate agents. The effective area of the enzyme, however, must be some small section of the molecular surface: and the only rational interpretation of the special activ- ity of the enzyme would seem to be that this active area is com- posed of material compatible with that which the enzyme speci- fically attacks that indeed it is this material, though conjoined perhaps with an acid radicle, which acts as the actual " tool " in hydrolysis. Starch may be regarded as a pavement or simple mosaic built up of many separate glucose-residues regularly ar- ranged in a definite pattern, and layer after layer is laid in this fashion: the enzyme as a template formed of a single layer thus composed; maybe as the starch layer increases in thickness there is a coincident up-growth at its margin of the protein constituent of the enzyme complex. Given such a mechanism, it is conceiv- able that starch might be almost directly produced: some ex- planation is required to account for the preponderance of glucose in the plant. The conception is one, moreover, which may be used to explain the action of enzymes in other cases.
With reference to the conditions under which enzymes (and acids) may act reversibly, it is to be noted that the manner in which action takes place, both rate and direction, is determined by the conditions of concentration. As hydrolysis proceeds water is used up; if the reverse action take place, water is produced. Usually a point of equilibrium is reached, when no further change seems to be taking place. This is true even of the hydrolysis of cane sugar by acid: as the concentration of the solution is raised and the opportunity for change is increased, the rate of change only rises up to a certain point, beyond which any further in- crease in concentration only serves to diminish the rate of the process. As the solution becomes more concentrated in the case of cane sugar, particularly through the increase of the num- ber of molecules in solution it becomes itself more attractive of water and hydrolysis is less promoted. The extent to which syn- thesis is effected is entirely a question of balance of affinity of desire for water. This point is one of extreme importance in connexion with vital phenomena. In plants, during the day time, synthetic actions prevail, as the tendency is constantly towards the concentration of the solutions in the leaf cells; when the influence of light is withdrawn water is attracted into the concentrated solutions and reversals set in, producing to an in- creased extent simpler molecules, which can wander out into the general circulation and be used elsewhere.
Thus far enzymes have been spoken of as influencing the hy- drolysis and formation only of compounds consisting of sugar units these are conveniently classed as Hologlucosides. Many sugar derivatives are known which are to be classed as Hetero- glucosides, being of more diverse origin: the methyglucosides may be taken as typical of this class, especially /3-methyglucoside, as most of these are more or less readily hydrolysed by the constit- uent of the mixture of enzymes in almond-emulsin to which j3- methyglucoside responds. Curiously enough, the few known natural a-glucosides are all hologlucosides; the known a-hetero- glucosides are all laboratory products.
The heteroglucosides are extraordinarily varied in composition. Little is known as to their precise function. Often they serve to give stability to a substance which could not well exist uncom- bined; or they mask one that would interfere if free; or they have the advantage of being far less soluble than the parent com- pounds. They form most of the colouring matters of flowers.
The most interesting member of the class perhaps is that first studied, amygdalin, present in considerable quantity in the fruit of the bitter almond and also in the fruit of most of the Rosaceae. It is resolved by emulsin which is equally well obtained either from the sweet or the bitter almond into two molecules of glu- cose, one of benzoic aldehyde and one of hydrogen cyanide:
QoHjyOnN +2H 2 O = 2C 6 Hi 2 O + C 7 H 6 O + HCN.
The two latter are present in direct association, as in the cyanhydrol, CeH 8 CH(O.H)CN in its dextro-rotatory form, the isomeric laboratory form being present in sambum'grin, from elder leaves which, however, contains only one glucose residue. By the action of one of the enzymes in emulsin, amygdalase, amygdalin is resolved into glucose and prunasin, the isomeride of sambunigrin; this heteroglucoside occurs naturally in the leaf of the almond and of the common cherry laurel in fact, in the leaves of all Rosaceae whose fruits contain amygdalin. Laurel leaves particularly are rich in an enzyme, prunase, which hydro- lyses prunasin; this is present together with amygdalase in all fruits containing amygdalin. The resolution of amygdalin there- fore, involves, it will be seen, the action of two enzymes in suc- cession. What appears to be amygdalase is present in some yeasts, together with maltase. The advantage to the plant is that the leaf contains the more soluble glucoside, that in the fruit being but slightly soluble; the presence of glucose and fruc- tose in the leaf and stem but of starch in the tuber of the potato is a parallel case. How the two glucose residues are united is not determined: the probability is that amygdalin is derived from gentiobiose. Prunase apparently is the /8-glucase in emul- sin which acts on the /3-methyglucoside and the /3-heterogluco- sides generally: to explain its indifference towards amygdalin and the varying degree of activity which it displays towards different /3-glucosides, it is necessary to assume that the group associated with glucose influences the fit of the enzyme. If the enzymes be, as suggested, but replicas, in part, of the hydrolytes they effect, in each particular class, the glucoside characteristic of the class may well be contained in the enzyme: thus prunase from the Rosaceae is conceivably x a prunasin derivative, whilst the linase of the Linaceae may be a derivative of the cyanhydrol of acetone, C(CH 3 ) 2 (OH).CN; consequently although both enzymes affect prunase they do not act with equal readiness; the addition of a second molecule of glucose to prunase, although it happens in the ^-position, may spoil the fit of prunase, entirely. The problem is one of extraordinary interest and importance.
Glucose and its congeners are of special value in the plant, as constructive materials, on account of their peculiar plasticity under the numerous enzymic and other influences simultaneously brought into action in nature. These are specially manifest in the phenomena of fermentation. In recent years the controversy which has long been waged over the fermentation process, as effected by yeast and other organisms, has been settled against the vitalfsts, as it is proved that it can be carried out apart from the living cell, in its entirety, by means of the juice expressed from yeast, and even in presence of substances, such as acetone and toluene, fatal to the life of the cell. The course of change is by no means ascertained: as yet only the main outlines are marked out, but these are of such significance that it is clear that a most delicate balance of forces comes into play.
When the formulae are contrasted it is obvious that the ulti- mate conversion of glucose into carbon dioxide and alcohol must involve much rearrangement within the molecule. Oxygen must be removed from some of the carbon atoms and its place taken by hydrogen; the reverse operation has to be effected at others. That such changes can be induced by mere contact with acids or alkalies is well known: thus lactic acid, CH 3 .CH(OH).CO 2 H, is easily formed by digesting glucose with alkali; reduction is car- ried still further in the production of laevulinic acid, CH 3 .CO.- CHz.CHj.COzH, by boiling either fructose or glucose with an acid, fructose being the far more easily attacked. This latter fact is perhaps not without significance.
A variety of factors come into play when fermentation is in- duced by yeast juice. Phosphate plays a part of fundamental
