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640 this glucophosphate is hydrolysed only fructose is recovered. It is noteworthy that phosphoric acid has a determining influence on the plant, especially during the ripening period: it may well be one of its functions to promote the interconversion of carbo-hydrates in the manner indicated; if it can convert glucose into fructose it should be able to produce the contrary change, and so supply the material for producing either starch or inulin.

The actual change in an alkaline solution is pictured as involving the production, by dehydration of an unsaturated " enolic " compound common to the three hexoses and the conversion of this by dehydration, only in part into the original form and in part into the other two. The process is one apparently which plays a preponderating part in the course of vital changes. The alteration is only in the first and second carbon systems of the sugar; the manner in which it takes place is simple, thus:

CH(OH), \ HC.OH

Glucose

CH(OH) 2 CH.OH t CH(OH) 2 Mannose

\ -OH 2 = II +OH 2 -^ \

HC.OH C.OH I HO.CH

C.HJ.OH C.(OH),

Fructose

It has been pointed out that, in the laboratory, sorbose is formed together with fructose, when formaldehydrol is con- densed, and that it is of rare occurrence in nature: if changed in solution as fructose is changed it would be converted into the sugars idose and gulose, but neither of these is met with. This fact and the rarity of sorbose is further proof that the vital syn- thetic process is narrowly controlled.

It remains to account for the production of galactose, which is very widely distributed and probably always present in plants, in small amount (as raffinose) ; this hexose is characteristic of mammalian milk, being coupled with glucose in milk sugar. Ga- lactose is closely related to glucose: to account for the conversion of one into the other, it is necessary merely to assume that the glucose is resolved, by hydrolysis, into two molecules of glyceral- dose, one of which is then changed in sign by the reversal of the position of the median OH group a change known to occur in solution; if the two molecules of opposite activity were then re- associated through the agency of a directing mechanism the change might well be complete.

Two pentoses are commonly met with in plants but only in combination, the one d-xylose, corresponding to glucose, the other, /-arabinose, to galactose; a third, d-ribose, is also found, which is the only pentose normally present in animal tissues, in both cases as a characteristic constituent of the nucleic acids. Arabinose and xylose are important components respectively of the gums and of straw and wood; at present, there is no clue to the manner in which they are formed from hexoses in the plant, if indeed they are so formed: it is not improbable that an oxida- tion process may be at work, by which the CH 2 .OH group is removed from the hexose molecule whilst it is held in combina- tion at the aldose end.

The higher carbohydrates are made up of hexose and fructose units in ways which we are only beginning to know: in fact, starch, cellulose and inulin are the only three of whose complete anatomy we have learnt anything, and the information does not carry us far. The labour involved in such work is immense, and methods of dissection are few. The most informative is that in- troduced by Purdie and developed by Irvine and his school, in- volving the methylation of the carbohydrate, the resolution of the complex into the constituent hexose fragments and the de- termination of the position taken up by methyl radicals in these: whence it is possible to infer, with more or less certainty, the manner in which the fragments were linked.

Whilst the primary unit of starch is glucose, into which it is resolved when completely hydrolysed, the chief secondary unit is the dihexose maltose, which is obtained as main product when starch is hydrolysed by the enzyme diastase; whether the sub-

sidiary more complex product, dextrin, is also composed of mal- tose units is uncertain. Maltose is formed by linking two mole- cules of glucose in direct apposition.

The primary unit of cellulose is also glucose; the secondary unit, however, is a dihexose isomeric with maltose, cellose, differ- ing from the former in that the two glucose bricks are laid, as it were, the one advanced a sixth of its length beyond the other. Moreover, the one is an a-glucoside hydrolysed by maltose; the other apparently is a /3-glucoside, as it is hydrolysed by emulsin. A third diglucose is known in gentiobiose, which is obtained, to- gether with cane sugar, when the trihexoside gentianose from gentian root is hydrolysed by invertase; it is not only hydrolysed by emulsin but has been reproduced from glucose by the action of this enzyme; it is therefore undoubtedly a /3-glucoside, and probably the 0-glucoside alternative to the a-glucoside, maltose.

The formula of the three sugars may be written as follows:

-O-

CH 2 (OH).CH(OH).CH.(CH.OH) 2 .CH

O-

O

CH(OH).(CH.OH) 2 .CH.CH.(OH).CH 2

Maltose (a) Gentiobiose (/3)

CH,(OH).CH.(OH).CH.(CH.OH.)CH

I O

CH(OH').(CH.OH) 2 CH.CH.CH 2 (OH) Cellose (0)

Trehalose, a gluco-dihexoside widely distributed in fungi, ap- pears to be the representative of the third type, but its structure is not yet ascertained. As it has no " aldehydic " properties, such as are shown by the three sugars previously considered. It is supposed that the two glucose components may be conjoined as shown by the formula:

. - O - , CH 2 (OH).CH(OH).CH.CH.(OH).CH(OH).CH

)o

CH 2 (OH).CH(OH).CH.CH.(OH).CH(OH).CH

It is to be expected that such a compound would be hydrolysed either by maltase or by emulsin; such is not the case but it is re- solved by a special enzyme present in fungi which appears to be peculiar to the sugar. The examples given may suffice to Illus- trate the manner in which hexose units may be linked together.

Inulin, the reserve material of the artichoke and dahlia tuber, is entirely composed, apparently, of fructose units in the 7-form. When acetylated it gives rise to a well-defined crystalline tri- acetate, which is clearly a simple derivative of the parent sub- stance as inalin may be reproduced from it by careful hydrolysis. The determination of the molecular weight of this compound shows that it contains nine fructose units a peculiar number.

In some plants, the monocotyledons especially, the place of starch is taken by cane sugar, little if any starch being formed; even in those in which starch is produced in considerable amount cane sugar is always present in the leaves, and it has been argued that cane sugar rather than starch may well be the primary prod- uct of assimilation. It is difficult at present to offer any rational explanation of the formation of cane sugar; the wish would be to regard it as traceable to enzyme activity.

All attempts hitherto made to synthesize cane sugar have been failures; it is completely hydrolysed by invertase. Either the point of equilibrium is so near to that of complete hydrolysis that it escapes detection, or the immediate products at once un- dergo change in solution and cease to be susceptible to the re- vertive influence of the enzyme: the fact that fructose is present in the 7-form in cane sugar and that this form does not persist in solution, either in fructose or dextrose, may not be without bear- ing on the problem. It is a matter of interest that cane sugar is usually present in leaves in considerable amount in the cellsap and together with invertase, but in some way separated from it: