the terminal one forms the
basis for the explanation of the existence of the so-called [alpha] and
[beta] modification of _d_-glucose (see page 46). However, the ordinary
aldehyde reactions of the sugars are more clearly indicated by the
open-chain formula. Some investigators are inclined to be that sugars
actually exist in the open-chain arrangement when in aqueous solution, and
in the closed-ring arrangement when in alcoholic solution. The closed-ring
formulas will be used in this text in the discussions of the birotation
phenomena and of biochemical properties, but for the explanations of the
stereo-isomeric forms and similar phenomena, the open-chain formulas are
just as useful in conveying an idea of the possibilities of different space
relationships, and are so much simpler in appearance and in mechanical
preparation, that it seems desirable to use these rather than the more
accurate closed-ring formulas.

CHEMICAL CONSTITUTION OF MONOSACCHARIDES

The term "monosaccharides," as commonly used, refers to hexoses. It applies
equally well, however, to any other sugar-like substance which either
occurs naturally or results from the decomposition of more complex
carbohydrates, and which cannot be further broken down without destroying
its characteristic aldehyde-alcohol groups and sugar-like properties.

All such monosaccharides, being alcohol-aldehydes, can easily be reduced to
the corresponding polyatomic alcohols, containing the same number of carbon
atoms as the original monosaccharides, each with one OH group attached to
it. All aldose monosaccharides are converted, by gentle oxidation, into the
corresponding monobasic acid, having a COOH group in the place of the
original CHO group. Further oxidation either changes the alcoholic groups
into COOH groups, producing polybasic acids, or breaks up the chain. When
ketose monosaccharides are submitted to similar oxidation processes, they
are broken down into shorter chain compounds.

The various monosaccharides which have thus far been found as constituents
of plant tissues, or as parts of other more complex compounds which occur
in plants, are shown in the following table:

_Trioses_ (C_{3}H_{6}O_{3}) _Tetroses_ (C_{4}H_{8}O_{4})

Aldose--Glyceric aldehyde, Aldoses--_d_- and _l_-Erythrose,
or glycerose _l_-Threose
Ketose--Dioxyacetone

_Pentoses_ (C_{5}H_{10}O_{5}) _Me