nonoses, respectively. Substances
corresponding in composition and properties with the artificial tetroses
and one or two derivatives of heptoses are occasionally found in plant
tissues, and a considerable number of pentoses and their condensation
products are common constituents of plant gums, etc.; but the great
majority of the natural carbohydrates are hexoses and their derivatives.

GROUPS OF CARBOHYDRATES

Since the simpler carbohydrates are sugars, i.e., they possess the
characteristic sweet taste, the name "saccharide" is used as a basis for
the classification of the entire group. The simplest natural sugars, the
hexoses, C_{6}H_{12}O_{6}, are known as _mono-saccharides_. The group of
next greater complexity, those which have the formula C_{12}H_{22}O_{11}
and may be regarded as derived from the combination of two molecules of a
hexose with the dropping out of one molecule of water at the point of
union, are known as _di-saccharides_. Compounds having the formula
C_{18}H_{32}O_{16} (i.e., three molecules of C_{6}H_{12}O_{6} minus two
molecules of H_{2}O) are _tri-saccharides_; and the still more complex
groups, having the general formula (C_{6}H_{10}O_{5})_n_, are called the
_poly-saccharides_. The mono-, di-, and tri-saccharides are generally
easily soluble in water, have a more or less pronouncedly sweet taste, and
are known as the _sugars_; while the polysaccharides are generally
insoluble in water and of a neutral taste, and are called _starches_. As
will be seen later, there are many natural plant carbohydrates belonging to
each of these groups.

In addition to these saccharide groups, there are other types, or groups,
of compounds which resemble the true carbohydrates in their chemical
composition and properties and are often considered as a part of this
general group. These are the pentoses, C_{5}H_{10}O_{5}, and their
condensation products, the pentosans (C_{5}H_{8}O_{4})_n_, and their methyl
derivatives, C_{6}H_{12}O_{5}; certain polyhydric alcohols having the
formula C_{6}H_{8}(OH)_{6}; pectose and its derivatives, pectin and pectic
acid; and lignose substances of complex composition. It is doubtful whether
these compounds are actual products of photosynthesis in plants, or have
the same physiological uses as the carbohydrates and it has seemed wise to
consider them in a separate and later chapter.

ISOMERIC FORMS OF MONOSACCHARIDES

Four sugars ha