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 having the formula C_{6}H_{12}O_{6}, namely, glucose, fructose, mannose, and galactose, occur very commonly and widely distributed in plants. In addition to these, thirteen others having the same percentage composition have been artificially prepared, while seven additional forms are theoretically possible. In other words, twenty-four different compounds, all having the same empirical formula and similar sugar-like properties are theoretically possible. In order to arrive at a conception of this multiplicity of isomeric forms, it is necessary to understand the two types of isomerism which are involved. One of these is _structural_ isomerism, and the other is _space_- or _stereo_-isomerism.