A complex saccharide, known as _stachyose_, which is found in the tubers of _Stachys tuberifera_, is said by some investigators to be a tetrasaccharide and by others to have the formula C_{36}H_{62}O_{31}·7H_{2}O (i.e., a hexasaccharide). It is a crystalline solid, with a faintly sweetish taste, and a specific rotatory power of +148°. When hydrolyzed it yields glucose, fructose, and two (or more) molecules of galactose. THE RELATION OF THE MOLECULAR CONFIGURATION OF SUGARS TO THEIR BIOCHEMICAL PROPERTIES As will be pointed out later (see Chapter XIV), all chemical reactions which are involved in vital phenomena, including those of plant growth and metabolism, are controlled by enzymes. The biochemical reactions which the soluble carbohydrates undergo afford such excellent illustrations of the relation of the molecular configuration of an organic compound to the possibility of the action of an enzyme upon it, that it seems desirable to discuss this relationship at this point, rather than to postpone it until after the nature of enzyme action has been considered. Undoubtedly, the student, after he has studied the nature of enzymes and their mode of action, as presented in Chapter XIV, will find it profitable to return to this section and review the facts here presented, as illustrating the principles and mechanism of enzyme action. But a consideration, at this time, of the relation of the molecular configuration of the sugars to their biochemical reactions cannot fail to add interest to the study of these matters from the chemical and biological standpoints. It has been known for a long time that the dextro- and levo-isomers of a compound which contains one or more asymmetric carbon atoms are affected differently by biological agents, such as yeasts, moulds, bacteria, etc. Pasteur, as early as 1850, showed that the green mould, _Penicillium glaucum_, when growing in solutions of racemic acid (a mixture of equal molecules of _d_- and _l_-tartaric acids) uses up only the _d_-acid, leaving the _l_-form absolutely untouched. Later, it was found that the same green mould attacks _l_-mandelic acid in preference to the _d_- form; whereas the yeast, _Saccharomyces ellipsoideus_, exhibits the opposite preference for these acids. These observations upon some of the earlier known forms of optically active organic acids led the way to a general study of this phenomenon as exhibited by the optically active soluble carbohydrates. The results of these studies may be considered in connection with the several different types of reactions which these sugars undergo, as follows: