Two opinions may be advanced as to the nature of the activity of the nectaries in the secretion of nectar. Either we can argue that the nectaries have to be considered as tissues with a real glandular function which actively secrete the nectar from substances applied by the phloem or we can consider the nectaries as a kind of passive hydathodes which only conduct the substances delivered by the translocation stream of the vascular bundle. The latter opinion has recently been defended by Frey-Wyssling (1933) and his collaborators (Aghte, 1951; Zimmermann, 1953 and Frei, 1955). These authors observed that the nectaries always possess an immediate anatomical connection with the adjacent vascular bundles. If the nectaries secrete a sugar-rich nectar this connection is made up mainly of sieve tubes while in nectaries with a more watery secretion xylem elements predominate. It was experimentally shown that excised flowers or even nectaries placed on a sugar solution secreted the sugar from the solution in the nectar. According to Frey-Wyssling the primary cause of the nectar secretion is the lack of a balance between the supply and the consumption of carbohydrates. In young, still growing parts of plants the supply and the consumption should be well balanced so that nectar secretion does not take place. In adult parts, however, this equilibrium should be disturbed in proportion as the supply of assimilates exceeds their consumption. The remainder is secreted as nectar. Though the observations of Frey-Wyssling and his collaborators show convincingly that the nectar secretion is connected with the translocation in the vascular bundle we still do not know whether the nectaries merely conduct the substances from the translocation stream or secrete the nectar from the substances in the translocation stream by a process of active secretion. If the nectaries merely conduct the above substances there is no reason to expect that the metabolic activity of the nectaries will be different from that of the adjacent tissues. If, however, the nectaries have a glandular function we may expect that in the nectaries specific metabolic reactions take place which finally lead to the formation of the nectar. In the latter case it is suggested that an eventual difference in metabolic activity between nectaries and the adjacent tissues could be histochemically demonstrated. Such an investigation should be directed towards enzyme systems taking part in the production of nectar. At present, however, we do not exactly know which enzyme systems are concerned in these reactions; moreover we have the disadvantage that only a very few enzyme systems can be determined histochemically. Therefore we have been limited in our choice to the phosphatases, as these enzymes can be convincingly demonstrated in a histochemical way and as they take part in certain reactions of the metabolism of the carbohydrates.