During the past half century, scientific effort has been made to determine the course of photosynthesis. Among the numerous lines of investigation pertaining to this subject, the occurrence and behavior of the simple sugars and sucrose have interested many investigators. Physiologists seem to agree now that the simple sugars are the first sugars formed by plants ; and since the pentoses do not occur generally in leaves, the hexoses, glucose and fructose, might logically be the first sugars formed. It is possible that only glucose is the first sugar synthesized and that fructose, as well as all the other plant carbohydrates, arise from it. If this be the case, then it might well be expected that mannose should be formed also, since structurally it is so nearly like glucose and fructose ; and, too, whenever glucose is converted by means of its enolic forms to fructose in the laboratory, mannose is produced as abundantly as fructose. In other words, these three sugars when existing under conditions favoring the enolic conversion will exist in equilibrium. To date, mannose has not been found in photosynthetic tissues, but an attempt to find it there has apparently not been made. It is the object of this study to make a rather extensive search for mannose in leaves of many species of plants. It is hoped that the results may be such as to be of value in determining the first sugar of photosynthesis. In 1903 Herissey (3), after studying mannans and galactans in seeds, concluded that these reserves are digested to form mannose and galactose respectively; but since neither of these sugars was positively identified in juices of living plants, if they exist at all they must exist as intermediate products of hydrolysis and must be changed very quickly to other compounds. Thus although mannan, which after hydrolysis yields mannose and mannitol (a hexahydric alcohol), has been found in many plant parts, mannose itself has never been found except in traces in those parts of plants in which the mannan is stored.