IT is customary to assume that an enzyme fulfils the function in vivo that it exhibits in vitro, however surprising the conclusions to which this leads; yet it is frequently overlooked that, were its true function unknown, haemoglobin might similarly be credited only with its powers of acting as a peroxidase, or absorbing carbon monoxide, neither of which would form a useful basis for a study of the physiology of blood. The extremely high concentration of urease found in a number of seeds is an interesting example of this type of problem; it can, for example, be readily calculated from the results below that the cotyledons of the seed of Citrullus (the water-melon) contain sufficient urease to decompose more than their own weight of urea every hour. Moreover, the technique used has been such that this result is almost certainly too low. An extensive literature on the occurrence of urease exists, virtually all of it directed to the production of pure or concentrated urease preparations for kinetic studies or urea determinations; the function of the urease in the seed receives in general no more than a puzzled mention.