Studies by the author and his associates lead him to conclude that growth substances are important for plants as well as for animals, and that one of the most important for plants is thiamin, all or almost all of them apparently requiring it. Some of the fungi studied which require no external supply of thiamin synthesize it. The green plant is thus not the sole source of this vitamin. Depending on the organism concerned, thiamin may be considered a hormone, a vitamin, or something which is neither. The author therefore for the present prefers the term "growth substance" for any specific organic substance needed in small amts. for normal development of an organism. Thiamin plays a significant role in carbohydrate metabolism and probably also in other metabolic processes. At least one of its functions is apparently to serve as a precursor of a part of an enzyme system involved in respiration, but probably this is not its sole function. It apparently has a multiple function as a coenzyme in nature. It is suggested that Phycomyces might be used advantageously in determining more completely the changes occurring in the intermediary metabolism of carbohydrates and in the quantitative determination of thiamin. Studies of this fungus, as well as of tomato and pea roots and staphylococci, have indicated thiamin to be quite specific, and this specificity is doubtless associated in part with its function as a precursor of a part of an enzyme system. Observations on growth in mixed cultures have proved most suggestive from the standpoints of parasitism and symbiosis, and attention is called to the promise in the plant-thiamin relationship of elucidating important questions in general physiology through the study of more easily controlled material than the higher animal. However, the point is stressed that thiamin is not the only growth substance concerned in plant development, and the functions of several others are briefly noted.