A relatively large amount of work has been done relevant to definite compounds which show growth-promoting activities. A review of the investigations dealing with the effects of these known growth factors for lower plants, exclusive of yeast, reveals several interesting aspects. The degree of the necessity of any factor referred to is limited by the conditions of the test conducted and by the strain of the organism used. Although many authors list certain substances as being essential for certain species, the possibility that said compound can be substituted for by another compound is not obviated. It is of interest to note that the bios-complex (including thiamin and its fractions) was the first discovered and that it has been shown to be of primary importance not only for yeasts, but also for many other fungi, bacteria, certain animals, and even non-chlorophyllous parts of higher plants. The inclusion of mesoinosite as a growth factor might be considered as fallacious. Mesoinosite has proved to be essential to none of the test organisms used ; it is inert to the majority ; and in those cases where it induces increase in growth, it is used in great enough concentrations to be classified as a nutrient. Biotin, on the other hand, does accelerate growth in certain organisms even when supplied in very minute quantities. This latter compound has been shown to be especially effective in conjunction with alanine and other growth factors. K?gl and T?nnis (4) report that biotin is the most potent growth factor known, in certain cases, being effective in concentrations of one part in 400 million. Thiamin (anuerin), or vitamin B_, has proved to be the most universally effective, not only with respect to the components of the bios complex, but also to all known growth factors yet tested on the lower plants. In the majority of cases, this compound has proved to be either essential or stimulatory for the growth of the test organisms. In several instances it exerts no effect, and in other cases it is reported as being inhibiting. Thiamin is a complex molecule composed of two ring nuclei each of which may exist as an individual compound ; viz., as pyrimidine and thiazole. A study of this compound and of its components reveals an interesting diversity among those fungi for which the vitamin is effective as an essential growth factor. In this respect, fungi may be divided into the following five categories: (a) Those which require the intact thiamin molecule. A substitution of the two components is non-effective; (b) that group which requires both com-