Growth of Rhizopus Suinus as Affected by Certain Growth Factors.

A relatively large amounit of w-ork has been done relevanit to definite comipounds which show growth-pronmoting aetivities. 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 liiiiited by the conditions of the test coniducted and bv the straini of the organiismii used. Although many anthors list certaini substances as beingc essenitial for certaini species, the possibility that said compounid can be substituted for byanother compound is niot obviated. It is of interest to n-ote that the bios-complex (including tliiamini alnd its fractions) was the first discovered ai(l that it has been showin to be of primarv imiiportance not only for yeasts, but also for miianiy other fungi, bacteria, certaini animals, anid eveni non-chlorophyllous parts of higher plants. The inclusion of imiesoinosite as a (rowth factor milight be considere(d as fallaciouis. Mesoiniosite has proved to be essential to nonie of the test oroalnisms used; it is inert to the majority; ancd in those cases where it inldulees inerease in growth, it is used in great enough concentrations to be classified as a nutrient. Biotin, on the other halni, does accelerate growth in certain organisms even when supplied in very minute qualntities. This latter compound has been shown to be especially effective in con1junlctioni with alanine and other orowth factors. KO(3L an(d T6iNNIS (4) report that biotin is the most potent growth factor known, in certain cases, beingo effective in concentrations of one part in 400 miillion. Thiamin (anuerin), or vitamin B1, has proved to be the mlost uniiversally effective, not only with respect to the comiponents 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 essenitial or stimuulatory for the growth of the test organisms. In several instainees it exerts o effect, and in other cases it is reported as being inhibiting. Thiamiii is a complex molecule composed of two ring nuclei each of which may exist as an individtual compounid; viz., as pyrimidine and thiazole. A study of this compound and of its components reveals aii interesting diversity aiiiongo those fungi for which the vitamin is effective as an essenltial orowth factor. In this respect, fungi may be divided into the followingy five categories: (a) Those which require the intact thiamin molecule. A substitutioni of the two componients is noni-effective; (b) that group which requires both coin278