The determination of the factors which are operative among the plants of a genus in the establishment of specific differences constitutes one of the interesting problems of biology. There are numerous approaches to this problem, but one approach which seemed particularly germane to the writers concerned the effect of zinc upon the general growth and metabolism of plants. Although zinc has been established for many years as an essential element in plant nutrition, it has only been fairly recently that sufficient evidence has been accumulated through the work of SKOOG (2), TsuI (5), and others to establish an hypothesis concerning its mode of action. In general the evidence seems to indicate that, in some manner, there is a relation between zinc and auxin production and/or destruction within the plant. Regardless of its precise method of action, there seems little question that in the absence of zinc the level of auxin is depressed. Not only has this fact been confirmed quantitatively through the work of SKOOG (2) and of TsUI (5), but the symptoms of zinc deficiency reported by many workers are all congruent with the concept of a low level of the growth hormone. Data have been obtained (6) which clearly indicate rather important inheritable physiological differences among at least four species with respect to effect of temperature on growth. It has also been reported by STEPHENS (3) that reciprocal grafts between various cotton species produced differential effects. It is considered that some of these effects might be associated with various levels of auxin production or sensitivity between the various species. This hypothesis might be tested by controlling the level of zinc, hence of auxin, in the plant and observing the general growth responses for differential effects. An experiment establishing the need for zinc by Gossypium hirsutum indicates the probable requirement of all species for this nutrient element (7). The species of cotton selected for this experiment were two American tetraploid (n = 26) species, Gossypium barbadense L. (Sea Island-Ex. Jenkins), and Gossypiurm hirsutum L. (Coker 100-W); and one Asiatic diploid (n = 13) species, Gossypium arboreutin L. (race-sinense). Observations were made on plants growing in an unshaded greenhouse. Plants were grown in two-liter Pyrex beakers which had previously been painted on the