Rubber contents are so frequently the focus of attention in investigations of guayule (Parthenium argent at urn Gray) that an account of rubber distribution in the plant may contribute to experimental studies as well as to purely descriptive literature concerning the plant. In the choice of plant ,material for study or for sampling, or in the interpretation of results, the relative amounts and concentrations of rubber in the various parts may be a pertinent factor. Even commercial scale harvesting and milling operations are aided to some degree by knowledge of which plant parts contribute significantly to rubber yields and which are insignificant or mere diluents. The pattern of rubber distribution as revealed qualitatively under the microscope has been thoroughly studied and described by Ross (8), Lloyd (6) and Artschwager (1). In plants older than one year, the vascular rays of phloem and xylem carry the major portion of the rubber. Other parenchymatous tissues, the pith, primary cortex, epithelial cells of the resin canals, and xylem parenchyma contain rubber; but, partly because they comprise a smaller fraction of the plant, the amount of rubber they contribute is less important. In younger plants, some of these latter tissues contribute a relatively larger part of the total rubber. Rubber is evident in the leaf parenchyma, but only in very small amounts. Only incomplete information is to be found concerning the macro-, but quantitative distribution of rubber in guayule. Whittlesey (13) separated several portions from native plants of uncertain but evidently advanced age. As determined by carbon tetrachloride extraction, following extraction with alcohol and water, the bark of the root and that of the trunk yielded approximately 20% rubber. Practically no rubber was obtained from the wood of these parts. This supported an impression, already current at that time, that the rubber in guayule is largely restricted to the bark. Above the trunk, which was apparently the sole remnant of the basal branching system typical