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THE PROBLEMThe rate of elongation of root hairs offers many advantages for a study of the responses of protoplasm to environmental conditions. Physiological experimentation consists in keeping such factors as temperature, light, gas, osmotic and chemical relations constant, and then varying one of them to a known degree and measuring with mathematical precision the alteration of the organism. Prime considerations in the choice of appropriate living material for this work are: the sensitivity of the organism; the reliability of the readings; and the simplicity of the process.Root hairs are cylindrical, increasing in size in only one dimension. They have definite termini, namely, the horizon of the root at one end and the dome-shaped apex at the other. Their diameter is, in many cases, about 10 microns, and their length during the first few hours of growth is usually less than two millimeters, hence they can be readily observed and measured within the field of the ordinary low-power objective.The elongation of the root hair involves a relatively simple physiological process. It consists of the projection of a portion of the exterior surface of a superficial cell, and elongates perpendicularly with respect to the longitudinal axis of this cell and of the entire root. The protoplasm contained within its cell wall consists of cytoplasm in the form of a pellicle enveloping and penetrating in strands the extensive vacuolar system. In most instances the nucleus is present in the hair rather than in the cell proper from which the hair protrudes. The process of root-hair enlargement seems thus simply to be a matter of the extension of the cell wall, increasing the volume of the cell. Its study is not complicated by the concomitant processes of cell division and cell differentiation, as obtains in the growth of multicellular tissues.Root hairs of some plants are readily produced either in a saturated atmosphere or in an aqueous medium. Thus is opportunity afforded for the study, not only of the effects of temperature, light, and other radiations, but of atmospheric gases directly, and also of osmotic, ionic and other conditions possible only in solutions. The present series of studies is directed toward the last-named objective.Sand and solution culture experiments have contributed greatly during the past few years to our information regarding proper balanced rations