Negative transport is the downward conduction of water in the plant. This phenomenon has been studied by several investigators, yet considerable controversy about several aspects of the problem still exists. The portion of the leaf through which water enters is obscure. Meidner (16) suggested that specialized epidermal cells of the plant, Chaetachme aristata were involved in the phenomenon. Gessner (8) decided that most of the water was absorbed directly through the cuticle. Most investigators (4, 23) have considered that no water enters through the stomates (except perhaps a small amount of water vapor). Breazeale, McGeorge, and Breazeale (2, 3) investigated the absorption of water by leaves and its subsequent transport through the plant to the soil surrounding the roots. They concluded that tomato plants could grow to maturity, flower, and set fruit with no other source of water than that absorbed through the leaves from an atmosphere of 100 % humidity. They demonstrated that tomato plants can absorb water from a saturated atmosphere, transport it to the roots, and build up the soil moisture to or above the field capacity. Other investigators repeated the experiments of Breazeale but could get no evidence of actual water secretion by roots (9, 10, 25). Stone, Shachori, and Stanley (22) concluded that negative transport occurs only when the temperature is allowed to fluctuate and is caused by vapor pressure gradients and not by any active secretive force within the plant itself. Slatyer (20, 21), who reviewed these studies, stated that the main reason for lack of transport into soil is lack of an adequate gradient. The movement of water in plants has been studied from two different approaches: I. Some investigators have considered the entire soil-plant-atmosphere system (1, 6, 19, 24). They applied an analogue of Ohm's law and showed that water transport is controlled by the potential difference across the section and the resistance within the segment. This theory also proposes the important consideration that the rate of movement is governed by the point or region of greatest resistance in the system. Those who have studied this theory agree that the greatest resistance under natural conditions is usually located at the leaf-atmosphere interface where the water is converted from liquid to vapor. Most of these studies seem to be based more upon theoretical arguments than direct experimental results. II. Other scientists have investigated the movement of water in plants by studying some particular part of the system, such as the flow of water in the roots, leaves, or stem. Resistance to water flow in the conducting tissue of the stem is generally considered to be small as compared to other parts of the plant (5, 13, 15, 17). Some researchers have found the resistance in the roots is much larger than in the stems (12, 13, 14). Others have observed that the resistance in leaves is larger than in stems and roots (26). The resistance in the vascular elements can become larger when very small diameters are encountered (7, 27). It has also been indicated that the resistance to water flow is uniform through the cell walls, membranes, and vacuoles of plant tissues (1, 19). The experimental evidence to support these concepts is meager and inconclusive. Experimental measurements of the relative magnitude of the resistance of the stem, leaves, and roots to water flow in the absence of a water phase change have been made. This gives evidence of the relative contribution of the several plant parts to water flow resistance without the complicating factor of vaporization. Once this contribution to water flow resistance is known then studies can be made to combine the vaporization and vapor diffusion resistance as well as the soil resistance to water flow to the absorbing root surface. These experiments have also produced some information about negative transport.