Roots can sense and then grow toward a water source. This extraordinary capability of roots - hydrotropism has not been well studied. The objective of this study was to examine the environmental and physiological factors that might affect hydrotropic response in the maize primary roots. A system that was first established by Takahashi and Scott in 1993 was adopted in this study. Maize seedlings with uniform root length were fixed on a vertically placed wet pad with a slanted angle at the bottom. The root tips were aligned with the upper edge of the slanted pad bottom. The pad with seedlings was then vertically placed in a small chamber. The humidity of the chamber was controlled by placing a reservoir of either saturated K₂CO₃ solution or pure water at the bottom of the chamber. The root continued to elongate downward in the darkened, closed chamber. Due to the higher humidity of the air near the slanted bottom of the wet pad (compared to the relatively dry air on the other side of the root), the root curved and grew toward the wet pad and grew away from the drier air. The curvature away from the vertical (gravity) axis was measured to quantify the strength of the hydrotropic response. To test the effect of the humidity gradient on the hydrotropic response, three experiments were conducted: 1) roots were grown on the wet pads with different slanted angels; 2) the roots were placed a few mm away from the wet pad; and 3) the roots were positioned at different distances from the saturated K₂CO₃ solution. The effect of root length and of the elongation rate on hydrotropic curvature was also tested. Because the results showed that hydrotropic curvature was greater, when the saturated K₂CO₃ solution was used to control the chamber humidity compared to pure water, all the other experiments in this study were conducted by using the saturated K₂CO₃ solution. When pads with different slanted degrees were compared (40°, 50°, or 60°), the root curvature was greater as the slanted angle increased (Note: The greater the slanted angle is, the closer the root tip is to the hydrostimulant - the wet pad.) Also, the roots showed the best hydrotropic curvature when the roots directly touched the hydrostimulant compared to when the roots were placed 3 mm or 6 mm away from the hydrostimulant. Finally, root curvature was greater when the distance between the root tip and the salt solution was 4 cm compared to 6 cm. When roots of different lengths (1.0, 1.5, 2.0, 2.5, or 3.0 cm) were compared, the shorter roots showed much better hydrotropic response than the longer roots did. Since maize primary roots elongate faster as the roots get longer during germination (before reaching a relatively steady rate), it is possible that a better hydrotropic response in the shorter roots might be explained by a slower elongation, providing more time for the roots to respond to the humidity gradient. To test this hypothesis, roots of the same length, but with different elongation rates, were tested for their hydrotropic response. The root elongation rate was controlled by varying growth temperatures, and the time was adjusted to allow the roots at different temperatures to reach the same final length before root curvature was determined. The results showed that root curvature decreased as the growth temperature increased. In summary, this study demonstrated that hydrotropic response in the maize primary roots is affected by environmental factors as well as the developmental stages of the roots. Maize roots showed a stronger hydrotropic response when the roots are closer to the hydrostimulant and when the humidity gradient around the roots becomes steeper. Moreover, roots with slower elongation showed greater hydrotropic response. Since roots elongate slower under drought, and the humidity gradient is steeper in dry soil, it can be hypothesized that hydrotropism may play an important role in water acquisition in maize plants under drought. The findings from this study also allow us to optimize an experimental system that can be used to study the mechanisms of hydrotropism in the future.