Abstract: This paper concerns mainly the midday depression of transpiration and water potential gradiant of the soil-plant system of some mesophytes and xerophytes in ordos Sandland of China. The conclusions are given below:1. A new concept of dividing the ecological types in light of plant water relations is provided based on the midday depression of transpiration, and three types of typical features of different plant species are given: a. non midday depression type; b. weak midday depression type; c. strong midday depression type.2. By analyzing the water potential gradiant of the soil-plant system, we hypothesized that population density is affected by the sensitivity of soil water availability change to the change of leaf stomatal resistance