This research represents the first systematic collection of the woody plants throughout the Cross Timbers. It provides the first keys to these plants in their vegetative condition, plant descriptions, distribution maps, and some quantitative measurements used for descriptive purposes. Descriptions of the woody plants were constructed as an aid in verification after a specimen has been identified by use of the keys. The measurements given pertain only to the woody plants as they occur in the Cross Timbers. Distributional maps are provided for all the taxa considered in this research. With the exception of those species which have the ecological amplitude to grow throughout the Cross Timbers, the distribution of the majority of the remaining species seems to be most strongly influenced by average annual precipitation. In a few instances, conditions associated with latitude appear to govern the distribution of species or varieties within the Cross Timbers. Throughout the Cross Timbers, post oak (Quercus stelta), blackjack oak (Quercus marilandica), and hickory (Caraa texan) dominate the upland forests. The streamside forests are dominated by willow (alix nigra), cottonwood (Populus deltoides), and hackberry (Celtis laevi ata). The variation in the vegetation of the Cross Timbers is not due to any change in …