This study addresses the nature of physiological and phenological evolutionary strategies of root growth dynamics and energy allocation followed by Atriplex confertifolia, Ceratoides lanata, and Artemisia tridentata growing in their natural cool desert environment. Root observation chambers with inclined Plexiglass windows were installed in monospecific desert communities of Atriplex confertifolia, Ceratoides lanata and Artemisia tridentata. Soil temperature and water potential measurements taken immediately adjacent to the observation windows indicated a minimal disturbance was caused by the presence of these chambers. For the three species initiation of root growth was observed before initiation of shoot activity, furthermore, active root growth extended over much longer periods during the year than shoot growth. Initial growth was observed for the three species in the upper soil layers in the spring. Later in the season most of the growth activity was measured at progressively greater depths in the soil. Measurable root growth was observed for Atriplexin August when the soil water potentials were in the range of -70 bars for 1972, and at water potentials of -60 bars for all three species during 1973. Detectable growth for these three species was recorded as late as January in 1974. Except for the main extension roots, individual apical meristems were seldom active for more than 2 weeks. Atriplex confertifolia and Ceratoides lanata plots were labeled during the growth season with 14Co2 in polyethylene enclosures to study both the partitioning of photosynthates to plant parts and their total allocation of carbon at the community level. A definite seasonal pattern of partitioning of recent photoassimilates corresponding to phenological events emerged. In the spring, photoassimilates were principally directed to shoot growth, especially expanding new leaves and vegetative buds. In terms of relative energy allocated to plant parts per unit dry weight basis, it appears that Ceratoides lanata expends less energy for reproductive organs. For both species, carbon used for new stems and previous years shoot growth appears to constitute a significant sink for energy use and storage. Relative translocation of carbon to roots was minimal during the spring for both species. It increased with the progression of the season reaching a maximum in July for Atriplex and at the end of the season for Ceratoides. Energy allocation at the community level for these species showed that approximately 60 and 40 percent of the recently photoassimilated 14C for the Atriplex-dominated community in July and September, respectively, appeared localized in the new shoot growth, the remaining was distributed in nearly equal amounts between previous year's shoot growth and the root system. The scheme of energy allocation in Ceratoides showed similar patterns of carbon utilization in July and September; approximately 80 percent of the fixed carbon was al located in approximately equal amounts to roots and new shoot growth with the remainder to the previous year's shoot growth. In the Ceratoides-dominated community 65 percent and 36 percent of 14C photoassimilated in April and July, respectively, and still remaining in the plant by September, was localized in the underground structures. Similarly, in the Atriplex community, 35 percent and 29 percent of the 14C incorporated in April and July appeared in the root system. From the total 14C photoassimilated in July for both communities, approximately 60 percent and 50 percent was retained in the plants by September in the Atriplex- and Ceratoides-dominated communities, respectively.