The increasingly widespread use of minirhizotrons, below-ground video cameras, and root image analysis software has generated information on fine root (< 1 mm in diameter) dynamics in many species and ecosystems (Johnson and others 2001; Eissenstat and Yanai 2002). These techniques have also revealed the daunting complexity of the fine root system. Within the fine root system of a single fieldgrown plant are roots of numerous branching orders, lengths, diameters, ages, colors, and degrees of mycorrhizal colonization (Afek and others 1990; Pregitzer and others 1997; Wells and Eissenstat 2001; Anderson and others 2002; Mihail and others 2002; Waisel and Eshel 2002). Fine roots continually emerge, age, and die throughout the favorable growing season at rates that differ among subsets of the root population and change in response to seasonal and environmental factors (Hendrick and Pregitzer 1993; Pregitzer and others 1993; Reid and others 1993; Forbes and others 1997; Ruess and others 1998; Majdi and others 2001; Wells and Eissenstat 2001; Wells and others 2002a). The demographic characteristics of the fine root population are constantly shifting, and the consequences of these shifts for whole-plant physiology and growth are very poorly understood. The heterogeneity of the fine root system should not be surprising. Above ground, individual leaves exhibit well-characterized differences in anatomy and physiology with age and position in the crown (Esau 1965; Nobel 1975; Coleman 1986). An appreciation of these differences is implicit in most aboveground sampling strategies and is incorporated into models of whole-canopy function. Until recently, however, physiological differences among individual roots less than 1 mm in diameter were not frequently acknowledged. Most water and nutrient uptake models use total fine root length as a model parameter and assume constant rates of uptake along this length (Chen and Barbar 1990; Smethurst and Commerford 1993; Teo and others 1995; but Gao and others 1998). However, a number of studies indicate that roots of different ages and positions are anatomically and physiologically dissimilar (Atkinson and Wilson 1979; Palta and Nobel 1988; Lopez and Nobel 1991; McKenzie and Peterson 1995; Pregitzer and others 1997; Bouma and others 2001; Kosola and others 2002). At present, the majority of our information on fine root physiology derives from studies of the young, white seminal roots of crop seedlings grown in solution culture (Clarkson 1985; McCully 1999). Such studies have been extremely valuable in elucidating the details of root anatomy, development,