In the move from the water to land, plants evolved traits that allowed them to cope with their new environment. Arguably, the most dramatic problem they faced was the relative dryness of air. Rapid water loss from photosynthetic tissues requires equally rapid water supply. Poor water transport capabilities limited the earliest nonvascular plants to small size [1]. The evolution of xylem vastly increased hydraulic conductance and contributed to the diversification of plant size evident today [2,3].

Xylem structure has changed and diversified considerably over time, presumably reflecting progressive adaptation [4]. "Measuring" this adaptation is a challenge because it is not always obvious what traits are being selected for and what constraints and trade-offs are limiting trait evolution. It seems likely, however, that a universally favorable trait for a water-conducting network is maximum hydraulic conductance per unit investment. Higher hydraulic conductance means greater volume flow rate of water per pressure drop across the network. The higher the hydraulic

FIGURE 4.1 A bifurcating blood vessel for which Murray's law is derived in the text. Shown are the radii (r) and lengths (l) of mother (subscript 0) and daughter (subscript 1) branch ranks.

conductance, the more leaf area that can be supplied with water at a given water status and the greater potential for CO2 uptake [5-10]. Minimizing vascular investment means that less of the assimilated carbon is required for the growth of vascular tissue, leaving more for reproduction and other functions. Evaluating the hydraulic conductance per investment criterion provides insight into the adaptive significance of diverse xylem anatomies throughout the plant kingdom.

In this chapter, we summarize the use of Murray's law for evaluating the conductance vs. investment trade-off across major xylem types. The results have appeared piecemeal elsewhere [11-14] but benefit from a unified summary. Of relevance is the high profile work of West and colleagues [15-17] who concluded that quarter-power scaling laws in biology (e.g., [18,19]) result from an energy-minimizing vascular structure that constrains metabolism. These models have been criticized as being mathematically flawed and based on inaccurate vascular anatomy [13,20,21]. Nevertheless, they have drawn attention to the "energy-minimizing" principle, which is equivalent to the maximizing of hydraulic conductance per vascular investment [22]. While West and colleagues assumed this principle holds empirically, we tested it using Murray's law.

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