It is widely recognized that many aspects of plant form and function are coupled to variation in water availability. This is because plant persistence is ultimately dependent upon the process of carbon fixation and it is physically impossible for a plant to transport CO2 to the sites of photosynthetic metabolism inside the leaf without, at the same time, loosing water to the surrounding atmosphere. How the efficiency of water use changes as leaves differ in size and longevity, and where leaves acquire their water from, are often times not well defined. In general, the water lost to the atmosphere by leaves is thought to originate from the soil via uptake by roots. However, previous research has shown that water deposited on leaf surfaces is often available for use via direct foliar uptake. Using field observations and a greenhouse experiment I show that leaf water interception can represent an overlooked water source for leaves that temporarily, but significantly, decouples leaf-level water and carbon relations from variation in soil water availability (Chapter 1). Additionally, within a particular environment water loss per unit leaf area is expected to increase with leaf size. Recent research suggests the construction cost of a leaf also increases with size and/or longevity. If leaves have maximized the ability to transport water to surfaces for energy and gas exchange in order to maximize CO2 uptake from the atmosphere, then vascular network efficiency (Leaf hydraulic conductance) should be size invariant. Using a survey of 60 angiosperm species I show that leaf hydraulic conductance is maximized for a given surface area (Chapter 2). By extension, if the lifetime return (carbon gain) on dry-mass invested in leaf area (construction cost plus maintenance respiration per unit leaf area) is maximized, then leaf hydraulic conductance per unit leaf dry mass should scale isometrically with leaf lifespan. Using plants from a common garden and previously published values of leaf lifespan and leaf hydraulic conductance for species inhabiting a broad range of vegetation types and climate, I explored the relationship between leaf longevity and leaf hydraulic conductance per unit leaf mass. I observed a negative correlation between leaf hydraulic conductance per unit leaf mass and leaf lifespan. Further, the slope of the relationship describing the covariation between leaf hydraulic conductance per unit mass and leaf lifespan is not significantly different from one. Isometric scaling (slope = 1) provides strong support for a constant net carbon gain per leaf despite significant variation in leaf size, longevity and environment. Therefore, variation in gross primary productivity is a function of the number of leaves a plant maintains over a given unit of time (Chapter 3).