Transpiration, the movement of water through plants from the soil to the atmosphere, is an important process in plant physiology, the hydrologic cycle, and the global energy balance. Transpiration at the scale of individual plants can be measured with weighing lysimetry, but this technique is limited to small plants and is generally impractical for trees and many field crops. Heat-pulse velocity methods offer an alternative, and several plant sap flow gauges have been marketed. Because these gauges use electrical resistance heaters to heat the stem, they present several problems: they are invasive, typically bulky, provide poor temperature control (killing the cambium), and have lengthy response times, so they cannot measure short-term transients. In this report, we describe a system for measuring sap flow in real-time and without the need to puncture the stem. Instead of a resistance heater, it uses a laser beam as a heat source, and instead of contact thermometers, it uses non-contact infrared thermometers. Used with a precision-mounting unit that insures constant alignment, it determines whole-plant transpiration. The laser has the added advantage of delivering a precisely controlled amount of heat for a discrete time period. The end product is a more accurate, less invasive way to gauge water flow through herbaceous plant stems.