Fluid Mechanics of Green Plants

Why is the study of the biomechanics of green plants important? First, it has been estimated that plant life comprises 99% of the Earth's biomass (Bidwell 1974). Second, green plants are virtually the only ultimate source of food for animals through photosynthesis (the process of conver­ sion of solar energy to stored chemical energy). A biofluid-mechanical overview of a typical green plant is shown in Figure 1. See Nobel (1974) for an extensive self-contained quantitative introduction and order-of-magnitude analysis; for a shorter quantitative introduction, see Merva (1975). Meidner & Sheriff (1976) have written a short introduction that uses engineering concepts with a minimum of mathematics, and Canny (1977) has written a brief nonmathematical introduction for fluid mechanicians. The leaves are the site of photosynthesis. This process requires sunlight, CO2 , and water, and produces glucose (a simple sugar) and oxygen. Sugars manufactured in the leaves are translocated to other parts of the plant via the vascular phloem tissue. Water and minerals absorbed in the roots are brought up to the leaves via the vascular xylem tissue. The upward xylem flow (called the transpiration stream) is driven by evapora­ tion at the leaves, while the largely downward phloem flow is thought to be driven by concentration differences created locally by active transport (e.g. the Munch hypothesis; see Bidwell 1974). Studies of each of these parts of the plant have involved special fluid mechanics problems based on the particular physiological function and geometry. This article introduces the reader to the concepts and problems