Physical Consequences of Ultrasound in Plant Tissues and Other Bio-Systems

With plant tissue, as well as with selected animal cells in suspensions and tissues, it has been possible to make direct observations by optical microscopy during irradiation with ultrasound. This visual inspection reveals a number of kinds of mechanical action by which ultrasound produces changes in biosystems. In general, cell membranes are set into vibration. These vibrations are especially vigorous near gas-filled intercellular spaces, which abound in plant tissues. Because of these complex oscillations, cytoplasmic structures behave as if acted on by time-independent forces and torques; also intracellular and extracellular fluids are caused to flow. Much of the mechanical action is understandable on the basis of theory for nonlinear acoustics. Second-order approximations to the solutions lead to the phenomena of acoustic streaming, radiation force (including aggregation tendencies) and sonic torque. Bio-systems present unique situations for applying this physical theory, as yet mostly on a qualitative basis. Examples of semi-quantitative agreement exist, and we can expect an increasing ability to make quantitative predictions as systematic experimental and theoretical studies proceed.