  Stochastic Schr\"odinger equations that govern the dynamics of open quantum
systems are given by the equations for signal processing. In particular, the
Brownian motion that drives the wave function of the system does not represent
noise, but provides purely the arrival of new information. Thus the wave
function is guided by the optimal signal detection about the conditions of the
environments under noisy observations. This behaviour is similar to biological
systems that detect environmental cues, process this information, and adapt to
them optimally by minimising uncertainties about the conditions of their
environments. It is postulated that information-processing capability is a
fundamental law of nature, and hence that models describing open quantum
systems can equally be applied to biological systems to model their dynamics.
For illustration, simple stochastic models are considered to capture
heliotropic and gravitropic motions of plants. The advantage of such dynamical
models is that it allows for the quantification of information processed by the
plants. By considering the consequence of information erasure, it is argued
that biological systems can process environmental signals relatively close to
the Landauer limit of computation, and that loss of information must lie at the
heart of ageing in biological systems.
