Nastic Structures: The Enacting and Mimicking of Plant Movements - Biomimetics: Biologically Inspired Technologies - page 474

movement taxis or tactic movement. Depending on the type of activating stimuli, such as light, ions,
sugars, or hormones, and electric fields we find directed translocations in the form of phototactic,
chemotactic, or even galvanotactic movements. The direction of the response to the triggering
stimulus is indicated by a positive or negative sign, for example, a single green alga moving
towards a light source carry out a positive phototactic move whereas light avoidance is referred to
as negative phototaxis. In higher plants, we find positive chemotaxis in fern sperms, which respond
to a malate signal from unfertilized archegonia. Similarly, pollen tubes change their direction of
growth towards a calcium or hormone signal from unfertilized ovules. Whether the last example
should be referred to as a tactic movement is disputable, since only the internal part of the original
organism — the pollen grain — is moving.
The locomotion of entire large organisms can be viewed as a zoological specialization that
derived from organ movements. It was useful only for the evolution of animals to coordinate these
organ movements with a purpose to translocate the entire organism as a unit. Primarily, both animals
and plants show only autonomous movements of their organs whether these are leaves, flower parts,
tendrils, legs, wings, or fins. In plants, there are two types of organ movements ; tropisms and nastic
movements. Tropisms are based on an induced difference in the irreversible expansion between two
flanks of a growing plant organ that cause the organ to bend and adopt a new direction of growth.
Accordingly, tropisms are growth responses that respond to the direction of the triggering stimuli.
Stimuli, such as light, gravity, hormones, mechanical stimulation, and electric fields induce directed
responses in the form of phototropic, gravitropic, chemotropic, seismotropic, and galvanotropic
curvatures. A well-known example for positive gravitropism is the growth of the main root towards
the earth's center of gravity. Negative gravitropism is the response that leads a growing organ away
from the earth's center of gravity and appears, for example, in the vertical reerection of trampled or
fallen plants. Since tropistic changes in growth direction are caused by different expansion rates on
the opposite flanks of plant organs, they resemble monomorph and bimorph actuators. Contrasting to
tropisms are organ movements that are independent from the direction of the inducing stimulus, for
example, raising and lowering of leaves, folding and unfolding of leaves, opening and closure of the
Venus flytrap. This second type of organ movement is called nastic . Like before, with tactic and
tropistic movements we can also specify photonastic, seismonastic, and chemonastic movements.
Nastic movements are often reversible and defined by joint-like structures that confine their mobility
options. Although these characteristics are shared by many human-made machines, we will find
nastic motors and structures to be uniquely arranged.
In engineering terms, plants are adaptive (smart) structures with remarkable capabilities that
were developed and perfected over millions of years of evolution in constantly changing and
increasingly complex environments. It is therefore smart to study, understand, mimic, and modify
nature's time-tested principles and mechanisms. Life originated in water, and nastic structures as
well as their motors are optimized to use the potentialities of this unique solvent. In addition to
ATP-dependent molecular motors, contracting and inflating molecules, such as the P-protein in the
phloem conduits, plants rely heavily on three hydration motors (osmotic, colloidal, and fibrous) that
figure as the major workhorses for nastic plant movements. After examining the three major types
of plant motors, we will review selected examples for their action in simple and then more complex
nastic structures.

19.2 MOTORS IN NATURE'S NASTIC DESIGNS

When exploring the designs of moving things, one usually starts with the force-generating units or
motors. Animal locomotion involves molecular motors called muscles, which consist of long
fiber cells with the ability to contract. With an almost ubiquitous force of contraction of about
30 g mm 2 , muscles develop their strength in pull but not in push. It follows that they must operate
any reversible joint in antagonistic pairs attached to different locations of the relevant bone levers.