Nastic Structures: The Enacting and Mimicking of Plant Movements - Biomimetics: Biologically Inspired Technologies - page 479

Figure 19.3 Cross section through a 3-mm-wide aerial root of a tropical orchid (left panel) shows the living central
part of the root (gray) and the nonliving hygroscopic fibrous layer, called velamen (white), surrounding it. Right side
picture shows velamen in higher magnification.

efficiency. We can combine this fact with the finding that quite a few plants developed hygroscopic
tissues that are able to extract water from the air. The list includes not only lichens, mosses, and tree
ferns but also higher plants like the gigantic mammoth trees ( Sequoiadendron ) and rootless epiphytic
orchids and bromeliads, including the so-called Spanish moss in the genus Tillandsia . Their hygro-
scopic tissues contain colloidal and matrix substances that hydrate easily and reversibly, character-
istics of prime interest for motor material. Another example is velamen, the dead tissue that covers the
aerial roots of epiphytic orchids (Figure 19.3). Although the hygroscopic swelling of this tissue has no
apparent mechanical function, it functions to transfer the humidity of air to the internal root tissue and
the plant. Even among plants, this is an incredible achievement. It is useful to study the hydration of
such materials in more detail to understand the aggressive mechanisms of such hygroscopic struc-
tures. In the case of velamen, the hydration material consists of easily accessible, hygroscopic, and
dead cell walls; that is, the material is fibrous.

19.2.3 Fibrous Motors

Fibrous motors are based on the adhesive absorption of water in internal capillary spaces of
parallel-arranged fibers. Fibers are used as reinforcing elements in both technical and biological
designs; their role as hydration motors identifies them as multifunctional materials. And yet this
multifunctionality has often been overlooked. With the exception of blotting paper and the wooden
stone splitters of ancient times, hydration-dependent form changes of wood have been mostly seen
as an annoyance rather than an opportunity.
The most common natural fibrous material is cellulose. Cellulose is a polymer with a clearly
defined hierarchical structure. Cellulose fibers are made of long chains of glucose molecules twisted
together in a micellar bundle. These bundles are often found in a parallel arrangement to increase
the breaking strength of the material. Unlike isometrically expanding colloids, parallel fibers swell
in a diametric fashion, that is, they expand only in the two directions that are perpendicular to the
direction of the fibers (Figure 19.4).
Diametric expansion occurs when water fills the inter-fibrillar spaces where it pushes the fibers
apart without altering their length (Figure 19.4 and Figure 19.5). The space between the microfibril
bundles resembles small-sized capillaries and is large enough for water to move and be adsorbed
(Figure 19.5; Frey-Wyssling, 1959; Robards, 1974). The large internal or specific surface ( S v )of
cellulose bundles and other fibers accounts for their rapid hydration. Fibrous hydration occurs in
the walls outside the living cell and therefore does not contribute to internal cell (turgor) pressure.
The obvious lack of operating osmotic motors in dead tissues led to an early acceptance of
operating fibrous or wall-based hydration motors in nonliving tissues. Although this has not been