Nastic Structures: The Enacting and Mimicking of Plant Movements - Biomimetics: Biologically Inspired Technologies - page 484

Figure 19.9 (See color insert following page 302) A stable, erect sunflower stem (left panel) depends on the
pressure of internal, easily expandable hydrostat tissue (pith ¼ transparent cells in center panel) that tensions the
stronger-walled surface layers of the stem giving them rigidity and stability. The well-known limp shape of wilting
young plant stems (right panel) occurs when the internal, thin-walled pith tissue dehydrates, shrinks (center panel)
and ceases to exert radial pressure on the surface layer and keeps it under tension. Left panel shows fully hydrated
sunflower stem and right panel a dehydrated stem; the center panel demonstrates volume reduction in pith during
dehydration of a segment slice. Pith parenchyma acts as a hydrostat motor that provides herbaceous stems with
stability and the driving force for expansion.
other colloid materials (for detailed reviews see Vincent, 1982; Taiz, 1984; Cleland et al., 1990;
Brett and Waldron, 1996).
Cellulose fibrils wrap the internal cell motor in either a single or many subsequently deposited
layers creating either a primary or secondary walls. If there are many layers and the microfibril
directions between these separate layers are diverse enough, the wall will provide multidirectional
support and the cell has no directional preference for expansion. Older cells with secondary walls
resist the vacuolar pressure equally well in all directions. The same principle applies to the
multidirectional strength of plywood (Figure 19.10). However, if a cell is young or part of a nastic
structure, it has only a thin primary wall with one cellulose layer with one dominant direction of the
microfibrils. Such anisotropic walls provide young cells with a preferred direction for their
expansion. Young tubular stem cells most frequently show preference for a transverse (radial)
orientation of the microfibrils or for a low pitch helix. Since such microfibril arrangements favor
axial and restrict diametrical extension, the cells are bound to elongate and develop into a narrow
cylindrical shape.
During the growth of plant stem cells have most of their microfibrils around them in rings or
helices with increasing pitch. The feature of helical microfibril bundles, particularly prominent
in the collenchyma cells of vascular tissues and tendrils, provides stems with an astonishingly
high resistance to tension, high strain, and breaking strength they are famous for (Wainwright,
1980; Vincent, 1982). Both osmotically driven expansion and external pull affect an increasingly
vertical reorientation of the cellulose microfibrils that finally terminates cell expansion (Robards,
1974). Additional processes like the deposition of multiple cellulose layers or the resin lignin, as
well as the disappearance of expansion-catalyzing enzymes, complete the termination of cell
expansion. Some older cells may also lose their liquid content and turn into dead wood or cork
cells. The stability of such cells no longer depends on internal pressure but exclusively on the static
stability of their walls (Gibson and Ashby, 1982). Even in dead wood cells it is the spiral
arrangement of the most prominent S2 cell wall layer that resists tension and deflects cracks and
makes wood ten times more resistant to fracture than plain fiber-resin composites (Gordon and
Jerominidis, 1980).