Biomedical Engineering Reference
In-Depth Information
Figure 19.8 A hydrostat network of thin-walled, elastic and expandable parenchyma cell walls internally pres-
surized by vacuoles. This kind of tissue is soft when dehydrated and rigid when fully pressurized (inflated) and
provides the basis for most reversible (cell walls undergo elastic expansion) and irreversible (cell walls yield to
stress and are plastically altered) volume changes in plant tissues. Volume changes of these thin-walled cellular
networks are the motor behind many nastic movements.
hydrostats (Niklas, 1989, 1992), sometimes as motor cells (Pfeffer, 1873). The term motor cell
refers to the ability of cells to rapidly expand or shrink their vacuoles, cell volume, and surface area.
The term hydrostat refers to the characteristic of cells and cell complexes (tissues) to undergo
striking changes in their mechanical properties in dependence on the degree of water uptake and
inflation. The soft walls and plasticity of hydrostat tissues turns into a very rigid structure upon full
hydration or inflation, which generates a high internal pressure and exposes the cell walls to high
tension and rigidity. The most common and basic function of such hydrostats is (i) to constitute the
expanding, growing parts of the plant and (ii) to provide rigidity, stability, and expandability to
these parts. Niklas (1992) calculated that only cells with a wall thickness considerably less than
20% of the cell radius will exhibit hydrostat and motor behavior. Reversibly expanding motor cells
need to have walls with sufficient elasticity, that is, the walls should stretch while building an elastic
tension that returns the cell to its original size when the initiating pressure is relieved.
A fitting example for a hydrostat motor tissue is the pith parenchyma in the core of younger
sunflower stems (Figure 19.9). When well-hydrated, the parenchyma cells of the pith exert radial
pressure on the peripheral cell layers of the stem and give it rigidity and straightness (Kutschera,
1989). Dehydration of the pith cells softens and bends (wilts) the upper, younger part of the stem.
Observation of stem slices shows that it is the lack of pressure from the shrinking pith cells that
causes wilted bending of the stem (Figure 19.9). In older, no longer growing parts of the stem, pith
cells die and fill with air (sectioned pith appearance turns to white) before disappearing altogether
(stem becomes hollow). Meanwhile the older stem is reinforced through lignin depositions in the
peripheral ring of vascular bundles that turns their walls into a stable fiber-resin composite. The
older stem is no longer a hydrostatically stabilized structure.
19.3.2
From Isotropic Cell Pressure to Anisotropic Cell Expansion
The form of cell expansions is always determined by a combination of motors and nonexpandable
materials surrounding them. While fiber motors are inherently structural by showing anisotropic
expansions, the common vacuole-based osmotic and colloid motors are unstructured in the sense
that they exert equal pressure in all directions. To convert the isotropic vacuolar pressure into a
anisometric cell expansion, plant cells use anisotropic depositions in their cell walls. The original
cell wall is made of cellulose and similar fibers, polyelectrolyte gels like pectin (made of galac-
turonic acid monomers that keep an unbalanced carboxyl group after polymerization), as well as
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