Nastic Structures: The Enacting and Mimicking of Plant Movements - Biomimetics: Biologically Inspired Technologies - page 477

100 MPa. Due to internal adsorption, the volume increase of colloidal systems is smaller than the
volume the adsorbed water occupies in its free form. Note also that the response of colloid-based
hydration motors is not limited to water alone but can also be strongly affected by the concentration
of Ca 2 þ , ATP, temperature, and pH (Pollack, 2004). This is one reason to consider colloid-based
hydration motors as more promising for biomimetic designs than osmotic motors.
Water-soluble proteins (albumins and globulins) from seeds are well-hydrating colloids that bind
1.3 g of water per gram. Although proteins have been studied for a long time, a recently isolated slime
or P-protein from beans generated a lot of interest because of its large, calcium-dependent expansion
(and contraction) that led to a rapid plugging of pores near the wound sites (Knoblauch et al., 2003;
Mavroidis and Dubey, 2003). Many other interesting colloids in plants remain unidentified; they
often contain both carbohydrate and protein polymers and are summed under the name of mucilage
(e.g., Wainwright, 1995). Mucilage is known to swell perceptibly in water and can be precipitated
(dehydrated) with alcohol. The water-holding capacity of mucilage is known and has been used in the
herbal sciences where mucilage coating soothes inflamed mucous membranes and keeps them
hydrated (emollients) during coughs and bronchial infections. Large amounts of mucilage are
found outside and inside cells from plants in highly dehydrating environments, for example, cacti
and halophytes (Englmaier, 1987). Interesting are recent findings of mucilage in the water-transport-
ing xylem vessels of plants. The adhesion of water molecules to capillary walls filled with mucilage
could become stronger than their cohesion in the column and so localized mucilage secretion may be
a way to segment the long and heavy water column that otherwise would stretch uninterruptedly from
the soil to the tip of the plant and require a pull of more than 0.01 MPa per meter to be moved
(Zimmermann et al., 1994). An increase in xylem-bound mucilage was found after mechanical
wounding and might also have a blocking function for pathogens (Crews et al., 2003). Having a
confirmed role in water storage and volume changes in desert, salt, and normal plants, we suggest that
mucilages are worth testing as potential motor material.

19.2.2.1 Macroscopic Swelling Bodies

One of the few investigated colloid-based plant motors are seeds, in particular pea seeds (Kuhne and
Kausch, 1965; Larson, 1968; see also Figure 19.1). The authors showed that the first phase of
hydration is not associated with the development or enlargement of vacuoles and is hence not an
osmotic but simple colloid-based hydration process. The hydration is easy to follow since the seeds are
big and their expansion can be measured macroscopically. The imbibition pressure of hydrating pea
seeds is very high. For centuries botany students in Europe have been impressed by their

Figure 19.1 Dry (left) and hydrated (right) natural swelling bodies in the form of split pea seeds (A, shells have
been removed) and corn grits (B, grits are particles of broken seed endosperm). The final volume increase is about
150% in split pea seeds and 200% in corn seed particles. Note that the smaller grit particles hydrate much faster
than the larger peas.