7.8 Plant Tissues
169
the same way. Bending movement during drying and rewetting is achieved in both
species by a bilayered structure in which one layer is soft and contracts lengthwise as
it dries, while the other layer is stiff and resists contraction. The direction of bending
is again controlled by the anisotropy of cell walls.
Desert plants, in contrast, need to scatter their seeds when more moisture is
available and they have better chances of germinating, and for them it is a matter
of life and death rather than just efficiency of dispersal. The desert ice plant seed
has evolved a sophisticated mechanism, illustrated in Fig. 7.37 (Harrington et al,
2011). In the dry state, seed compartments, partitioned by septa, are covered by
keels (bottom petals) that serve as protective valves preventing premature dispersion
of seeds. When the hygroscopic keel tissue absorbs water and swells, as shown in
the bottom panels, each valve, consisting of two halves separated in the dry state but
coming into contact when swollen, unfolds in a way that depends on the anisotropic
deformation of the keel cells.
Seed dispersal is most effective when motion is fast, which is not easy to achieve
without muscles. Effective operation should rely on a mechanism that stores elastic
energy gradually by integrating activities across different spatial scales, but releases
it rapidly. Hofhuis et al (2016) thoroughly investigated such a mechanism in C.
hirsuta, a plant commonly called popping cress, due to the explosive shatter of its
fruit pods, which takes just a few milliseconds and fires seeds over a two-meter range
(Fig. 7.38a–c). The firing structure of the two valves enclosing the seeds consists
of three elastic layers, each with a different reference geometry: an active soft outer
layer, the exocarp, a passive middle layer, and an inner layer, the endocarp, stiffened
by lignin. They are attached together in such a way that, in the closed state, the outer
layer is in tension and the other two are in compression.
During fruit maturation, the growing seeds deform the valve (Fig. 7.38d), so that
its cross-section is not flat but bowed outward. Explosive opening is brought about
Fig. 7.38 (a)–(c) Explosive seed dispersal by C. hirsuta. The two valves detach from the fruit
(a), curl back with seeds adhered to the inner valve surface (b), and launch seeds while coiling
(c); arrowheads indicate seeds. (d), (e) Triggering the energy release. (d) The three-layered valves,
with the exocarp, endocarp, and passive middle layer colored red, blue, and green, respectively,
are curved in cross-section and build up tension while attached to the fruit (brown). Dehiscence
zones (orange) form along the valve margins, weakening the attachment. (e) Valves flatten in crosssection via opening of the endocarp hinges, as shown in the insets, and release the tension by coiling
(Hofhuis et al, 2016)
169
the same way. Bending movement during drying and rewetting is achieved in both
species by a bilayered structure in which one layer is soft and contracts lengthwise as
it dries, while the other layer is stiff and resists contraction. The direction of bending
is again controlled by the anisotropy of cell walls.
Desert plants, in contrast, need to scatter their seeds when more moisture is
available and they have better chances of germinating, and for them it is a matter
of life and death rather than just efficiency of dispersal. The desert ice plant seed
has evolved a sophisticated mechanism, illustrated in Fig. 7.37 (Harrington et al,
2011). In the dry state, seed compartments, partitioned by septa, are covered by
keels (bottom petals) that serve as protective valves preventing premature dispersion
of seeds. When the hygroscopic keel tissue absorbs water and swells, as shown in
the bottom panels, each valve, consisting of two halves separated in the dry state but
coming into contact when swollen, unfolds in a way that depends on the anisotropic
deformation of the keel cells.
Seed dispersal is most effective when motion is fast, which is not easy to achieve
without muscles. Effective operation should rely on a mechanism that stores elastic
energy gradually by integrating activities across different spatial scales, but releases
it rapidly. Hofhuis et al (2016) thoroughly investigated such a mechanism in C.
hirsuta, a plant commonly called popping cress, due to the explosive shatter of its
fruit pods, which takes just a few milliseconds and fires seeds over a two-meter range
(Fig. 7.38a–c). The firing structure of the two valves enclosing the seeds consists
of three elastic layers, each with a different reference geometry: an active soft outer
layer, the exocarp, a passive middle layer, and an inner layer, the endocarp, stiffened
by lignin. They are attached together in such a way that, in the closed state, the outer
layer is in tension and the other two are in compression.
During fruit maturation, the growing seeds deform the valve (Fig. 7.38d), so that
its cross-section is not flat but bowed outward. Explosive opening is brought about
Fig. 7.38 (a)–(c) Explosive seed dispersal by C. hirsuta. The two valves detach from the fruit
(a), curl back with seeds adhered to the inner valve surface (b), and launch seeds while coiling
(c); arrowheads indicate seeds. (d), (e) Triggering the energy release. (d) The three-layered valves,
with the exocarp, endocarp, and passive middle layer colored red, blue, and green, respectively,
are curved in cross-section and build up tension while attached to the fruit (brown). Dehiscence
zones (orange) form along the valve margins, weakening the attachment. (e) Valves flatten in crosssection via opening of the endocarp hinges, as shown in the insets, and release the tension by coiling
(Hofhuis et al, 2016)