Biomedical Engineering Reference
In-Depth Information
Figure 19.7 Dehydrated and twisted (top) and smooth hydrated halves (bottom) from the same seed pod of
Lathyrus japonicus, a legume. The wall is made of two hydration layers with the fibers pointing in different directions
(angle of
90
8
). When dehydrating (top), this torsion motor rips the pod apart and releases the seeds.
from the soil). In spite of their different complexity, all these structures show movements that are
reversible, independent from the direction of the triggering stimuli, and powered by some type of
hydration motors. While the definition could be expanded to suborgan levels of structure, it is
general enough to include two different types of nastic structures: (i) autonomous structures that
are driven by their own hydration motor and (ii) nonautonomous structures that depend on external
energy sources to work. Nonautonomous nastic structures are most frequently found in flowers
where they work as strictly mechanical machines that force pollen on unsuspecting visitors. It is
the transmitted energy of the weight and impetus of a landing insect (legume flowers) or bird
(
Strelitzia reginae
or bird-of-paradise flowers) that causes the movement of stamens (legume
flowers) or the opening of a protective envelope around the stamens (
S. reginae
). Details of these
structures can be found in many reviews and monographs on plant reproduction (e.g., Meeuse and
Morris, 1984).
The following part focuses on examples for the even more intriguing, autonomously working
nastic structures that contain their own motors and hence depend on external factors only for
stimulation. Such autonomous nastic devices consist of a motor and structures that modify and
direct the forces it generates. Therefore, nastic structures could be well defined as asymmetric or
heterogeneous elastic restraints of hydration motors. Whether they surround osmotic, colloidal
or fibrous motors, the unequal restraints are almost always realized by heterogeneous depositions of
the cell wall material and in particular by a different direction of the deposited layers of cellulose
microfibrils. Depending on the complexity of the nastic structures, the asymmetry can be located
between the opposite walls of one cell (e.g., guard cell), between cell layers (Venus flytrap) or
between internal and external tissues (growing stems). Nastic cells or organs are characterized
by asymmetric capacities for elastic expansion that result in asymmetric expansions in the form
of curved responses. The following part of the chapter describes the functioning of nastic
structures through different levels of complexity and provides useful examples of how plants use
their motors.
19.3.1
Hydrostat Motor Cells — Source and Location of Movements
Most plant movements and growth are driven by
osmotic motors
. These motors are located in either
young cells or specialized cells that feature thin, expandable walls and a large, osmotically
pressurized central vacuole (Figure 19.8). If not part of specialized units (guard cells of stomatal
pores, pulvinus cells of bendable petioles, etc.) these young and undifferentiated cells are constitu-
ents of a tissue called parenchyma. The behavior of these cells compares well to pressure-dependent
mechanical softness and rigidity of human-made inflatables and they are referred to sometimes as
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