Nastic Structures: The Enacting and Mimicking of Plant Movements - Biomimetics: Biologically Inspired Technologies - page 489

Figure 19.16 Longitudinal and cross sections of the swollen base (pulvinus) of a leaf stalk of Mimosa pudica.
Cross sections (pictured above the stalk) show that a centralization of the vascular bundles allows bending of this
organ in an almost joint-like fashion. Loss of ions from the hydrostat parenchyma cells below the central vascular
tubing leads to simple and reversible buckling of the structure and drops the leaf into a vertical position. (From
Fitting H, Harder R, Schumacher W, Firbas F, (1954) Lehrbuch der Botanik fur Hochschulen. Gustav Fischer
Verlag, Stuttgart. Reprinted with permission of Specktrum Verlag GmbH, Heidelberg.)

19.3.2.3 Leaf ''Muscles'' or Pulvini

Leaves of Phaseolus , Oxalis , Desmodium , Mimosa and many other plants can undergo slow or
sudden nastic ''sleep movements,'' in which the drooping leaf stalk brings the leaf blade from a
horizontal into a more vertical position. Although reasons for this movement are still unknown, the
fact that several unrelated plant species independently developed similar designs, makes one safely
conclude that it is worth the effort. The organ involved in elevating and lowering the antenna-like
structure of the leaf blade is a so-called leaf muscle or pulvinus, a swollen joint-like structure at the
basal part of a lever (the leaf stalk or petiole) that attaches the leaf to the stem. The functioning of
this unusual crane rests solely with the hydrostat motor cells of the parenchyma below the central
vascular bundle of the pulvinus (Figure 19.16). As these cells dehydrate and shrink the leaf drops
rapidly. It rises slowly again when they regain a pressure large enough to overcome the gravita-
tional force of the leaf blade. An antagonistic course of events in the cells on top of the vascular
bundle of the petiole help these movements but is less critical for the mechanism (Satter and Morse,
1990). Intriguing is that so far no ion channel has been found that is rapid and large enough to
account for the rapid loss of pressure and volume in the motor cells of the lower petiole of Mimosa
and Oxalis species. It is instructive to note that the petiole construction (cross sections pictured
above the leaf stalk in Figure 19.16) changes from a statically stable design supported by fibrous
bundles in the periphery to a statically unstable structure (after the vascular support moved from the
periphery to the center) that now has to be supported by the hydrostatic pressure of the parenchyma
cells at the lower half of the pulvinus.

19.3.2.4 External Structures Allowing Large Volume Changes

The expansion and shrinking of entire plant bodies qualifies as a slow nastic movement. A truly
remarkable example was first mentioned by Paturi (1976). He refers to the impressive adaptation of
the mescal cactus Lophophora williamsii , which converts the dehydration-induced shrinking during
the beginning of the dry season to reduce the shoot length and submerge below the desert floor.
After just one seasonal rainfall the hydrating shoot reemerges by pushing the photosynthesizing
apex out of its soil cover into the light and open air (Figure 19.17). Although smaller-sized volume
oscillations occur also in fruits, leaves, and stems, they are most prominent in the strange, leafless