Pulvini are swollen parts at the base of leaf stalks or petioles which move leaves. Since pulvini can also house mechano- and photoreceptors [12,20], they are autonomus organs, which perform induced movements also when, detached from stem and leaf [21]. Anatomically all pulvini consist of thick-walled waterconducting vascular tissue surrounded by thin-walled motor cells which can undergo visible swelling and shrinking. In
analogy with animal joints, the motor cells on the lower site of the pulvinus are called flexor cells and on the upper site extensor cells. A lifting of the leaf is actuated by an increase in tugor pressure and volume caused by the uptake of K+ ions in the extensor cells. Upon darkness K+ channels in the extensor cells close but open in the flexor cells which loose turgor pressure and shrink, the pulvinus joint looses its rigidity and lets the leaf droop. In Mimosa pulvini like in animal muscles the flexor undergoes a measurable shortening upon stimulation by an action potential [21]. Its response can be triggered by touch, sudden darkness and the arrival of action potentials. Pulvini-based leaf movements occur in many plants of the bean family (Fabaceae), the fastest responses can be found in Mimosa pudica, and Desmodium gyrans. While most plants can move their leaves only up and down, lupines move them in any direction.
Recently Nakano [22] studied experimentally the size distribution of motor cells in the upper and lower parts of main pulvinus of Mimosa pudica before and after it is subjected to mechanical touch on the petiole, Fig. 6. The results of the motor cell size distribution are shown in Fig. 7 where (a) and (b) denote the case before and after the stimulus at the petiole. The darker color in Fig. 7 denoted the larger sized motor cells, thus a comparison between Fig. 7 (a) and (b), demonstrated clearly the shrinkage of the motor cells in the lower part of the main pulvinus and the turgor the upper part, resulting in a rapid bending of the petiole.