260 - Multicellular Plants 



Fig. 14-1. Response of the "sensitive plant," Mimosa pudica: A, before being 

 touched; B, five seconds after being touched (E. H. Runyon). 



minute, the plant slowly regains its original 

 posture. 



The receptors, conductors, and effectors of 

 this response of Mimosa are strictly localized. 

 The excitation (p. 190) originates in the epi- 

 dermal cells of the leaf, which possess numer- 

 ous tactile hairs projecting from the surface; 

 and the excitation spreads, mainly along the 

 sieve tubes. The excitation, as indicated by 

 the action potential, follows the course of the 

 vascular bundles through the petiole to the 

 point where the petiole joins the stem. Here 

 lies the pulvinus, a cushion of cells that 

 normally supports the leaf in a horizontal 

 position. When an excitation reaches the 

 pulvinus, the pulvinus cells become more 

 permeable, and losing turgor they relax their 

 support of the leaf. Thus the pulvinus is the 

 effector of the drooping response, and the 

 sieve tubes and epidermal cells, respectively, 

 are the conductors and receptors. 



Specialized receptor - conductor - effector 



structures in plants are less clearly defined 

 than in animals, and consequently the re- 

 sponses of plants are less rapid and precise. 

 The drooping of the mimosa leaf is very 

 rapid compared to most other plant re- 

 sponses, but even in this case the propagated 

 excitation — as timed by the spread of the ac- 

 tion potential — is transmitted at a rate of onlv 

 about 5 centimeters per second (compared to 

 120 meters per second along the nerves of 

 man). Nevertheless the basic nature of ex- 

 citation and conduction appears to be similar 

 in plants and animals. In both cases, the 

 excitation may be blocked by narcotic agents, 

 and in both plants and animals excitation 

 appears to be accompanied by a bioelectric 

 discharge, increased permeability of the ex- 

 cited cells, and a temporary alteration of the 

 local metabolism. 



Other Turgor Movements. Although the 

 evidence is somewhat scantv, it seems prob- 

 able that other rapid and reversible responses 



