PULVINI 569 



Perception of the mechanical (seismic), gravitational, photic or other 



effective stimulus, may take place within the motor-tissues itself, 

 as in the case of nyctitropic movements. The perceptive faculty 

 may, on the other hand, be located in special sense-organs ; this 

 more advanced condition is exemplified by geotropically sensitive 

 organs in general, and also by Mimosa pudica and Biophytum sensi- 

 tivum, though in these two plants the motor-tissues are also capable of 

 direct stimulation. Mimosa and Biophytum, besides exemplifying the 

 co-existence of direct and indirect stimulation of motor-tissues, also 

 illustrate the fact that the same motor-tissue may respond to several 

 different stimuli. In the n case of heliotropically sensitive pulvini, the 

 stimulus is often perceived by the leaf- blade, and transmitted to the 

 motor-organ through a more or less extensive intervening region. 



Curvature of pulvini may take place in several different ways. 

 Heliotropic (phototropic) and geotropic curvatures are effected by means 

 of the relative acceleration of growth on one side of the organ, whereas 

 nyctotropic movements depend upon alterations of turgor ; in the 

 latter case the precise nature of the mechanism is not yet understood. 

 Pfeffer believes that the two antagonistic halves of the pulvinus 

 react in the same sense, but with unequal velocities or intensities. 

 Schwendener, on the other hand, maintains that the two halves respond 

 in opposite senses, a rise of turgor in the adaxial half being accompanied 

 by a fall of turgor in the abaxial half, and vice versa. According to 

 this latter view, when a leaf-blade which is horizontally extended in 

 the diurnal position, sinks downwards in order to take up the nocturnal 

 or " sleep " position (as happens, for instance, in the case of Phascolus, 

 Amicia, Oxalis, etc.), turgor increases in the adaxial half, and diminishes 

 in the abaxial half of the pulvinus ; the converse changes take place 

 when the leaf returns to the diurnal position. 



Changes of turgor in the motor-tissue must also be held responsible 

 for the seismic and traumatic responses of the pulvini of Mimosa pudica 

 and Biophytum sensitivum. The paratonic movements of the leaf of 

 Mimosa pudica constitute one of the most interesting problems in the 

 whole field of plant physiology, and have been studied by a whole 

 series of investigators. The main external features of these movements 

 are well known : the pinnules fold together in pairs (in the upward 

 direction), the sub-petioles approach one another laterally, and the main 

 petiole sinks suddenly downwards. All these separate movements are 

 due to appropriate curvatures performed by pulvini. The pulvinus at 

 the base of the main petiole is larger and more accessible than the 

 others, and is therefore best adapted for experimental investigation ; 

 as a matter of fact the most important experiments have been per- 

 formed upon these main pulvini. Briicke and Pfeffer agree that the 



