270 BOTANY 



ments. Anatomically considered, the pulvinus consists, for the most part, of 

 strongly turgescent parenchyma with very elastic cell walls. The vascular bundles 

 and mechanical elements, which, in other portions of the leaf -stalk, have an 

 approximately circular arrangement, unite in the pulvinus in the form of a single 

 flexible strand, and so offer little opposition to the movement of the leaf resulting 

 from the curvature of the motile region (cf. Fig. 165, i). The parenchyma of the 

 pulvinus forms a thick enveloping layer about this axial strand, by means of which, 

 through the pressure arising from a difference in the turgescence of its opposite 

 sides, a movement of the whole leaf-blade is brought about, similar to that of the 

 outspread hand by the motion of the wrist. 



These variation movements are either autonomic, when the 

 variations of turgor are due to no recognisable external influence, or 

 paratonic, when the turgor is regulated in a definite way by the action 

 of external stimuli. 



Autonomic Variation Movements. — A remarkable example of this 

 form of movement is furnished by the small lateral leaflets of 

 Desmodium (Hedysarum) gyrans, a papilionaceous plant growing in the 

 damp Ganges plains. In a moist, warm atmosphere (22°-25°) these 

 leaflets make circling movements which are in no way disturbed by 

 variations in the intensity of the light, and which are of such rapidity 

 that their tips describe a complete circle in 1 - 3 minutes. The auto- 

 nomic variation movements of Trifolium and Oxalis take place, on the 

 contrary, only in darkness. Thus the terminal leaflets of Trifoliwn 

 pratense exhibit oscillatory movements in the dark with an amplitude 

 which may exceed 120°, and are regularly repeated in periods of 2-4 

 hours ; but on exposure to light the leaflets cease their oscillations and 

 assume a fixed light position. 



Paratonie Variation Movements are chiefly induced by variation 

 in the intensity of the light, by the stimulus of gravitation, and by 

 mechanical irritation (shock, friction), and also, but more rarely, by 

 variations of temperature. The pulvini of leaves may be affected by 

 several different stimuli ; the leaves of MiniAsa pudica, for example, 

 are set in motion by the action of light, and also by the stimulus of a 

 shock, and in addition, exhibit autonomic movements. 



A change from light to darkness, as from day to night, occasions 

 nyctitropic movements, or the so-called SLEEP MOVEMENTS. In the 

 day or light position, which is the same as that of diaheliotropic foliage- 

 leaves, the leaf-blades are perpendicular to the incident rays of light. 

 With the commencement of darkness the leaves or the single leaflets 

 fold either upwards with their upper surfaces inward, or downwards with 

 their lower surfaces together, and so remain until the diurnal position 

 is again assumed on recurring illumination. The turgor of the 

 whole motile organ of the Bean, for instance, increases with darkness, 

 but in the upper half more (4-5 atmospheres) than in the lower ; 

 while the turgor of the motile organ is decreased by illumination, 

 the upper half in particular loses the rigidity acquired by the tissue- 



