DIV. ii PHYSIOLOGY 337 



splitting the portion of the pulvinus longitudinally as in Fig. 277, 2, the tendency 

 towards expansion of the parenchyma, especially of its middle layers, is very 

 clearly shown. 



It will now be readily seen that an increase in turgescence on all 

 sides will increase the tension between the vascular bundle and the 

 parenchyma and thus increase the 

 rigidity of the pulvinus. On the 

 other hand, an increase of turges- 

 cence on one side or a diminution 

 on the other side, or the occurrence 

 of both these changes together, will 

 cause a lengthening of the one side 



and a Shortening, of the Other Side FIG. 27". Pulvinus of Phasedus (after SACHS). 



Which naturally Curves the pulvinus. * ***** cut longitudinally from the middle 



m, i r ji -i of the pulvinus ; 2, the same cut up ; 3, 



The Vascular bundle IS passively transverse section, g, Vascular bundle. 



bent, and undergoes no alteration 



in length. The passive movement of the part of the leaf attached to 



the pulvinus is due to the curvature of the pulvinus. 



Autonomic variation movements are probably present in all leaves 

 provided with pulvini, but only attain a striking degree in a few 

 plants. 



Thus the small lateral leaflets of Desmodium gyrans move uniformly or move 

 interruptedly in elongated ellipses. At higher temperatures (30-35 C.) the move- 

 ment is very rapid, the course being completed in half a minute. The movement 

 of the leaflets of Oxalis hedysaroides is still more rapid, the tip moving through 

 O'5-l'o cm. in one or a few seconds. While the autonomic movements of these 

 two plants do not appear to be affected by light, those of Trifolium pratense are 

 completely suppressed in light. In the dark, however, the terminal leaflet 

 exhibits oscillatory movements with an amplitude that may exceed 120 ; these are 

 regularly repeated in periods of two to four hours. 



2. Paratonie Movements (Stimulus Movements) ( 97 ) 



In the induced or paratonic movements an external factor always acts 

 as a stimulus and starts the movement. By means of these movements 

 attached organisms bring their organs into the positions in which 

 their functions can be best carried out. If the organs of a seedling 

 continued to grow on in the directions which have been accidentally 

 brought about on sowing the seed, the root would often grow into the 

 air and the shoot into the soil. 



Light, heat, gravity, and chemical or mechanical influences of the 

 most various kind enable the plant to orientate itself in its environ- 

 ment. The different organs of a plant often show quite different re- 

 actions to the same external stimulus. Thus the stem and root, while 

 both tending to place themselves in the direction of the rays of light, 

 grow towards or away from its source respectively ; the leaves, on the 



z 



