SECT, n PHYSIOLOGY 299 



A pulvinus of one of the Leguminosae, such as the Kidney Bean, has the 

 vascular bundle and tlie sclerenchyma, which are peripherally arranged in the leaf- 

 stalk, united to form a central and easily bent strand ; this is surrounded by a 

 thick zone of parenchyma (Fig. 226, 3). If from a pulvinus isolated by two 

 transverse sections the middle sheet of tissue is cut out (Fig. 226, 1) the bulging of 

 the cortical parenchyma both above and below shows the considerable tension. On 

 sjilitting tlie }iortion of the pulvinus longitudinally as in Fig. 226, 2, the tendency 

 towards expansion of tlie parenchyma, 

 especially of its middle layers, is very 

 clearly shown. 



It will now be readily seen that 

 an increase in tnrgescence on all 

 sides will increase the tension he- 

 tween the vascular bundle and the 



narPTirhvina ind thns inorease the ^'°- 22(5. -Pulvinus of P/iaseoZws (after Sachs). 



parencnyma ana tnus inciease me i, Layer cut longitudinally from tiie middle 



rigidity of the pulvinus. On the of the pulvinus ; 2, the same cut up ; 3, 



other hand, an increase of turges- transverse section, g, vascular bundle. 



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 

 which naturally curves the pulvinus. The vascular bundle is j)assively 

 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. 



A remarkable example of this form of movement is furnished by 

 the small lateral leaflets of Besmodium gyrans, and still more strikingly 

 by Oxalis hedysaroides. In a moist, warm atmosphere (22°-25^) these 

 leaflets make circling movements which are of such rapidity that the 

 tips describe a complete circle in 1-3 minutes ; in the case of the 

 leaflets of Oxalis a path of i-l| cm. is traversed in one or a few 

 seconds. Variations in illumination do not disturb these movements. 

 The autonomic variation movements of Trifolmm and of the Wood- 

 Sorrel take place, on the contrary, only in darkness. Thus the 

 terminal leaflets of Trifolium prattnse 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. 

 In a less striking form similar autonomic variation movements are 

 exhibited by all leaves provided with pulvini. 



B. Paratonic Movements (Stimulus Movements) 



The induced or paratonic movements are of much greater interest to 

 the physiologist ; in these an external factor acts as a stimulus and starts 

 the movement. By means of these movements freely motile organ- 

 isms seek out the situations in which they find the optimal conditions 



