260 MOVEMENT IN PLANTS. 



Invert one plant, and examine it from time to time. Note that 

 the blades of the leaves rise considerably in six to eight hours, while 

 the petioles also rise but more slowly. Instead of measuring the 

 angles, it will suffice to invert the erect control plant (^4) from time 

 to time and hold it alongside the plant (B) that is being kept in- 

 verted, comparing the positions of the leaves and leaflets in the two 

 plants. 



Next morning, bring B back into the upright position, and note 

 that the leaves resume their normal positions in the course of the 

 day. 



372. Influence of Gravitation and of Darkness. Now 



invert the plant B of the preceding experiment, and keep it inverted 

 for four or five days. The leaves show periodic sleep movements, 

 but the day and night positions are now in the reverse direction 

 with reference to the stem that is, the movements retain their 

 relation to the direction of gravitation. Now turn the plant into 

 the upright position, and note that the leaves either regain their 

 normal position very slowly or not at all geotropic curvature of 

 the pulvini has been fixed by growth. 



373. Autonyctitropic and Geonyctitropic Movements. 



From the preceding experiment, and from the fact that the "sleep" 

 movements of Phaseolus cease when the plant is rotated (with its 

 stem horizontal) on the clinostat, it is clear that we have here not 

 a " nyctitropic " but a geotropic movement the plant shows 

 different geotropic reactions according to whether it is exposed to 

 light or to darkness. Similar changes in the geotropic reaction are 

 seen when rhizomes and roots are exposed to light. In Mimosa 

 and most other plants showing sleep movements, these movements 

 retain their original direction after continual rotation on the 

 clinostat for many days. Hence we can distinguish between 

 autonyctitropic and geonyctitropic ' ' sleep " movements. 



374. Structure of Pulvinus of Phaseolus. In a transverse 

 section from the middle of the petiole of Phaseolus, note the ring of 

 vascular bundles, enclosing the central pith and surrounded by 

 a narrow zone of cortex which is largely collenchj'ma. In trans- 

 verse sections of a pulvinus, note that the bundles are collected 

 towards the centre, while there is a broad zone of thin-walled 

 cortex parenchyma. The movements of the petiole (and of each 

 leaflet) are due to changes in the turgescence of the parenchyma on 

 the upper and lower sides of the pulvinus. An increase in turges- 

 cence on one side, or a decrease on the other side, or both these 

 changes occurring together, will result in elongation of one side and 

 contraction of the other, the vascular tissue bending passively and 

 undergoing no change in length. 



