66 EXPERIMENTAL PLANT PHYSIOLOGY. 



50. Circumnutation. If the tip of a shoot of some rapidly- 

 growing plant, such as the Pea or Bean, is kept under observa- 

 tion for several minutes, it will be seen that it slowly changes 

 position ; and if the time of observation is extended, it will be 

 found that it inclines successively toward every point in the 

 horizon. In some plants the movement is in the same direc- 

 tion as the hands of a watch, and in others it is in the contrary 

 direction. This nutatory movement of growing tips is quite 

 generally distributed among plants, but it is most marked in 

 twining stems. The causes which produce the movement are 

 chiefly inequalities in growth extension of the sides of the 

 stem, and the reaction to the influence of gravity. 



EXPERIMENT 82. 



CIRCUMNUTATION OF SHOOTS AND TENDRILS. 



Note the positions of a growing tendril of the Gourd, Pea, 

 Bryony, or Wild Balsam Apple at intervals for three hours. 



Plant three or four seedlings of the Scarlet Runner or common 

 Bean at equal distances from one another in a circle around an 

 upright post. Mark the successive positions of the tips of each 

 until it becomes twined around the support. 



51. Hygroscopic Movements. Many plants are provided with 

 cells which take up or lose water in such manner as to give 

 rise to very marked movements in the organs of which they 

 form a part. Such cells are found in the leaf-blades of a large 

 number of Grasses, and other plants which inhabit arid regions. 

 In such plants this is a provision for rolling up the leaves in 

 a form which will prevent undue loss of moisture from the 

 organ. By a similar action many anthers open and allow the 

 escape of the pollen, and fruit-capsules allow seeds to escape. 

 In the latter instance sufficient force is sometimes furnished 

 to throw the seeds to a distance or bury them in the soil. 



