584 LIFE MOVEMENTS IN PLANTS 



The question may now be asked : Why should the 

 pulsations occur preferably in the morning ? In connec- 

 tion with this I shall refer to the suppression of the 

 pulsatory activity of Desmodium gyrans when the leaflet 

 was pulled up by the action of light (cf. Fig. 188). 

 The leaf of Mhnosa executes a very rapid movement of 

 erection at night, and the natural pulsations are thereby 

 rendered very inconspicuous. These pulsations may, how- 

 ever, be found in the night record of young leaves. 

 The general occurrence of pulsations in the forenoon is 

 probably due to the fact that the resultant force which 

 causes the down-movement is at the time relatively feeble — 

 the operative factors being: (1) the action of the rising 

 temperature which induces down-movement, and (2) the 

 action of light which in the forenoon opposes this move- 

 ment. It will thus be seen that the forces in operation 

 in the forenoon are more or less in a state of balance 

 hence conditions for exhibition of natural pulsations are 

 more favourable in the morning than in other parts of 

 the day. 



Experiment 222. — I next tried to discover conditions 

 under which the plant would exhibit its normal rhythmic 

 activity during the whole course of 24 hours. The ex- 

 ternal stimuli which may interfere with the exhibition 

 of its automatic pulsations are those due to gravity 

 and light. They act most effectively on the pulvinus, 

 when that organ is more or less horizontal and therefore 

 at right angles to the direction of the incident stimulus ; 

 they act least effectively on the pulvinus when the organ 

 is parallel to the direction of the external force. This 

 latter condition may be secured by holding the plant upside 

 down, when the pulvinus bends up and the leaf becomes 

 erect and almost parallel to the vertical lines of gravity and 

 to vertical light from above. The leaf, now relatively free 

 from the effects of external stimulus, was found to exhibit 



