246 CHAP. XXII. DIURNAL MOVEMENTS OF PLANTS 



thermonastic movement in one direction, while fall of 

 temperature causes movement in the opposite direction. In 

 many ways the movement of the Palm appeared to be ther- 

 monastic. Thermonasty has, however, been defined as the 

 characteristic of differentially growing organs the anisotropy 

 of which is natural and ingrained. But since I have shown 

 (i) that there is a continuity of reaction between growing 

 and non-growing organs, and (2) that anisotropy may be 

 induced in an organ by the prolonged action of geotropic 

 stimulus, it follows that the phenomena of the diurnal move- 

 ments of trees and other organs may well be included under 

 the wider generalisation of Thermonasty. 



It will be convenient to use the short term thermo- 

 geotropism as a descriptive phrase for the diurnal move- 

 ments under variation of temperature of plants rendered 

 anisotropic by geotropic stimulation. 



Effect of Transpiration on Periodic Movement 



A tree laid horizontally would exhibit a passive down- 

 ward droop due to loss of turgor, resulting from excessive 

 transpiration, this loss being very great at midday ; at 

 night there might be an opposite movement on account 

 of diminished transpiration. But such movements, more 

 or less passive, would not account for the active force which 

 enabled the Faridpore Palm to lift a man off the ground 

 (p. 229). An additional factor, not entirely dependent on 

 transpiration, thus appears to co-operate in the diurnal 

 movement. Whether this is so or not is demonstrated by 

 the following experiments. 



Experiment 133. Periodic movement after abolition of 

 transpiration. — A Kentia Palm was placed horizontally in 

 my greenhouse under a canvas tent in semi-darkness ; the 

 tent also protected the plant from mechanical disturbance 

 caused by gusts of air. After taking these precautions it 

 was easy to obtain very accurate records of the diurnal 

 movement of the Palm by means of the Automatic Recorder. 



