THE PLANT AND THE ANIMAL 269 



leaves. Let us concentrate our attention on some par- 

 ticular leaf which arrests us by such unexpected move- 

 ment, and we shall soon perceive one of the most striking 

 phenomena in the vegetable world. At first the two 

 leaflets are in a horizontal position. Suddenly one of 

 them changes this position by an abrupt movement, 

 by a bound, so to speak, and it is found at a considerable 

 angle with the horizon. Another and yet another such 

 abrupt movement, and it is raised vertically. Mean- 

 while the opposite leaflet is lowered by a similar series 

 of abrupt movements, by internal shocks. Then the 

 leaflets change their parts. The raised one falls and 

 the lowered one rises. This movement seems to be 

 regular, produced by inner pulsation, provided the plant 

 has sufficient light and heat. As the temperature falls, 

 the intervals between every two movements will become 

 longer, and eventually the movement will be no more 

 jerky, but slow and continual, and only to be noticed 

 when attention is drawn to the relative position of the 

 leaflets. Finally, if the temperature fall, say, to 68° F., 

 the movements will cease altogether, the plant will 

 become chilled. But warmed again it will recommence 

 to wave its small leaflets. 



The study of all these phenomena of motion leads of 

 necessity to the question as to the purpose they serve 

 for the plant. Apparently their significance varies 

 according to the case. The movements of spores and 

 antherozoids, of the stamens of barberry, etc., are 

 obvious : they are useful, if not necessary, for purposes 

 of fertilisation and the reproduction of plants. The 

 sleep movements of flowers, the folding of their outward 

 wrappings during the night, probably saves them from 

 cooling, which is so dangerous to them. The sleep 

 movements of leaves have probably the same eff^ect : by 

 folding or turning their edges upwards in the condition 

 of sleep the leaves present a comparatively small surface 

 from which heat can radiate ; consequently they escape 



