IHi: AMl-RIC.W }U>'IAXIST 163 



I'.k'ctr. >lri'|»isni i-^ a ccTtain ad jiislnifnl wliicli a plant will 

 make to a niiki electric cnneiil. Thennolropism is the turn- 

 ing of the roots towards warmth. It is very rare and possibly 

 has been confused with other stimuli. Rheotropisiu is the 

 turning against a water current. This illlustrates a confusion 

 of stimuli, for undmihtedly the roots mistake the {)ressure of 

 the flowing water for a hard object in the soil and attenij^t to 

 turn away therefrom. There are others which might be men- 

 tioned, but they are of lesser importance. 



NYCTITROPIC MOVEMEXTS 



These are commonly callxl "sleep movements" in plants 

 and are brought about following variations in the intensity of 

 illuminati(^n, so that parts have a diurnal and nocturnal posi- 

 tion. The sleep of leaves is a phenomenon so conspicous that 

 it is observed as early as the time of Pliny. Since Linnaeus, 

 the scientist of anti(|uity pui)lished his essay. "Somnus Plan- 

 tarum" it has been the subject of many investigations. 



Thq movements which result from circumnutation. are 

 modified and regulated by the alternations of day and night 

 or light and darkness; but they arc to a certain extent inher- 

 ited. Leaves when they go to sleep move either upwards or 

 downwards, or in the case of leaflets of compound leaves, for- 

 wards or backwards, — that is, towards the apex or the base. 

 In almost every instance the plane of the blade is so placed as 

 to stand vertically or nearly so at night. The upper surface 

 of each leaf and more especially of each leaflet, is often 

 brought into close contact with that of the opposite one. This 

 is 'sometimes effected by singularly complicated movements. 

 This fact seems to suggest that the ujjper surface requires 

 more protection than the lower. There are plants in which 

 the terminal leaflets after turning up at night so as to stand 

 vertically often continue to bend over until the upper surface 



