DIURNAL MOVEMENTS 429 



to the more excitable side of the organ causes predominant 

 contraction of that half, which cannot be neutralised by the 

 transverse conduction of excitation to the feebly excitable 

 other half of the organ. These facts explain the effect of 

 strong light from above causing downward folding of the 

 leaflets of the Averrhoa and upward folding of the leaflets 

 of Mimosa. 



Diurnal Movements of Plants 



The diurnal movements are the outcome of the co- 

 operation of numerous factors, the most important of which 

 are : (i) thermonastic movement caused by differential 

 growth of two opposite sides of an anisotropic organ under 

 the hourly variation of temperature ; (2) the movement 

 due to change in the transpiration-current ; (3) the thermo- 

 geotropic response, under variation of temperature, of 

 organs rendered anisotropic by gravitational stimulation ; 

 (4) the response of organs sensitive to light under recurrent 

 alternation of light and darkness ; and (5) the movements 

 of plants like Mimosa which are sensitive to variation of 

 both light and temperature, and which are also affected by 

 both the direct and the after-effects of light. 



Thermonasty. — Thermonastic movements are of two 

 types. The positive is a movement of opening during rise 

 of temperature, the inner side of the organ growing the 

 faster on account of its being the more sensitive. Examples 

 of this are found in Crocus, Zephyranthes, and in European 

 and a few Indian Nymphaeas. In the negative type, rise 

 of temperature induces a movement of closure by the 

 acceleration of the growth of the more sensitive outer side 

 of the organ. The flower of a blue Nymphaea growing 

 in India belongs to this type. 



Thermo-geotropism. — This phenomenon was discovered 

 in the investigation of the remarkable periodic up and 

 down movement exhibited by the ' Praying ' Palm of 

 Faridpore (p. 224). It was afterwards found to be of wide 

 occurrence, being exhibited by rigid trees as well as by young 



