IRRITABILITY. 593 



position in which the leaf receives light of that intensity 

 which is most favourable to the performance of its functions. 

 That this is the case is proved by the changes in position 

 which some motile leaves undergo under variations in the 

 intensity of light. Thus, it has been mentioned (p. 552) 

 that, when exposed to light of moderate intensity, the leaf- 

 lets of Robinia are horizontal, but that when the light is 

 intense they move upwards until they present their margins 

 to the incident light. In light of moderate intensity, then, 

 these leaves are diaheliotropic ; in intense light, they are 

 paraheliotropic. 



We may now conveniently consider the nyctitropic 

 movements of leaves whether growing or motile. These 

 consist in the assumption by the leaf or leaflet, on a dimi- 

 nution in the intensity of light, of a position in which the 

 surfaces are more or less nearly vertical, the apex being 

 directed either upwards or downwards (see pp. 405, 539.) 

 That this movement is not essentially connected with any 

 relation between the function of the leaves and the intensity 

 of the light, is shewn by the fact that a similar movement 

 may be induced by a fall in the temperature of the air 

 (P- 378)- The significance of it probably is, as Darwin 

 suggests, that it prevents the excessive lowering of the tem- 

 perature of the organs by radiation during the night ; clearly 

 the radiation from a leaf with its surfaces vertical must be 

 much less than when its surfaces are horizontal. In the case of 

 floral leaves the movement has, in some instances, the further 

 significance, that by the closure of the petals the essential 

 floral organs are protected not only from cold, but also from 

 becoming wetted by dew or rain. 



With regard to geotropism and hydrotropism, it is so ob- 

 viously to the advantage of the plant that its shoot should 

 be negatively geotropic, and its root positively geotropic, 

 and further, that its root should be positively hydrotropic, 

 that the facts need only to be mentioned to be understood. 



Finally, though it is not possible in most cases to see 

 what advantage accrues to the plant from the spontaneous 

 movements of its organs, for instance, the movements of 

 V. 38 



