50 i SUMMARY AND CHAP. XII 



Heliotropism prevails widely throughout the vege- 

 table kingdom, but whenever, from the changed habits 

 of life of any plant, such movements become injurious 

 or useless, the tendency is easily eliminated, as we see 

 with climbing and insectivorous plants. 



Apheliotropic movements are comparatively rare in 

 a well-marked degree, excepting with sub-aerial roots. 

 In the two cases investigated by us, the movement 

 certainly consisted of modified circumnutation. 



The position which leaves and cotyledons occupy 

 during the day, namely, more or less transversely to 

 the direction of the light, is due, according to Frank, 

 to what we call diaheliotropism. As all leaves and 

 cotyledons are continually circuinnutating, there can 

 hardly be a doubt, that diaheliotropism results from 

 modified circumnutation. From the fact of leaves and 

 cotyledons frequently rising a little in the evening, it 

 appears as if diaheliotropism hud to conquer during 

 the middle of the day a widely prevalent tendency to 

 apogeotropism. 



Lastly, the leaflets and cotyledons of some plants 

 are known to be injured by too much light ; and when 

 the sun shines brightly on them, they move upwards 

 or downwards, or twist laterally, so that they direct 

 their edges towards the light, and thus they escape 

 being injured. These paraheliotropic movements cer- 

 tainly consisted in one case of modified circumnuta- 

 tion ; and so it probably is in all cases, for the leaves 

 of all the species described circumnutate in a con- 

 spicuous manner. This movement has hitherto been 

 observed only with leaflets provided \vith pulviui, in 

 which the increased turgescence on opposite sides i.s 

 not followed by growth ; and we can understand why 

 this should be so, as the movement is required only 

 for a temporary purpose. It would manifestly be dis- 



