IRRITABILITY. 43 1 



until they point to the south-east ; they remain in this posi- 

 tion until the evening, and then curve so that their superior 

 surfaces are directed upwards. The explanation of this is 

 simple. The rays of the rising sun cause a positively helio- 

 tropic curvature of the peduncle, so that the flower points 

 towards the east, and for a time the peduncle continues to 

 curve so that the flower follows the sun. When the light 

 becomes intense, the growth, and consequently the curvature, 

 of the peduncle is arrested, and the flower is fixed in the 

 position which it then occupies, a position usually such that 

 it points to the south east. In the evening the peduncle 

 straightens itself under the influence of gravity, so that the 

 flower is directed vertically upwards. A more striking case 

 is afforded by Tragopogon orientate. In this plant the growth 

 of the peduncle is not arrested by the intense light, so that 

 the positively heliotropic curvature continues all day, and the 

 flower follows the sun, though the movement is less active in 

 the afternoon than in the morning. In the evening the 

 peduncle begins to straighten itself under the influence of 

 gravity. Wiesner mentions as other instances of more or less 

 well-marked movement, Leontodon hastilis, Papaver Rhoeas, 

 Ranunculus arvensis. 



In many cases, finally, the ultimate position which the 

 organs assume appears to be independent of the influence of 

 light. Wiesner mentions as examples the inflorescences of 

 Verbascum, of Dipsacus, Gentiana, and others. Even when 

 light falls laterally upon them they continue to grow erect. 

 This can only be accounted for by regarding the organs as 

 being almost destitute of heliotropic irritability. 



These are the general facts which have been ascertained 

 as to the heliotropism of radial organs. In entering upon a 

 closer study of them, we will, in the first place, direct our 

 attention more especially to the influence of light in pro- 

 ducing them. There are three principal points for us to con- 

 sider : first, the relation of the direction of the incident rays 

 to the curvature ; secondly, the relation of the intensity of 

 the light to the curvature ; thirdly, the relative activity of 

 rays of different wave-length in producing the curvature. 



