430 LECTURE XVII. 



the peduncle is negatively heliotropic, and. Wiesner, that the 

 peduncle of Helianthemum vulgare is negatively heliotropic 

 after fertilisation has taken place. 



It will be readily understood that, in the course of its 

 growth, a heliotropic organ may take up a definite position 

 with reference to the direction of the incident rays of light, 

 a position which we will term the fixed light-position. The 

 most striking examples of this are afforded by inflorescences, 

 particularly those which are capitulate or umbellate. In most 

 cases when these inflorescences stand in the open, so that 

 they are fully exposed to light, they stand erect, and when 

 they are shaded on one side, as when they grow in a hedge, 

 they curve so that the upper surface of the inflorescence is 

 exposed to the brightest incident light. Wiesner mentions, 

 as examples of this, Chrysanthemum Leucanthemum, Achillea 

 millefolia, Anthriscus vulgaris, Aegopodium Podagraria, etc. 

 In other cases the fixed light-position is a different one. The 

 flowers, or more correctly the inflorescences, of the Sunflower 

 (Helianthus annuus), even when the plant is growing quite in 

 the open, direct their superior surfaces, not upwards, but to 

 some quarter of the compass, usually to the south-east. This 

 peculiarity cannot as yet be fully accounted for, though it 

 doubtless depends upon some special form of heliotropic 

 irritability. A clue to the explanation of it is afforded by 

 investigations which have been made upon the assumption of 

 their various fixed light-positions by dorsiventral organs, a 

 subject which we shall fully discuss hereafter (p. 447). 



Some radial organs do not, however, assume a fixed light 

 position, but follow the daily course of the sun to a greater 

 or less extent. It is usually accepted as a fact that this is 

 the case in the Sunflower, but Wiesner has found that it 

 is not so. Under normal conditions the inflorescences of the 

 Sunflower assume a fixed light- position as described above; 

 it is only when the peduncles are partially etiolated that any 

 daily movement can be detected. Such a movement does, 

 however, occur in various degrees in different plants. Thus, 

 in Sonchus arvensis, according to Wiesner, the flowers are di- 

 rected early in the morning towards the east, and they travel 



