HELIOTROPISM 355 



(1) Certain animals and free-swimming plant-organisms move 

 towards or away from the source of light. These are said to be 

 positively or negatively heliotropic respectively. Caterpillars of 

 Porthesia chrysorrhoea are of the former class. They move towards 

 the light and may starve, with abundance of food just behind 

 them. 



(2) If positively and negatively heliotropic animals are placed 

 in a trough covered half with red and half with blue glass, those 

 that are positively heliotropic collect at the blue end and the 

 others at the red end of the trough. Red glass is practically 

 opaque, as every photographer knows, to the photo-chemical rays 

 of light. The most efficient rays for heliotropic reactions are 

 (a) the blue between 460 and 490/x/z and (b) the yellow-green 

 between 520 and 530^- Now, most blue glass permits not only 

 the passage of the blue rays but of the yellow-green rays also 

 (cf. Fig. 1). 



(3) That the heliotropic animal is oriented in relation to the 

 source of light is shown by a simple experiment due to Loeb. 

 Direct sunlight is allowed to fall from the upper half of a window 

 on to a table and diffused daylight from the lower half on to the 

 same table on which is placed a test-tube in such a way that it 

 lies at right angles to the window and is illuminated over one-half 

 of its length (room half) by direct sunlight and over the remainder 

 by diffused daylight. Positively heliotropic animals are intro- 

 duced into the sunny end of the tube. They promptly and invari- 

 ably move towards the window, i.e. out of the sunlight into the 

 shade towards the source of light. 



(4) To explain these facts (and others) Loeb has put forward 

 an interesting theory. " Animals possess photosensitive elements 

 on the surface of their bodies, in the eyes or occasionally also in 

 the epithelial cells of their skin. These photosensitive elements 

 are arranged symmetrically in the body and through nerves are 

 connected with symmetrical groups of muscles. The light causes 

 photochemical changes in the eyes (or photosensitive elements of 

 the skin). The mass of photochemical reaction products ' so 

 formed " influences the central nervous system and through this the 

 tension or energy production of the muscles. If the rate of photo- 

 chemical reaction is equal in both eyes, this effect on the sym- 

 metrical muscles is equal and the muscles on both sides of the 

 body work with equal energy ; as a consequence the animal will 

 not be deviated from the direction in which it is moving. This 

 happens when the axis or plane of symmetry of the animal goes 



