456 TROPISMS—THE SLAVES OF THE LAMP 



ill 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 ra>^s 

 of light. The most efficient rays for heliotropic reactions are 

 {a) the blue between 460 and 490^ju, and (/>) the yellow-green 

 betwTcn 520 and 580/x/;t. 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 orientated 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 introduced 

 into the sunny end of the tube. They promptly and invariably 

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

 towards the source of linht. 



(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 

 photochemical reaction is equal in both eyes, this effect on the 

 symmetrical 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 

 through the source of light, provided only one source of light be 

 present. If, however, the light falls sidew^ays on the animal the 

 rate of photochemical reaction will be unequal in both eyes and 

 the rate at which the symmetrical muscles on both sides of the 

 body work will no longer be equal ; as a consequence, the direction 

 in which the animal moves will change. This change will take 

 place in one of two ways, according as the animal is either positively 

 or negatively heliotropic." 



In manmials, at least, the rods of the retina (g.v.) appear to be 



