2 STUDIES IN GENERAL PHYSIOLOGY 



second, those belonging to the structure of the organism. 



So far as the light is concerned, the circumstance which 

 controls the orientation of the animal and the direction of its 

 movements is the direction of the rays falling upon the 

 animal. 1 The condition which is of importance on the part 

 of the animal is the symmetrical shape of the body. 



Sachs discovered that all plant organs which have a 

 radial structure are orthotropic (this means that they bend, 

 when light strikes them on one side, until their longitudinal 

 axes lie in the direction of the rays of light), but that all 

 dorsiventral structures are plagiotropic, i. e., they place their 

 surfaces perpendicular to the rays of light. Symmetrically 

 situated points at the surface possess a quantitatively and 

 qualitatively equal irritability. In this way the organ of a 

 plant is mechanically forced to orient itself in such a way 

 that the rays of light strike symmetrical points at equal 

 angles to the surface. If the plant, as for example the 

 swarm spore of algae, is capable of a progressive motion, it 

 must of course, in order to maintain this position, move in 

 the direction of the rays of light. This is, indeed, found to 

 be the case. 



I shall now show that quite generally in animals the 

 direction of the rays of light controls also the direction of 

 those movements which are caused by light; that, in addi- 

 tion., quite generally in animals their orientation depends 



i In these experiments it is presumed that the animals move under the influence 

 of only one source of light. It is explicitly stated in this and the following papers 

 that if there are several sources of light of unequal intensity, the light with the 

 strongest intensity determines the orientation and direction of motion of the animal. 

 Other possible complications are covered by the unequivocal statement, made and 

 emphasized in this and the following papers on the same subject, that the main 

 feature in all phenomena of heliotropism is the fact that symmetrical points of the 

 photosensitive surface of the animal must be struck by the rays of light at the same 

 angle. It is in full harmony with this fact that if two sources of light of equal 

 intensity and distance act simultaneously upon a heliotropic animal, the animal 

 puts its median plane at right angles to the line connecting the two sources of light. 

 This fact was not only known to me, but had been demonstrated by me on the larvae 

 of flies as early as 1887, in Wiirzburg, and often enough since. These facts seem to 

 have escaped several of my critics. [1903] 



