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.’ 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, 7. 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 alge, 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 
1 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 heliotropie 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 larve 
of flies as early as 1887, in Wiirzburg, and often enough since. These facts seem to 
have escaped several of my critics. [1903] 
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