THE ORGANISM AND ITS ENVIRONMENT 373 



to the response of animals in general and in the hght receptor or- 

 gans is undoubtedly to be referred to the presence of some chemical 

 that is sensitive to light and which requires a certain amount of 

 energy in order to undergo its reaction. Just how the chemical 

 transformation of a light-sensitive substance in the receptor organ 

 actuates the nerve is not known. 



Phototropism. An example of phototropic response was given 

 above to show it is dependent on light-sensitive chemicals in the 

 light receptors. Phototropism, or response to light, includes helic- 

 TROPisM, or response to sunlight. Heliotropism is most strikingly 

 shown in the growth of plants; all are familiar with the fact that 

 plants in a partially shaded area bend toward the sunlight. So far 

 as known, there is no special receptor organ in the plant; this 

 behavior must be attributed to a general property of plant proto- 

 plasm, an influence of light on plant growth. Among all phyla of 

 the animal kingdom are representatives that respond to light by 

 some type of immediate contractile eflFort, not by growth. 



Graviotropism. All animal forms in a way respond to gravity, 

 for all exhibit righting reactions and either move or stand in more 

 or less definite positions or balance with respect to gravity. The 

 human balancing mechanism has already been described (p. 232). 

 Another type of response to gravity is shown by some animals. For 

 example, paramcecium always tends to move upward, responding 

 negatively to gravity. If certain fresh water planaria are placed in 

 water that has an abundant supply of oxygen they are indififerent 

 to gravity; but when the oxygen supply decreases they become nega- 

 tive to gravity and persist in creeping upward. This is of course an 

 adaptation, associated with the fact that oxygen dissolves in the 

 water at the zone of contact between the water and the air, and 

 tends to bring the animals into a region of oxygen supply. 



Galvanotropism. It is usually outside the ordinary experience 

 of animals to enter into electrical fields that are of appreciable 

 strength. Biologists have made much of the responses of animals to 



