372 ELEMENTS OF BIOLOGY 



mediate response by animals permits their division into several 

 groups: Light, Gravity, Electricity, Contact, Temperature, and 

 Chemicals. Frequently more than one of these act together and the 

 response on the part of the animal is the sum of their influences. 

 Responses to such stimuli that are reflex and in a manner forced are 

 spoken of as tropisms. The term is also used to refer to the orienta- 

 tions that occur when an animal is attempting a trial-and-error 

 solution. If a planarian is exposed to a bright light it moves away 

 from the lighted area; such an act is described as a negative 

 PHOTomopisM. The roots of a plant respond to gravity by growing 

 downward, a response known as positive graviotropism. Thus a 

 response is given in terms of the nature of the stimulus, gravity in 

 the latter case, and the eflfect, for example, positive. 



There is no uniformity among animals in response to these ex- 

 ternal factors; some are positive and some negative to light; most 

 of the lower forms when placed in an electric field go toward 

 the anode, but some behave in the opposite fashion. It is not profit- 

 able here to attempt to review the nature of the tropisms in the 

 various animal groups; a basic principle that determines whether 

 or not an animal within a given group is positive or negative to 

 these stimuli, has not yet appeared. The major facts concerning the 

 nature of the receptor organs have already been given (p. 223). 



Certain characteristics of the tropisms reveal their basis in the 

 energy-transforming processes of metabolism. For example, if the 

 siphon of a long-neck clam is exposed to a beam of light, it is 

 withdrawn with considerable promptness. It has been found that 

 the response is elicited either by long exposure to light of low 

 intensity or by brief exposure to a bright light. Furthermore, if the 

 intensity in candle-power is multiplied by the time, the product is 

 always a constant figure, which means that there is an accurate and 

 quantitative relation between the time and intensity required to 

 produce the response. This in turn means that the amount of energy 

 required is always the same. The same law has been found to apply 



