56 



FUNDAMENTAL PRINCIPLES 



Tropisms are named with respect to the stimulating agent. The 

 following are generally recognized: 



1. Thigmotropism, or response to contact. 



2. Thermotropism, or response to temperature. 



3. Phototropism, or response to light. 



4. Chemotropism, or response to chemical stimulation. 



5. Rheotropism, or response to mechanical currents. 



6. Electrotropism, or response to currents of electricity. 



7. Geotropism, or response to the force of gravity. 



Animals in nature are subject to all of these forms of stimuli except 

 that of the electric current, which is purely an artificial stimulus. 



If the response is such as to cause the animal either to turn toward the 

 source of stimulation or to approach it, it is termed a positive response. 

 If, however, it is such as to cause the animal to turn away from the source 



Fig. 18. — Sketch showing a long, rectangular glass aquarium partly covered by an 

 opaque hood, and containing three species of organisms responding differently to light. 

 The direction of the light is shown by the arrows and the positions assumed by the animals 

 at + (positive), - (negative), and ± (gradient). 



of stimulation or to avoid it, it is termed a negative response. The mini- 

 mum strength of the stimulus which is necessary to produce an effect is 

 termed the threshold of stimulation. 



Some animals do not seem to respond positively either to the strength 

 of the stimulus immediately above the threshold or to a maximum 

 strength of the stimulus but do seem to be attracted to a position in which 

 they receive the stimulus in a degree intermediate between the maximum 

 and minimum. In this case they are said to exhibit a response to a 

 gradient of the stimulus, corresponding to an intermediate strength 

 expressed in degrees of temperature, intensity of light, or concentration of 

 chemical solution (Fig. 18). 



87. Conductivity. — Though the stimulus may be received at a particu- 

 lar point on the animal, the effect is not limited to that point but is con- 

 ducted more or less throughout its body. This power of living matter 

 to transmit the effect of the stimulus is termed conductivity. Irritability, 

 or reactiveness, which is the power to respond to stimuli, as well as con- 



