Reactions to the Substratum 61 



realized that the same intensity of attachment for these fouhng or- 

 ganisms extended throughout great areas of Biscayne Bay, the magni- 

 tude of the reaction is appreciated. 



Reactions to the Substratum 



Other animals and plants, rather than relying on chance to reach a 

 suitable substratum, actively seek it. Climbing plants often exhibit 

 a tendency for the growing parts to keep in contact with solid surfaces. 

 This differential growth in response to contact with a surface is termed 

 stereotropism. Thus, the tendrils or stems of climbing vines twine 

 around or adhere to objects with which they come into contact. In 

 other species, such as euonjTnus, the vine simply presses against the 

 solid surface without any special structure for attachment. Root tips, 

 on the other hand, turn away from stones and other solid objects 

 which they may encounter as they grow through the soil. This 

 reaction might be thought of as a negative stereotropism. 



Certain animals exhibit a locomotory orientation to surfaces, a 

 response known as tliigmotaxis, by means of which they keep in con- 

 tact with, or avoid, solid objects. This reaction can be observed in 

 many insects, and among higher animals it is demonstrated by rats 

 and house mice when they tend to keep in contact with a wall 

 (Fraenkel and Gunn, 1940). 



Many worms and insects are stimulated to continue moving about 

 until their bodies are in contact with surfaces of the environment as 

 they would be when in burrows or under stones. In the laboratory 

 insects will come to rest between the surfaces of glass slides, and 

 this fact shows that the reaction is a positive response to touch 

 rather than an avoidance of light. In nature, since there are no 

 transparent, solid objects, the organism would also be hidden from 

 view. If a caterpillar is placed on its back, it will immediately go 

 through righting reactions until its feet are again in contact with the 

 substratum. However, while the caterpillar is still on its back, if a 

 leaf is placed in contact with its feet, the animal makes no further 

 attempt to right itself. It will be perfectly content to remain upside 

 down as long as its feet are firmly attached to a solid surface. 



A great many animals obviously exhibit the reverse reaction, 

 avoiding solid objects, and ordinarily do so by the use of sight or 

 touch. A most unusual method of avoiding collision in the dark is 

 now known to be employed by bats (Galambos and Griffin, 1942). 

 At night and in dark caves bats can fly rapidly about without running 

 into each other or into jutting rocks or other objects. The success 



