EXTERNAL INFLUENCES AS CAUSES OF VARIATION 247 



Influence of light upon the direction of locomotion or of growth ; 

 heliotropism. Irritability to light is one of the properties of 

 protoplasm. This reaction is generally in the form of contrac- 

 tion, as with muscle fiber 1 resulting in a shortening of the side 

 next the source of light. With free-moving plants and animals 

 this gives direction to locomotion, and they gradually swing 

 about until both sides are equally lighted, when, if motion 

 continues, the creatures will of necessity progress toward the 

 light. This is positive heliotropism. Quick-moving forms are 

 often carried into the source of light by their very impetus, 

 before the repellent effect of great heat has time to act. In 

 this way moths fly into the candle and are killed, while slower- 

 moving forms are checked by the heat in time to save them- 

 selves. In this latter case the future movements will be a 

 resultant of the positive heliotropism and negative thermotrop- 

 ism, by which the insects are held at a certain radius circling 

 about the source of both light and heat, as if not able to leave 

 it, as indeed they are not. 



Negatively heliotropic forms, like earthworms, are those whose 

 protoplasm does not contract in the presence of light, but, on the 

 contrary, expands. These are carried away from the light, and 

 if, in their wanderings, a lighted area is approached, they are 

 unable to enter it. 



When the organism is not free to move, as in the case of 

 stems of higher plants, the effect will be manifested in the direc- 

 tion of growth, which is all the response to heliotropism pos- 

 sible under the circumstances. In cases of this kind the stem 

 will bend toward, or away from, the source of light, according 

 as the plant is negatively or positively heliotropic, until all 

 sides are equally lighted, in which position it will remain during 

 growth, as do other forms during locomotion. 



This placing of the body (or stem) with reference to the 

 various " tropisms " is technically known as "orientation," and 



1 As most of the examples to follow are confined to the lower animals, and to 

 plants in which protoplasm is comparatively exposed, it is well to remind the 

 student that the higher animals are not destitute of the same properties, and that 

 they have one exposed region peculiarly sensitive to light, namely the iris of the 

 eye, whose muscles contract promptly under its influence. 



