RESPONSE TO CERTAIN STIMULI 39 



plants are to grow symmetrically. This is due to 

 certain movements of the stem and the leaf-stalks, and 

 these movements are called heliotropic. 



Certain plants, such as the Mimosa, close the leaves at 

 sunset and open them again only at sunrise. Such also 

 is the case with a large number of flowers which close 

 their petals at night, and open them again only when 

 the sunlight strikes them in the morning. These helio- 

 tropic movements are especially important for the higher 

 plants, as they bring the essential parts of the plant 

 into the proper position with regard to the light, and 

 thus make the normal healthy life of the plant possible. 



In animals evidence of heliotropism is seen in the 

 greater general activity during the day. Among the 

 lower forms of animals heliotropic movements almost 

 identical with those exhibited by plants are shown in a 

 number of cases such as the worm Spirographis spallan- 

 zani. The most striking example of light-irritability 

 among animals is seen in the specialised organs of 

 vision, where cells are formed for collecting and in- 

 tensifying the light. 



It must be remembered that there are numerous 

 examples among living organisms of avoidance of light ; 

 these are cases of what is called negative heliotropism, 

 and one of the best examples is the root of any of the 

 higher plants. 



For sufficient illustration of the effect of sunlight on 

 the human skin, we need only allude to sunburn and to 

 freckles, which are simply small collections of pigment 

 due to the action of the sunlight on the skin. 



That chemotropism plays an important role in the 

 life of plants and of animals goes without saying, and 



