58 



BIOLOGY: GENERAL AND MEDICAL 



The roots of plants, whether terrestrial or aerial, con- 

 tain no chlorophyl and, like the fungi, are negatively 

 heliotropic. A majority of the flowers that open in the 

 sunshine are more or less colored. The negatively 

 heliotropic flowers that open only at night are invariably 

 white. 



Heliotropism among animals is evidenced by a general 

 activity during the daytime as compared with quiescence 

 at night. Like the flowers, the animals that enjoy the 

 sunshine are apt to be variously colored; those that live 

 in the dark are apt to be white. 



The varying intensity of the sun's rays provoke interest- 

 ing temporary changes in the skins of many animals 



FIG. 12. Positive heliotropism of Spirographis spallanzani. The source of 

 light being on the left, the polypi all turn in that direction. (Loeb.) 



through their stimulating effects. Thus the skin of the 

 squid contains a great number of small chambers in 

 which an inky fluid is contained. These chambers 

 communicate with one another; and when the animal 

 is upon a dark object the superficial chambers dilate to 

 receive added fluid, making the skin dark; when it is upon 

 a pale surface, they contract, driving the fluid into the 

 deeper chambers and making the skin pale. 



The chameleon is well known because of its ready 

 change of color as it moves from object to object. Here 

 the mechanism is different and the color change depends 

 upon certain migratory pigmented cells of the skin which 

 change their positions with surprising rapidity according 



