THE MANIFESTATIONS OF LIFE 59 



to the intensity of the light reflected upon the animal 

 by surrounding objects. The same change of color ia 

 seen among many other reptiles and some batrachia. 



The attractive influence of a lamp upon nocturnal 

 insects is a striking example of positive heliotropism, 

 many of the insects actually flying into the light to meet 

 destruction. 



But in the animal world the most striking example of 

 the irritability of the cells toward light is shown in that 

 particular and adapted form known as the sense of 

 vision, where the light rays are caught and intensified so 

 as to act upon a special organ, the retina. Animals 

 living in perpetual darkness are either devoid of visual 

 organs or possess their rudiments only. 



The cells of the higher plants contain peculiar granules 

 known as chloroplasts whose office is the production of 

 the chlorophyl and other colored substances peculiar to 

 the leaves and flowers. When these are present in the 

 deeper cells of the plants, into which light cannot pene- 

 trate, or when the plants are kept in darkness, they 

 develop into leucoplasts, but if at any time light reaches 

 them, a change is effected through its stimulation, and 

 they become changed into the chromoplasts which give 

 the fruits and flowers their varied colors. 



The effect of light in the transformation of these chro- 

 moplasts can be studied by placing a photographic film 

 upon the surface of a growing fruit a large apple an- 

 swers the purpose well exposed to the sunlight. The 

 admission or exclusion of the sun's rays, by the denser 

 or lighter portions of the negative, results in a photo- 

 graphic picture upon the fruit caused by the formation 

 of perfect chromoplasts where the light penetrated and 

 imperfect formation where it was withheld. Interesting 

 and beautiful pictures may thus be produced. 



The effects of the sun's rays upon the human skin are 

 well known, though it is difficult to differentiate between 

 those attributable to heat and those due to the light 

 alone. Exposure to the direct rays of intense sunlight 



