84 



Introduction to Botany. 



leaves are borne in whorls at the nodes, the illumination of 

 the lower leaves is brought about by an elongation of the 

 internodes (see Fig. 33). In short, after whatever plan the 



plant may be built, the exposure 

 of the leaves to the light is always 

 provided for. 



63. Sun the Source of Energy. 

 ^The sun is, of course, the one 

 important source of light which 

 reaches the earth from the out- 

 side universe. The heat and light 

 of the sun are due to the intense 

 energy of motion of its mole- 

 cules ; this is communicated to 

 the ether, which fills interplane- 

 tary space, and in which all ob- 

 jects are embedded. (We do not 

 know what the ether is, but there 

 are good reasons for assuming 

 its presence.) The ether trans- 

 mits the motion communicated 

 to it by the sun with such swift- 

 ness that in 8 minutes the dis- 

 turbance is being felt on the 

 earth, having traversed a distance of more than 93 milHon 

 miles. 



The vibrations from the sun are manifest to us as light 

 and heat. Those vibrations which give us the sensation of 

 light succeed each other with inconceivable quickness, 

 ranging from 399 million miUion to 750 million million per 

 second; those of less frequency than 399 million million, 

 while not producing light, can be detected by their heating 

 effect. In this way, vast amounts of energy are brought 



Fig. 33. 



Veronica Virginica, showing 

 leaves in whorls separated by 

 long internodes. The leaves in 

 each whorl stand opposite the 

 interspaces between the leaves of 

 the next higher or lower whorl. 



