io6 'SCIENCE PRIMERS. [xxvi. 



Grow hyacinths of different colours in a completely 

 dark cellar. They will expand their leaves and 

 flowers. The former will be pale, but the latter will 

 develop their proper colours. 



163. Heliotropism. Place a pot of germinating 

 cress before a window. It will be found after a few 

 days that the stems are turned in the direction of the 

 light. This is due to the fact that light retards growth, 

 and therefore the sides of the stem turned to the light 

 and away from it come to have different lengths, and 

 the stem consequently bends. 



It follows from this that if a pot of germinating 

 cress be shaded equally all round, the plants will grow 

 faster than if not so shaded. 



Grow some germinating cress in a closed box in 

 one side of which a piece of dark blue glass has been 

 introduced ; there will be no curvature in the growth. 

 Repeat the experiment substituting a piece of deep 

 red glass ; curvature will take place as with ordinary 

 daylight. This proves that the heliotropic effect of 

 light is due to the rays belonging to the red end of 

 the spectrum. 



