The Light Relation 



427 



of light and darkness upon the form and growth of plants 

 has been much investigated. In most cases no adequate 



FIG. 119. Sun print showing difference in opacity (thickness and chlo- 

 rophyll content) of celery leaves grown in half-shade (left) and in sun- 

 light (right). 



consideration has been given to other factors than light, 

 but in general the response to light is so much more marked 



