19 

 Lecture 6, July 17, 1900. 



Kerner's Light Theory . Iiisiii.. 



Light an aerial factor in the development of leaves. No green 

 leaves found except in air or water. Scales foitnd in soil. Green 

 leaves called eoraraonly foliage. Red leaves also, since green 

 is masked by red. 



A third type is the floral leaf. Stems are also often green 

 and do same work as leaves. Sometimes leaves are absent and 

 stems act as leaves. Most stems of herbacious plants are green. 



Chlorophyll depends upon light. 



Form jQl. igaveg. 



Greater surface in proportion to volume, the greater the 

 possibility of Protosynthesis. A spherical form has smallest 

 surface. All gradations between greatness of bulk and smallness 

 of volume and the opposite. A large thin leaf the best type. 



A stem must have a large volume to its surface, since it must 

 support the leaves. The less mechanical work a leaf has to do, 

 the fewer organs it must develop. 



1. Leaves are plastic. Variations in size and shape under 

 different conditions. Power of adaptation to environment. 



2. Leaf direction. Perpendicularity to rays of greater intensity. 

 Position of leaf on tree and movement of the sun. Also variation 

 of the position of sun during different seasons. Absolute horizon- 

 tally best for leaves at EquatoB — varies as we come further north. 



