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Yert.ina.litv. 

 If a leaf faces north and south, the south side would receive light 

 all through the da^r. The east and south facing as in the compass 

 plant. This receives light morning and evening hut not at noon. 



A perfect compass plant — Sylvium. This shows an ideal 

 condition for prairie plants. A single leaf receives the most 

 light when it is perpendicular to rays of greatest intensity. 

 Since stems are usually vertical, this is another advantage of 

 leaves over stems with regard to protosynthesis. 



Direction of leaf also plastic. Heliotropism — tendency of a 

 plant to face or turn wway from the sun. Stems turn directly 

 to'^ard the light. Although diffused light seems hest, trees grow 

 toward the intense light. This is not true in sand spit or desert 

 plants where leaves are positively heliotropic. 



Structure of leaf . 



Palisade cells, on upper with most chlorophyll. 



Spongy tissue, on lower with less chlorophyll. 

 In weak light, upper side most effective, in strong light lower. 

 Chlor. bodies change their position with respect to sun. 

 Lab. ex. with prickly lettuce. Cottonwood since leaves move, 

 has same structu.re on both sides of leaf. 



Effect of light on color. 

 Light or dark green probably dependent on physical properties. 



Shade leaves have darker color, but are so thin that the chlor. 

 shows better. The sun leaves have more chlor. but it is masked 



b" ha 



