Leaves in Relation to Light 



45 



to the woods, the leaves of plants growing in the shade are 

 usually darker and more bluish-green than the leaves of 

 plants growing in full sunlight. 

 This difference in color is ac- 

 counted for in part by the 

 amount of chlorophyll near the 

 surface and in part by a slight 

 difference in the color of the 

 chlorophyll itself (page 63). 

 In a few shade plants the depth 

 of the green color is increased 

 by the presence of chloroplasts 

 in the epidermal cells. Shade 

 plants are not subjected to 

 drying, as are plants growing 

 in exposed situations, and gen- ^^ ^ ^ , . 1 .. r i 



^ ^ ' o riG. 32. Pulvinus and section of pul- 



erally speaking their leaves are vinus from leaf of sensitive plant, both 



broad and thin. The leaves ^""^^'scd. When the leaf is touched, the 



r 1 • ^'^ter in the cells on the side A passes 

 of these plants differ further outward into the intercellular spaces, 



from the ordinary leaf in that causing the cells partially to collapse. 



, . , . . , , The pressure of the cells on the side B 



the cuticle is less developed, then forces the leaf downward. 



the raesophyll is composed al- 

 most entirely of spongy tissue, and usually stomata are 

 present on both surfaces of the leaf. 



Submerged leaves. Every one who has gone fishing or 

 rowing knows that a great deal of sunlight is reflected from the 

 surface of water. This means that the amount of light that 

 penetrates the surface is reduced by just the amount that is 

 reflected. The penetration of the water by the sun's rays is 

 further interfered with by the fine sediment that clouds our 

 ponds and lakes. Every one who has dived and opened his 



