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TEXTBOOK OF PLANT PHYSIOLOGY 



with a maximum absorption in the green rays, will be found at the 

 greatest depth; the brown algae are more common at a higher 

 level, and near the surface live the green algae, which have no 

 additional pigments in their chloroplasts and, therefore, do not 

 differ in their selective absorption from the land plants. 



11. Leaf Structure and Assimilation. Absorption of Carbon 

 Dioxide by the Leaf. — The process of photosynthesis takes place 

 in the chloroplasts. Hence, in order that the carbon dioxide may 

 serve as material for the synthesis of carbohydrates, it must be 

 absorbed by the chlorophyll-bearing cells. These cells, forming 



Upper Epidermis 



Palisade Cells 



Cuticle 



Sponffy 

 Tissue 



Lower E\ 



Fig. 11. — Cross-section of a sunflower leaf {after Smith, et al.). 



the mesophyll or chlorenchyma of the leaf, are not in direct contact 

 with the outer atmosphere, as either side of the leaf is covered by 

 an epidermis containing no chloroplasts and, along its outer 

 wall, with a cuticle which is little permeable to gases (Fig. 11). 

 The only way by which the carbon dioxide is able to penetrate into 

 the leaf and to reach the absorbing cells is through the stomata. 

 These are small but numerous openings in the epidermis, whose 

 structure is known from anatomical studies of plants. That the 

 carbon dioxide enters the leaf only through the stomata may be 

 shown by a simple experiment : If on a certain portion of the leaf 

 the stomata are coated with vaseline, and the leaf is then exposed 

 to light and afterwards treated with iodine, according to the 



