LEAF STRUCTURE 



75 



gases diffuse through small openings similar in size and 

 arrangement to stomatal openings almost as rapidly as they 

 do through single openings the size of the entire leaf. 



Some leaves, notably tree leaves, have all their stomata 

 on the lower surface. Most of our herbaceous plants have 

 more stomata on the lower than the upper surface; a few 

 have them about equally distributed on both sides; and a 

 few, such as water lily, have them only on the upper surface. 



The number may vary rather widely on the same plant, 

 under different conditions, but the variation in number is 

 much greater among species, as indicated by the following 

 table (from original data and from the references at the end 

 of the chapter) giving general average numbers of stomata 

 per square inch of leaf surface for some of our common 

 plants: 



Plant Upper surface Lower surface 



Spanish oak 750,000 



Scarlet oak 650,000 



Red oak 425,000 



White water lily 287,000 



Sunflower 110,000 200,000 



Apple 190,000 



Rose 62,000 



Common pea 63,000 135,000 



Petunia 62,000 87,000 



Tomato 62,000 131,000 



Zinnia 75,000 137,000 



Potato 33,000 104,000 



Corn 45,000 57,000 



Oats 26,000 28,000 



It should be remembered that all molecules tend to dif- 

 fuse from regions of relative abundance of one particular 

 kind of molecule to regions of relative scarcity. This ex- 

 plains the movement of the three kinds of molecules through 

 the stomatal openings. The carbon dioxide is used in the 

 leaf in photosynthesis, the oxygen is given off in the same 

 process (see Chapter 12), and the water is lost from the 

 humid air of the air spaces in the leaf to the atmosphere 



