Respiration and Transpiration of Plants. 143 



lower portion of Fig. 42, which shows a section of barley 

 leaf with the epidermis removed and much magnified. 



172. Breathing Pores Leading into the air chambers 

 are many breathing pores through which the air enters. 



Eight of these are repre- 

 sented in Fig. 42. They 

 are most numerous on the 

 under sides of leaves 

 where evaporation may be 

 least. 



The breathing pores or 

 stomata are very small 

 and numerous, Weiss es- 

 timating, from an average 

 of 40 plants, as many as 

 209,000 in each square 

 'centimeter of surface, an 

 area equal to the square 

 shown in Fig. 43. In 

 the case of a corn leaf 

 21 per cent, of the surface 



Fio. 42 Structure of barley leaf. CAfteriq nppnrn'pfl V>v thp flnor- 

 Sorauer; sp is a breathing pore ; m, chlo- 1S 

 rophyll cells ; i, respiratory chambers, ways to the breathing 



chambers. 



173. Chlorophyll Cells. Surrounding the air chambers 

 in every leaf there are multitudes of tender, thin-walled 

 cells in which are found the green chlorophyll grains, giv- 

 ing color to the leaf, which absorb the sunshine and use it 

 in breaking down the carbon dioxide for the carbon, which 

 is one of the chief constituents of plant tissues, and of the 

 starches, sugars and most other compounds. 



174. Guard Cells. In order that the loss of water may be 

 as little as possible each breathing pore is surrounded by a 

 pair of guard cells, represented in Fig. 42, and on a much 

 larger scale in Fig. 43. These guard cells have for their 

 function the regulation of the amount of evaporation from 



