[Chap. XIV THE RATE OF PHOTOSYNTHESIS 121 



If the stomates are closed, the entrance of CO- is diminished. But 

 stomates that appear closed to the human eye may sometimes be open 

 wide enough to admit the passage of molecules of CO2, which are much 

 too small to be seen through the best microscopes. The length of the 

 shortest diameter of a fully opened stomate of average size is several 

 million times the diameter of a molecule of CO2. 



When the stomates are open or partly open, the relative rates at which 

 COl' passes through them are proportional not to the respective areas of 

 the stomates, but to their respective boundaries (the distance around 

 each pore). Because of this fact and certain related ones, the amount of 

 COl' that passes through the stomates is about 50 times greater than 

 would be expected on the basis of their total area. When the stomates 

 are closely spaced, less CO- passes through each one than when they are 

 farther apart. From such facts as these it is obvious that the stomatal 

 mechanism of leaves may indirectly affect the rate of photos)aithesis. 



Stomatal behavior. The opening and closing of stomates are largely 

 dependent upon alterations in the intensity of light, moisture, and tem- 

 perature. As we shall see in more detail in Chapter XXIII, these external 

 factors initiate a sequence of physical and chemical processes in the 

 guard cells. These processes ultimatelv result in changes in the volume 

 and shape of the guard cells, and consequentb' in the size of the stomate 

 between them. 



Under excellent growing conditions the stomates of man\' plants are 

 closed at night and open during the greater part of the dav. But during 

 hot dry weather the stomates of many of these plants mav be open mainlv 

 at night and closed during the day. In other plants they may be closed 

 except for a few hours in the early morning. In certain cacti and other 

 succulents they may remain open twenty-four hours each day. 



The entrance of CO- into the intercellular spaces of the chlorenchyma 

 of young stems, green fruits, flowers, and seeds may also occur through 

 stomates in the epidermis of these organs. Chlorenchvma in the stems 

 and exposed roots of woody plants, however, soon becomes covered by 

 a la)'er of cork, the presence of which is a partial obstruction to both light 

 and the entrance of COj. The chlorenchyma in the roots of orchids and 

 also in the leaves of certain plants is separated from the epidermis by 

 one to several layers of non-green cells. Stomates are absent in algae and 

 the so-called green leaves of mosses. The submerged leaves of aquatic 

 plants mav have rudimentarv stomates or none. Stomates may be present 



