RELATION OF CARBON DIOXIDE TO PLANT GROWTH 195 



various pai'ts of the plant. It is very noticeable in the root of the 

 carrot and the petals of many orange and yellow-colored flowers. 



The leaves of many plants contain other coloring materials, 

 sometimes in such abundance that we do not see the chlorophyll, 

 but this is no evidence that they do not contain chlorophyll. 

 The chlorophyll is present and active, but is concealed by the 

 other colors. 



Relation of the Stomata to Carbon Dioxide. — ^Although we 

 have already referred to the stomata in connection with transpi- 

 ration (page 172), their relationship to the intake of carbon 

 dioxide is of as great or even greater importance. Therefore, 

 their greatest activity in this connection will be in the presence 

 of sunlight, i.e., the time when photosynthesis is most active. 

 The guard cells contain protoplasm and chloroplasts and are 

 therefore capable of photosynthetic action. The formation of 

 sugar in the cell sap of the guard cells causes a rise in their os- 

 motic pressure, which results in the mthdrawal of water from the 

 neighboring cells into the guard cells and increases their tur- 

 gidity. The outer wall of the guard cell appears to be more 

 flexible than the inner wall ; therefore, the increase in turgidity 

 results in the opening of the stomata. There are many other 

 factors that influence the action of the stomata, but in general it 

 may be said that they are open during periods of photosynthetic 

 activity (i.e., during the day) and closed during darkness ajid 

 as a result of wilting of the plant 



Photosynthesis. — This may be considered as the most im- 

 portant process in nature. Briefly stated, it is the process by 

 which the plant manufactures organic food. It is the re- 

 sult of the action of the sunlight on the chlorophyll in the 

 presence of carbonic acid (i.e., a mixture of water and 

 carbon dioxide) at a favorable temperature. Therefore, 



