PLANT NUTRITION 243 



among which may he mentioned atmospheric hnmidity, air tempera- 

 ture, air movements, light intensity, and soil conditions. Such morpho- 

 logical factors as distribution and placement of stomata, thickness and 

 character of cuticle, and amount of leaf surface phiy important parts 

 in the rate of transpiration. 



Carbon Dioxide 



The role of carbon dioxide in photosynthesis has been discussed in 

 Chapter 9. It has been established that the growth of a plant is 

 related to the amount of carbon dioxide available to it for photo- 

 synthetic purposes. Experiments have shown that most plants manu- 

 facture more carbohydrates and grow faster if exposed to higher- 

 than-normal amounts of carbon dioxide. However, over an extended 

 period of time an elevated percentage of carbon dioxide in the 

 atmosphere will be deleterious to the plant. Carbon dioxide is given 

 off by plant roots and is known to play an important part in plant- 

 soil relationships. 



Oxygen 



The requirement for oxygen during biological oxidation and the 

 production of oxygen in the photosynthetic process have been dis- 

 cussed in Chapters 7 and 9, respectively. Thus it can be seen that, 

 while the plant releases oxygen as a by-product of the photosynthetic 

 reaction, the same plant has oxygen requirements for the biological 

 oxidations taking place within it. This need for oxygen is greatest 

 at night when the rate of photosynthesis is nil, and the majority of 

 reactions taking place are enzyme-catalyzed energy-releasing reactions. 

 Such reactions require molecular oxygen for the final transfer of 

 electrons and the formation of water. 



MINERAL ELEMENTS 



Although a large number of elements have been shown to exist in 

 plant tissues, only fifteen of these have been proved to be essential 

 for higher plants. The roles of carbon, hydrogen, oxygen, and nitrogen 

 in the synthesis of carbohydrates, lipides, proteins, and related com- 

 jiounds have been discussed in preceding chapters. The essential 

 elements that remain to be disctissed are usually classified as the 

 mineral elements and include phosphorus, sidfur, calcium, magnesium, 

 potassium, iron, boron, manganese, copper, zinc, and molybdenum. 



The precise functions of many of the mineral elements in plant 



