GROWTH REGULATION 277 



such items as species competition, animal grazing, soil bacteria, fungi, 

 and insects. A number of the environmental factors have been consid- 

 ered in detail in other chapters or are obvious in their effect on plant 

 growth. The following discussion is limited therefore to a few of 

 the other environmental factors relating to plant growth. 



Temperature 



All chemical, physical, and physiological processes in a plant are 

 directly aifected by temperature. Such processes, in turn, govern both 

 character and rate of plant growth. Since each process can be consid- 

 ered critical and limiting, modification of any one of them may affect 

 growth of a particular part of the plant or the overall growth of the 

 whole plant. 



Most of the reactions taking place in a plant are enzyme-catalyzed. 

 A rise in temperature increases the velocity of such reactions, but 

 only within definite limits. Since an enzyme is heat sensitive because 

 of its protein nature, two processes are affected by a rise in tempera- 

 ture: (1) an increase in the velocity of the catalyzed reaction due to 

 greater molecular activity, and (2) an increase in the rate of destruc- 

 tion of the enzyme by heat denaturization. Therefore, the tempera- 

 ture at which maximum velocity takes place is not necessarily the 

 optimum temperature for enzyme action, since this rapid velocity 

 can be maintained for only a short time. The optimum range of 

 temperatures for many plant enzymes lies between 70 and 80°F. 



Temperature is an important factor in initiating flowering in plants. 

 In the case of beets, radishes, and carrots, of bulbs such as tulips and 

 hyacinths, of trees such as apple and peach, and of cereal crops such 

 as winter rye and winter wheat, low temperatures are particularly 

 important for flowering. The induction of flowering as a result of 

 low temperatures is known as vernalization. 



Light 



When referring to light as an environmental factor influencing 

 plant growth, one includes all radiant energy that reaches the plant 

 from the sun or from artificial sources. Light affects plant growth in 

 many ways, including among others the processes associated with 

 photosynthesis, the temperature of that portion of the plant above 

 ground, the direction of growth of certain plant organs, and the form 

 and anatomy of the leaves produced. Thus it is evident that light 

 intensity, quality, and duration play important parts in influencing 

 plant growth. 



Some plants flourish in dense shade (light of low intensity), whereas 



