216 Plant Physiology 



food-making in weaker light. In this connection, how- 

 ever, it is possible that an important factor limiting high 

 production in intense light is the increased evaporation 

 then resulting, which would tend to dry out the plant 

 and induce a closure of the stomata, as well as otherwise 

 affect photosynthesis. The control of light intensity is 

 important in crop work, as more particularly discussed 

 under shading. 



n6b. Temperature.- -Temperature is just as impor- 

 tant in food-making as in any other physiological process. 

 According to Blackman the best temp'erature for sustained 

 photosynthesis is generally about 25 to 30 C., and this 

 in spite of the fact that at a temperature of about 10 the 

 cell-sap may absorb and hold practically twice as much 

 CO 2 as at the former temperature. The effect of higher 

 temperatures upon respiration complicates the heat 

 relation. It may be expected that plants adapted to 

 diverse environmental conditions will not respond alike 

 to heat, especially under field conditions. High tempera- 

 ture is an important factor in the early maturity of wheat. 

 The grain then contains relatively little starch, and the 

 yield of straw is lessened. On the other hand, with ade- 

 quate soil moisture, corn requires a distinctly higher tem- 

 perature for abundant starch formation and maximum 

 yield. 



117. Organic matter, rate of production. - - A vigorous 

 vine of the Concord or Niagara grape may expose to the 

 light about 10 square meters or more of surface. Careful 

 experiments with other plants indicate that the produc- 

 tion (taking no account of respiration) per square meter 

 of surface may be about 1 gram of organic matter per hour, 



