374 



PHYSIOLOGY 



The best estimates as to the amount of photosynthesis carried on by 

 thin-leaved plants are given in the following table : 



Carbohydrate made in i hr. by i sq.m. or leaf surface 



Nos. 1-9, after Brown and Escombe, in part recalculated; nos. 10-14, after 

 Blackman and Matthaei, especially intended to show the effects of temperature 

 on photosynthesis. An effect of excessive temperature is to be seen also in no. 4. 



Using such results as the basis of calculation, it would be easy to show 

 how enormous a weight of food is made in a growing season by the foli- 

 age of meadows and forests. But unknown allowances must be made for 

 leaves unfavorably situated or lacking in vigor, and such estimates are 

 of little value except for their impressiveness. The value and volume 

 of the annual crops of cultivated plants is even more impressive; and 

 to this must be added in imagination the unknown but huge volume of 

 wild vegetation, all dependent upon photosynthesis for at least 85 per cent 

 of its dry substance. 



The following are the approximate values of some of the more important crops 

 of 1909 in the United States: corn, $i,720,ocx>,ooo; wheat, oats, rye, and barley, 

 $1,280,000,000; cotton, $850,000,000; hay, $665,000,000; potatoes, $212,000,000. 

 Together the weight of these marketable products is something like 175,000,000 

 metric tons; and of course this is but a small fraction of the vegetation that pro- 



