BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 91 



except that taking place through the leaves, while the heat of 

 combustion may be found by finally burning in a bomb calorim- 

 eter a representative sample of the plant substance produced. 



Numerous investigations have been made of the water re- 

 quirement of plants, i.e., the ratio of the weight of water absorbed 

 by the plant during its growth period to the weight of dry matter 

 produced. It will accordingly be convenient to transform our 

 efficiency equation to include this term. If a mass M w of water 

 is transpired in the production of a mass M p of plant tissue, and 

 if hv and h c represent the heat of vaporization of water and the 

 heat of combustion per gram of plant substance respectively, 

 then 



Q e = MJu (2) 



Qc = M P hc (3) 



Substituting these quantities in equation (1) and remember- 

 ing that Mw/M v is by definition the water requirement R w , we 

 have 



E = -J— (4) 



he 



Determinations of the heat of combustion of the dry matter of 

 various plants do not appear to have been made. In the absence 

 of more specific data, we may assume the heat of combustion 

 to be represented approximately by that of cellulose, namely, 

 4200 gram-calories per gram. By substituting this value to- 

 gether with that of the heat of vaporization of water (536 gram- 

 calories per gram) in equation (4) we have as a first approxima- 

 tion to the efficiency of the plant system 



E = (5) 



0.13 # w + l 



The water requirement of field crops in the Great Plains as 

 measured by Dr. H. L. Shantz and the writer ranges from 200 

 to 1000, depending upon the species and the evaporation-rate. 2 



2 Briggs, L. J., and Shantz, H. L. Relative water requirement of plants. 

 Journ. Agr. Research, 3: 1-63. 1914. 



