90 BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 



not at present explainable on a physico-chemical basis would be 

 termed by the vitalistic school as " vital;" by the physico- 

 chemical school, "unknown." 



The efficiency of plant systems. I shall ask you in what fol- 

 lows to consider the plant from the point of view of a physical 

 system, a system which is absorbing energy and performing 

 useful work. Radiant energy is absorbed by this system from the 

 sun and sky, and under some conditions heat is absorbed from 

 the surrounding air as well. This energy is used to do work upon 

 water and nutrients taken in through the roots and upon car- 

 bon dioxide absorbed through the leaves. A part of the energy, 

 through the agency of remarkable catalyzing substances — the 

 chlorophyll bodies and the protoplasm — is employed in the 

 transformation of the materials absorbed into plant tissue. The 

 latter represents the useful work performed by the system, 

 which may be quantitatively determined by measuring the heat 

 of combustion of the total plant substance formed. The re- 

 mainder of the absorbed energy is spent in lifting and vaporizing 

 water, except in those cases where the temperature of the plant 

 rises above that of the surrounding air. In such instances heat 

 energy is also transferred to the air. 



Let us for the present assume that the temperature of the 

 plant does not exceed that of the air, an assumption which leaf 

 temperature measurements have shown to be amply justified 

 in the case of actively growing plants in a dry atmosphere. The 

 total energy used by the system in a given time will then be 

 represented by the total heat equivalent of the water evaporated, 

 plus the total heat of combustion of the plant substance devel- 

 oped during this period. If we represent the former by Q e and 

 the latter Q c , both expressed in gram-calories, the efficiency (E) 

 of the system may be represented by the relation 



• E = —Q°— (1) 



Qe+Qc 



Both quantities are capable of direct measurement. The 

 water evaporated can be determined by means of suitably con- 

 trolled field experiments designed to prevent all loss of water 



