BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 93 



The growth-rate. We pass now to the consideration of our 

 plant system from the standpoint of the growth-rate. The 

 plant system is remarkable in that the useful work done is ex- 

 pended in increasing the size of the system, at least during the 

 earlier stages of growth. Furthermore, the quantity of cata- 

 lyzer available for transforming the absorbed substances also 

 increases with the size of the system. Therefore, while the 

 efficiency of the system may not change during the growth of the 

 plant, the quantity of useful work which it is capable of doing may 

 theoretically increase at a rate proportional to the size of the 

 system, assuming that an adequate supply of the products used 

 in synthesis, i.e., carbon-dioxide, water, and nutrients from the 

 soil, are present. This hypothesis is equivalent to saying that 

 the rate of change of the weight m of the plant system is pro- 

 portional to the weight itself, or, put into mathematical form : 



dm fa . 



— = a x m (6) 



at 



Let us now see how nearly this assumption may be realized in 

 nature. 



Since it is not possible to determine directly the dry weight 

 m without destroying the plant, we must have recourse to some 

 indirect means of determining the mass of material present at 

 any time. We shall, therefore, make a further assumption that 

 the increase in size of the plant from day to day is accompanied 

 by a corresponding increase in the quantity of water which it 

 transpires, a quantity which as we have seen is readily meas- 

 ured. This latter quantity, however, would obviously depend 

 upon the weather of each day, so that we must either maintain 

 the plant under constant conditions from day to day during the 

 course of the experiment, or we must correct the transpiration 

 in accordance with the intensity of the weather factors each day. 

 The latter procedure is the one which we have actually em- 

 ployed. This correction may be made in a simple manner by 

 dividing the observed transpiration during a day by the observed 

 evaporation from a shallow blackened tank for the corresponding 

 day. This gives us a series of numbers which are proportional 



