THE ORGANISM AS A MECHANISM 71 



converted into heat. We do not know just what are 

 the steps in the transformation, though it is clear that 

 solar radiation is absorbed and that the chlorophyll of 

 the leaf is instrumental in converting this energy of 

 radiation into chemical potential energy. But the im- 

 portant thing to notice is, that we have here a process 

 closely analogous to that of a reversed Carnot engine. 

 Energy (that of the carbon dioxide and water) passes 

 from a state of low potential to a state of high potential 

 (that of the energy of starch) , and work is done on the 

 plant in producing this transformation. 



Work is not done hy the green plant. This state- 

 ment is not, of course, quite rigidly true, for a certain 

 amount of mechanical work is done by the plant. 

 Flowers open and close ; tendrils may move and clasp 

 other objects ; there is a circulation of protoplasm in 

 the plant cells, and a circulation of sap in the vessels 

 of stems, etc. Also work is done against gravity in 

 raising the tissues of the plant above the soil, while 

 work is also done by the roots in penetrating the soil. 

 But when compared with the work done by radiation 

 in producing the chemical transformations referred 

 to above, these other expenditures of energy must be 

 insignificant. Speaking generally, then, we may describe 

 the green plant as a system in which available energy 

 is accumulated in the form of chemical compounds of 

 high potential. It is, further, a system in which energy 

 becomes transformed without doing mechanical work, 

 except to a trifling extent, and in which there is no 

 formation of heat, or at least in which the quantity of 

 heat dissipated is only perceptible during very re- 

 stricted phases, is relatively small during the other 

 phases, and tends to vanish. 



Let us now combine the processes of plant and 

 animal ; we start with the latter. In it we have a 



