BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 111 



but beyond that it can not go. As the air follows the water back 

 into the imbibing walls minute capillary surfaces of great curva- 

 ture are developed, which successfully withstand the pressure of 

 the gas from within, and what is far greater, the stresses from 

 without. This compartment then simply becomes inoperative. 

 The transpiration stream flows around it, as the waters of a 

 river flow around an island in its center. 



When the world's supply of coal and oil is exhausted, man 

 will be reduced to the extremity of dependence upon solar engines, 

 water power, and wood as sources of energy, unless his ingenuity 

 has meantime been equal to the task of liberating the energy of 

 the atom. So far as we can see at present, wood and plant 

 products will then constitute, as in fact they do now, the only 

 means of storing in a readily transportable form the energy re- 

 ceived from the sun. Hence, efficiency in plant production 

 from the standpoint of both food and fuel promises to be a 

 problem of rapidly increasing importance as the years progress — 

 a problem which demands for its complete solution the fullest 

 possible understanding of the physical and chemical processes 

 associated with plant growth. 



