26 DISEASE IN PLANTS. 



in the plant. To take a single instance only. If 

 the carbohydrate is rapidly burned off to carbon- 

 dioxide and water, very little is got out of it in 

 the way of work most, if not all, of the energy 

 set free escapes as heat : whereas if the carbo- 

 hydrate is slowly and gradually oxydised, passing 

 through various stages and giving rise to power- 

 fully osmotic bodies in the process, or if it is built 

 up into protoplasm, or into the structure of a 

 cell-wall, relatively enormous quantities of work 

 may be got out of its surface-energy, and heat 

 may be absorbed. Whence it follows that we 

 cannot measure the power for physiological work 

 of a body by merely obtaining its heat of com- 

 bustion, any more than we can infer its significance 

 in metabolism from its chemical properties. 



The general conclusion that the plant stores 

 large quantities of energy may of course be arrived 

 at by simply estimating the enormous quantities 

 of food-material which we obtain annually from 

 agricultural plants. 



Modern physiologists have attempted to proceed 

 further than this, however, in their essays to form 

 an estimate of the relations between the available 

 energy in the solar rays and that used and stored 

 in the plant. 



If we reflect on such phenomena as the cool 

 shade of a tree, and the deep gloom of a forest, 

 and on experiments which show that an ordinary 

 leaf certainly lets very little of the radiant 

 energy of the spectrum pass through it, it 

 becomes evident that many of the rays which 



