106 



THE BIOLOGY OF A PLANT. 



INCOME OF PTERIS. 



the chlorophyll-bodies; for plants which, like fungi, etc., are 

 devoid of chlorophyll are unable thus to acquire energy. En- 

 tering the chlorophyll-bodies, the kinetic energy of sunlight is 

 applied to the decomposition of carbon dioxide (CO 2 ) and water 

 (H a O). After passing through manifold but imperfectly known 

 processes, the elements of these substances finally reappear as 

 starch (C B H 10 O 5 ) in the form of granules imbedded in the chloro- 

 phyll-bodies, and free oxygen (O), most of which is returned to 

 the atmosphere. Thus the leaf of Pteris in the light is contin- 

 ually absorbing carbon dioxide and giving forth free oxygen. 



Carbon dioxide and water contain no potential energy, since 

 the affinities of their constituent elements are completely satis- 

 fied (cf. p. 33, Chap. III.). Starch (C 6 H 10 O 6 ), however, contains 

 potential energy, since the molecule is relatively unstable, i.e., 

 capable of decomposition into simpler, stabler molecules in which 

 stronger affinities are satisfied. And this is due to the fact that 

 in the manufacture of starch in the chlorophyll-bodies the ki- 

 netic energy of sunlight was expended in lifting the atoms into 



* It is generally stated that plants are unable to make use of free atmos- 

 pheric nitrogen, but recent investigations have thrown doubts upon this view. 

 (Cf. Atwater, American Chemical Journal, Vol. VI. [1885] p. 365.) 



