150 



THE BIOLOGY OF A PLANT. 



facture of starch, or to the bulk of the water of evaporation, 

 which passes straight through the plant without undergoing any 

 chemical change. Energy likewise leaves the plant continuously 

 both as heat and in the doing of mechanical worh^ both of which 

 are involved in every vital act. 



Respiration. It has been remarked that ui the light (i.e., 

 when manufacturing starch) Pteris takes in carbon dioxide and 

 gives off free oxygen. But if the plant be dejDrived of light, as 

 at night, the reverse is true, and the plant takes in a small 

 amount of oxygen and gives ofl" a corresponding amount of car- 

 bon dioxide. This latter process is the true hreathing or resj>i- 



ration of the plant, and it must not be confounded with that 

 taking in of carbon dioxide and gi\dng off of oxygen which is an 

 incident in the manufacture of starch. Resj^iration goes on in 

 the light also, probably with greater energy than in darkness, 

 but it is then largely obscured by the other and more conspicu- 

 ous process. We have seen that energy is set free in living mat- 

 ter by a decomposition of its own substance, which is really a 

 process of oxidation or combustion, where free oxygen plays 

 an important part (p. 32, Chap. III.) ; hence the absorption of 

 free oxygen in respiration. Among the products of the combus- 

 tion, water and carbon dioxide are the most im^^ortant ; and this 



