Ch. IV, 8] RESPIRATION OF PLANTS 165 



cordingly, since photosynthesis is completely dependent 

 on light, there must exist a certain light intensity, at which 

 the photosynthesis and respiration of a leaf exactly balance 

 one another. This must happen twice a day, morning and 

 evening, and perhaps during very dark days. In such case 

 it is probable that the two processes use reciprocally one 

 another's products, and neither of the gases passes into or 

 out of the leaf. It is because photosynthesis stops, while 

 respiration continues, in the dark, that plants are unhealth- 

 ful in sleeping rooms at night, though in truth the effect is 

 quantitatively small, else one could not camp overnight in 

 the woods. 



In respiration the oxygen comes from the air ; but the car- 

 bon of the carbon dioxide, and the hydrogen of the water 

 come from the food, in which they were incorporated, in the 

 original grape sugar, by photosynthesis. Since respiration 

 thus withdraws solid material from the body, it is always 

 accompanied by loss of weight, and, unless compensated by 

 addition of food, it ends in emaciation and starvation. 



Viewing the process now in the large, and centering atten- 

 tion upon the end substances involved, we find that respi- 

 ration may be expressed in an equation as follows, — 



C 6 H 12 6 + 60 2 = 6 C0 2 + 6 H 2 0. 



This equation, which may be termed the respiratory equation, 

 is the exact reciprocal of the photosynthetic equation (of 

 page 23). 



The exchange of gases here described is not, however, the 

 important feature of respiration, but merely an incidental 

 accompaniment thereof. The central fact, and explanation 

 of the great physiological importance of the process is this, — 

 that it releases the energy latent in potential form in the 

 food, — which energy, set free at precisely the points of need, 

 supplies the power which does the work of the plant in its 

 growth and other processes. Respiration is in fact identical 

 in nature with combustion, which, in a steam engine, pro- 



