Biology of Higher Plants — Anatomy and Physiology 239 



energy; (5) through assimilation and growth they may build new tis- 

 sues; (6) they may be stored in roots, seeds, or stems for future use or 

 for the use of animals. 



It is believed that coal, peat, petroleum, natural gas, and similar fuels 

 are the result of decomposition of living organisms of many years ago. 

 These remains originally were made by the plant through the process of 

 photosynthesis and the radiant energy stored in these fuels in the form 

 of potential energy. When these fuels are used today, this energy is 

 released. The amount of photosynthesis which has taken place in all 

 the green plants of the past is beyond our imagination. The amount of 

 material produced each year by present-day plants through the process 

 of photosynthesis is beyond computation. 



IX. RESPIRATION BY PLANTS 



During respiration there is an absorption of oxygen and a liberation 

 of carbon dioxide. Respiration occurs continuously in the living proto- 

 plasm of all animal and plant cells. In this respect it differs from 

 photosynthesis which occurs only in chlorophyll-bearing cells of green 

 plants in a proper source of light energy. These two phenomena have 

 been contrasted in a previous chapter. During respiration the molecules 

 of the plant materials are broken down into simpler forms, and the 

 stored chemical energy is liberated in such a form as to be utilized 

 by the plant. During photosynthesis, light energy is absorbed and used 

 by the plant. Animals confiscate energy when plants are consumed. 

 All the energy used by plants and animals in their activities is derived, 

 directly or indirectly, from the sunlight. For instance, when such a 

 sugar as glucose is oxidized by respiration, the equation is as follows: 



Glucose (sugar) Oxygen _. Carbon dioxide Water Energy 



(CeHiiOe) (6O2) (6CO2) (6H2O) released 



In many respects this equation is the reverse of the equation of photo- 

 synthesis. 



X. CORRELATION AND PLANT HORMONES 



Correlation in plants by means of chemical hormones recently has 

 been realized as being of utmost importance. The presence of specific 

 chemical substances in plants (of certain species, at least) is known to 

 play an important role in plant metabolisms and in the correlation of 

 the plant as a whole. Plant hormones might be defined as chemical 



