150 . TEXTBOOK OF BOTANY 



growth soon ceases and there is little or no change in temperature in the 

 bottle. But if oxygen is allowed to enter the bottle before the seeds suf- 

 focate, the effects of respiration become evident and growth is renewed. 



All these facts are in some way related to the chemically bound 

 energy of the food in the plant cells, and to its transformation into other 

 kinds of energy through the process of respiration. Yet none of the 

 phenomena mentioned above is respiration; they are merely the recog- 

 nizable external effects of respiration. These statements will become 

 much clearer as we consider the relations of certain facts, some of which 

 we have already met in previous chapters. 



Thus far nothing has been said about the formation of water from 

 food during respiration. Water formed in this way is also an accessory 

 consequence of respiration, but of little importance except to clothes 

 moths and a few other animals which are dependent on water from 

 this source. These easily detectable consequences of respiration are often 

 referred to as external evidences of respiration. 



Sugar the source of chemically bound energy in living organisms. All 

 plants and animals are dependent directly or indirectly upon the syn- 

 thesis of sugar as the sole source of their supply of chemically bound 

 energy. This is a broad generalization, and thus far we have encountered 

 only a few of the facts upon which it is based. But a clear understanding 

 of the energy transformations noted in the last two chapters is a necessary 

 first step toward a full appreciation of this generalization. 



The available bound energy in a green plant is obtained by photo- 

 synthesis. Experiments have shown that sugar and other soluble organic 

 matter may pass from the environment into the cells of a green plant, 

 but the amount of available bound energy obtained in this way by 

 green plants growing in natural conditions is negligible. A possible ex- 

 ception is the absorption of soluble organic substances by algae that grow 

 where these substances are abundant. 



Animals and non-green plants obtain this chemically bound energy 

 when food passes from some external source into the cells of their bodies. 

 No additional chemically bound energy is obtained in any other way 

 by these organisms, except by the few groups of bacteria that oxidize 

 reduced atoms of iron, nitrogen, and sulfur in certain compounds. But 

 even the reduction of nitrogen and sulfur in living organisms is depend- 

 ent upon the potential energy in sugar. Some of the evidence for this 

 last statement was encountered in the chapter on protein synthesis; addi- 

 tional evidence is presented under the discussion of nitrogen fixation. 



