88 FOOD ELABORATION AND RESPIRATION 



soluble carbohydrates and products of photosynthesis. In 

 the building-up or reconstructive and other processes it is 

 therefore available. There is a series of changes, gradually 

 increasing in complexity. There are formed substances 

 containing nitrogen, in addition to carbon, hydrogen and 

 oxygen. Others will contain also sulfur and phosphorus, and 

 the process may be thought of as culminating in protoplasm. 

 Protoplasm is the living matter in plants. It is in the cells, 

 and is usually semifluid. Starch is not living matter. The 

 process of building up the protoplasm is called assimilation. 



187. Respiration. In the maintenance and growth of 

 the plant, energy is required. This energy is derived from 

 the food that the plant has manufactured; and its ultimate 

 source is the sunlight. For the release of this energy, chemi- 

 cal changes are involved which require oxygen; as by- 

 products, carbon dioxid gas is given off and water is formed 

 in the cells; this whole process is respiration. This process 

 of respiration is similar in animals. All animals require oxygen 

 and give off carbon dioxid. Likewise, all living parts of 

 the plant must have a constant supply of oxygen. 



188. In green plants, at night, carbon dioxid is given off 

 into the air and oxygen is taken into the cells. In the day- 

 time, respiration goes on, but the required oxygen is derived 

 from the supply released in photosynthesis; and the carbon 

 dioxid released in respiration supplies a part of the carbon 

 dioxid used in photosynthesis. In the daytime, the plants 

 tend to purify the air because they use carbon dioxid and 

 give off oxygen. At night, like animals, they tend to make 

 the air foul because they use oxygen and give off carbon 

 dioxid. The carbon dioxid given off by a few plants at 

 night, however, is so slight that it need not disturb one at all. 



189. The oxygen that the plants need may come into 

 the plant through the stomata, through pores in the stems or 

 trunks of trees, or it may diffuse through the cell walls. 

 All rapidly growing plants respire very freely. Germinating 



