CHAPTER XXXVIII 

 WASTES AND BY-PRODUCTS OF ORGANISMS 



232. The origin of wastes. Every chemical process results 

 in the formation of substances that did not exist before. In 

 the chemical processes that take place in the cells of a liv- 

 ing organism, substances are produced that are directly related 

 to the protoplasm's being ''alive." Other substances, which 

 are of no direct use to the living body or to the living process, 

 are produced incidentally. The latter are called wastes, and may 

 be compared to the sawdust of a mill, or to the smoke that goes 

 up the chimney, or to the ashes that drop through the grate. 



233. Removal of wastes from cells. In our study of pho- 

 tosynthesis (p. 54) we found that one of the wastes or 

 by-products is oxygen, which diffuses out of the chlorophyl- 

 containing cells through the cell walls. In our study of ener- 

 gesis (p. 143) we found that carbon dioxid, water, urea, and 

 other substances may be produced. These also diffuse out 

 of the cell. 



In plants water and carbon dioxid are usually eliminated in 

 the form of vapor. The carbon dioxid given off by the cells 

 of the roots usually remains in solution, forming so-called 

 carbonic acid (see Fig. 42, p. 144). 



Plants often dispose of their waste substances in a way that 

 seems to be beneficial to them ; and the same is true of animals 

 (see p. 203). 



234. Accumulation of wastes in plants. The waste sub- 

 stances produced by plants (outside of water and carbon dioxid) 

 are generally not eliminated from the body. They are usually 

 combined into insoluble or non-diffusible compounds, and in 

 this condition they are accumulated in dead cells in parts 



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