44 TEXTBOOK OF BOTANY 



67. Living Matter. — Proteins, complex as they are, are not 

 themselves living matter, but living matter is built up in turn 

 out of proteins and some other food substances that the cell 

 has either taken in or made for itself. The living matter of 

 the Spirogyra cell is found in the slimy cytoplasm, in the 

 chloroplasts, and in the nucleus. It is a very unstable sub- 

 stance ; and just as it is being continually built up, so it is 

 being constantly torn down. This continual change that 

 living matter is undergoing is one of the most striking things 

 about it. We must think of a living cell as something like a 

 chemical laboratory in which there are going on at the same 

 time one long series of changes that lead from the simple 

 substances that come in from the outside, through the car- 

 bohydrates and proteins, to living matter itself ; and another 

 equally long series of changes that lead from living matter 

 downward again to very simple substances (including carbon 

 dioxid and water), which in turn may be given off by the cell 

 as waste materials, or may be used once more in rebuilding 

 living matter. Even in the living parts of the cell there is a 

 good deal of water ; the cell wall is thoroughly water-soaked ; 

 and since the cell sap is mostly water, it follows that water 

 makes up a very large proportion of the whole brdk of the 

 Spirogyra cell. 



68. Respiration. — Like the yeasts and other plants that 

 have been studied, Spirogyra must respire — that is, it must 

 take in oxygen, which like carbon dioxid is present in solu- 

 tion in the water in which the plant lives. Oxygen is not a 

 food as carbon dioxid is, because it is not used in building 

 up new substances. Its use is rather to assist in the tearing 

 down which, as we have seen, goes on side by side with the 

 building-up process. In this tearing down, ^energy is lib- 

 erated which the cell uses in various ways — for instance, in 

 growth, as well as in other activities that are continually 

 going on in the plant. Thus res])iration is a source of 

 energy to the cell. One substance that is formed in large 



