PROTOPLASM 17 



physical changes, or both, in the protoplasm. It is well known 

 that complex chemical substances, such as are found in proto- 

 plasm, can be made to yield energy when they are torn down 

 into simpler ones by some element which has an affinity for 

 some of the elements constituting the substance. The result 

 of this action is, always, simpler and more stable compounds 

 than the original, and therefore of less use in the further 

 freeing of energy. This tearing-down process is the opposite 

 of assimilation and is sometimes called dissimilation or katab- 

 olisni. Oxygen is one of the most important agents in nature 

 for the freeing of energy by breaking down the complex chemical 

 substances. It unites with the carbon and hydrogen particu- 

 larly, and these unions are ' among the principal sources of 

 energy which animals show. The process is called oxidation 

 and is essentially the same thing that occurs when wood or coal 

 is burned. The energy belonging to the wood by virtue of its 

 chemical constitution is partly freed by the action of the oxygen 

 in uniting with the carbon and hydrogen, reducing the wood to 

 ashes, water, and carbon dioxid. In the stove the principal 

 form of energy secured is heat; but in appropriate engines, 

 locomotion and other forms of mechanical work, or light, or 

 electrical energy may be secured by the oxidation. So in 

 protoplasm, various types of energy may result from the tear- 

 ing down of the complex substances. Among these are animal 

 heat, motion, nervous energy and electrical energy. The living 

 body with its protoplasm is really a mechanism, — an engine. 



27. Secretion and Excretion. — As a result of the constructive and destructive 

 work already mentioned as characteristic of protoplasm certain substances, not 

 themselves protoplasm, may be produced. If these products are of further use in 

 the animal economy, they are usually described as secretions; if they represent the 

 final reduction in the process of tearing down, they are called excretions. Such 

 materials may be deposited either within the protoplasm or at its surface. In the 

 latter case it may be deposited in a uniform sheet and produce a protective mem- 

 brane (cell wall). The presence of such a covering to the protoplasm very mate- 

 rially modifies all the elementary activities which have been described. 



28. Demonstrations. — It is easy to make microscopic demonstration of secre- 

 tions and excretions: — as starch grains formed in the leaves of plants; fat in adi- 

 pose tissue; cell- walls in plants; crystals in plant cells (see Botanies); intercellular 

 substance in cartilage or bone. 



29. Supplementary Topics for Library Work. — Find and examine some of the 

 classic definitions of life. Examine more completely the theories of protoplasmic 



