PROTOPLASM. 15 



veloped by a cell-wall or is naked. If without a wall, it may 

 send out foot-like projections into which there passes a stream 

 of granules, as in the Amoeba (see Fig. i) ; if enclosed, the 

 protoplasmic mass may rotate within the cell wall, or there 

 may be narrow channels in which the currents move between 

 banks of more stationary material. The latter motion is de- 

 scribed as circulation. (Fig. 2.) 



24. Demonstrations. The teacher should, if possible, demonstrate 

 protoplasmic motion to the students with a compound microscope of good 



/-magnification. The Amoeba will serve to illustrate the naked streaming 



) y motion; Paramedum, rotation; the hairs from the stamens of Tradescan- 



^ tia beautifully illustrate circulation. (There is a cultivated species which 



may be kept blooming in greenhouses at all seasons of the year.) Ciliary 



motion may be shown in several of the large Protozoa, or by living cells 



scraped from the oesophagus of the frog. 



25. Dissimilation. Motion and the other responses which 

 protoplasm makes to stimuli necessarily represent chemical or 

 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- 

 olism. 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 particularly, 

 and this union is one of the principal sources of energy which 

 animals show. The process is called oxidation and is essen- 

 tially 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, 



