14 ZOOLOGY. 



division. At any rate division furnishes a way out of the 

 dilemma and allows a renewal of growth of the daughter units. 

 23. Contractility. A body of living protoplasm seems 

 always to possess the ability to change its form in greater or 

 less degree. This results in motion of parts or of the whole, 

 and is called contractility. Movement or contractility is closely 

 related to irritability, and results from the action of stimuli, 



FIG. 2. 



s 



FIG. 2. The circulation of protoplasm (p) in a cell of a stamen-hair of Tradescantia. 

 In the channels the granules move back and forth to the various parts of the cell. 

 The remainder of the cell is filled with cell-sap (j) which in these cells is colored. 



Questions on the figure. In what respects are the activities of the 

 protoplasm necessarily limited in this cell as compared with the condition 

 in Amoeba? Why is circulation an appropriate term? 



external and internal, upon the complex protoplasm. It is 

 made possible by the assimilation of food substances. These, 

 in being broken down, furnish the energy shown in motion. 

 The nature of .the motion resulting from contraction differs 

 somewhat, depending upon whether the protoplasm is en- 



