PROTOPLASM 15 



the food must be taken, and the volume, which represents the 

 mass to be fed. The surface increases as the square of the 

 diameter, whereas the volume increases as the cube of the 

 diameter. It is apparent that the nourishing surface does 

 not increase as rapidly as the mass to be nourished, and in con- 

 sequence the time will come when the nourishment possible to 

 be absorbed will just nourish the volume, and growth must 

 cease. This condition may constitute an internal stimulus to 

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

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



P 



FIG. 2. Streaming of Protoplasm in the .Amoeba. The forward motion of the granules takes 

 place more rapidly in the centre of the pseudopodium (p) . Those at the margin fall behind those 

 in the centre as the pseudopodium advances. 



Questions on the figure. Why may the amoeba readily change its form? Do 

 its internal parts preserve a constant relation to each other? 



24. 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, 

 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- 

 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. 2); if enclosed, the 



