PROTOPLASM 15 



merely growth beyond the limits of the individual. The cell 

 cannot continue to grow indefinitely. Its size is limited by the 

 necessity of physical support on the part of the soft protoplasm, 

 and by the relation between the outer surface, through which 

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

 mass to be fed. As a cell grows its surface increases as the 

 square of the diameter, whereas the volume increases as 

 the cube of the diameter. It is apparent that the nourishing sur- 

 face does not increase as rapidly as the mass to be nourished, 

 and in consequence the time will come when the nourishment 

 possible to be absorbed will just nourish the volume, and growth 

 must cease. This condition is in some way related to the imme- 

 diate causes of division. At any rate division furnishes a way 

 out of the dilemma and allows a renewal of growth of the 

 daughter units. 



Pig. 2. 



•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 (^). 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- 



