28 



PROTOPLASM AND THE CELL. 



direction of the flow. If particles of food be met with, the 



protoplasm flows around them, and when thej have been digested 



within tlie body, the protoplasm flows onward, leaving the refuse 



behind. Hour after hour and day after day this flowing may 



go on, and there is perhaps no 



more fascinating and suggestive 



spectacle known to the biologist. 



A similar change of form is ex- 



liibited by the colorless corjDuscles 



of amphibian and other blood, in 



which it may be observed, though 



far less satisfactorily, if Afnoehce 



cannot be obtained. Among plants, 



protoplasmic movements of perhaps 



equal beauty may be observed. 



One of the simplest is known as the 



rotation of protoplasm, wliich may 



n- 



'?,»•■*■ •••.,'ov .•.-.'<:"- <J ■■:■■*. 0- 



Fig. 17.— a cell of a stonewort (NiteU 

 la) showing the rotation of proto- 

 plasm; the arrows show the direc- 

 tion of the flow, m, membrane of 

 the cell; n, nucleus, opposite to 

 which is a second ; p, protoplasm ; v, 

 large central vacuole filled with sap. 



i 





5>~ .:-;.■ .-V^^i.^: •,'^:f> 



r 



Fig. 17a.— Two cells and a part of a 

 third from the tip of a "leaf" of a 

 stonewort, showing rotation of the 

 protoplasm in the direction of the 

 arrows. 



be studied to advantage in rather young cells of stoneworts {Cliara 

 or Nitella). These cells have the form of short or elongated 

 cylinders which are often pointed at one end (Fig. 17). The 



