THE CELL 



241 



situated and approximately spherical in form. The 

 protoplasm is divided into an outer hyaline and an 

 inner granular portion, the former being limited exter- 

 nally by a very delicate membrane. The shape of the 

 animal is irregular, and, moreover, undergoes gradual 

 alteration owing to the characteristic amoeboid move- 

 ments. These consist in a slow protrusion and with- 

 drawal of processes of the body, enabling the animal 

 to change its position by a kind of flowing movement, 

 and also to engulf its food, which consists of various 



FIG. 17. AMCEBA. 

 n, Nucleus ; /, food particle. 



FIG. 1 8. PLEUROCOCCUS. 



n, Nucleus ; w, cell -wall ; 

 chl, chloroplast. 



minute organic particles, by the simple process of 

 flowing around it. 



In contrast with Amaba the unicellular plant Pleuro- 

 coccus is motionless, and is surrounded by a firm wall 

 of cellulose. In addition to a central nucleus, the 

 plant contains, embedded in its peripheral protoplasm, 

 several plastids which bear the chlorophyll concerned 

 in the assimilation of carbon from the gases of the 

 atmosphere. This chlorophyll lends a green colour to 

 the whole contents of the cell, and in its natural habitat 

 the plant is quite conspicuous. The green powdery 



substance often to be seen on the bark of trees, 



16 



