THE PLANT CELL 159 



The Amoeba feeds upon smaller organisms. These may be 

 drawn in at any point on the surface of the cell, whose proto- 

 plasm simply flows around the bodies and thus takes them into 

 the interior. The oxygen gas held in the water which bathes 

 the Amoeba is also absorbed all over its surface. Food materials 

 which cannot be digested, together with the waste products, are 

 left behind by the protoplasm as it moves from place to place. 



When the Amoeba reaches a certain size there takes place 

 the interesting event called cell division. The cell divides, 

 by a process of constriction (Fig. 167, B), into similar halves, 

 which separate from one another as two independent daughter 

 Amoebae. Previous to the division of the cell there has been a 

 division of the nucleus, so that each daughter Amoeba is pro- 

 vided with a daughter nucleus, and therefore has exactly the 

 same structure as the parent cell, but is, of course, only about 

 half as large. Cell division is the method of cell reproduction. 

 It is interesting to note that in this process of reproduction 

 there has been no loss of protoplasm, no death of any region of 

 the parent Amoeba, but from the division of one have come two. 

 There is, therefore, no death from old age in one-celled organ- 

 isms. They are being killed constantly, of course, by adverse 

 conditions, or eaten by other animals. These are the accidents 

 of life. However, the Amoeba and other one-celled animals and 

 plants need never die of old age ; that is, there is nothing in the 

 constitution of such an organism to prevent its living forever. 



195. The plant cell. The plant cell generally differs from 

 the animal cell in two important respects. 



First. The protoplasm is inclosed in a little box-like chamber 

 with transparent walls. The substance of the walls is called cellu- 

 l ose> a compound belonging to the great group of the starches 

 and sugars (carbohydrates). Such an envelope is termed a cell 

 wall, and is peculiar to plants. Indeed, the term cells, as used in 

 biology, was first applied to the chambers inclosed by cell walls, 

 which may be seen in thin sections of cork, pith, and other 

 plant structures. 



