The Cellular Structure of Plants 19 



paper and most book papers are made of cellulose derived from 

 wood cells. The walls of cells may be modified in various ways 

 by changes in composition, and thickened by the deposit of 

 additional layers. Other substances may be added to the cellu- 

 lose which render the wall hard and rigid, as is found in the 

 shells of nuts, or impervious to water, as in the outer cells of 

 leaves. 



Animals, as well as plants, are composed of cells ; but the 

 animal cell, instead of having a stiff cellulose wall like a plant 

 cell, has a soft wall, or it may lack a wall entirely, as in nerve cells 

 and white blood corpuscles. Consequently, the tissues of animals 

 (except the skeletal tissues) are usually softer and more pliable 

 than plant tissues. This makes it easy for an animal to bend 

 and to move about. The difference in cell walls and in the 

 phability of tissues is so general throughout the plant and animal 

 kingdoms that it is one of the important distinctions between 

 plants and animals. 



Cell division and enlargement. Among simple one-celled 

 plants, new individuals are formed by the division of the cell into 

 two. The cell first enlarges ; then the nucleus divides and the 

 two newly formed nuclei separate. The cytoplasm then divides, 

 the division beginning at the outside and gradually extending 

 to the middle of the cell. As the cytoplasm divides, a new 

 division wall is formed between the two daughter cells. In one- 

 celled plants this wall sphts and the two cells separate. In the 

 more complex plants the same kind of cell division takes place 

 when growth occurs and when new parts are formed, but the 

 cells remain together. 



Cell division is accompanied, or immediately followed, by an 

 increase in the amount of protoplasm and the taking in of addi- 

 tional water. These two processes lead to the enlargement of the 

 newly formed cells. Cell division and cell enlargement are first 

 steps in the growth of all plants, whether the plants be small and 

 simple or large and complex, 



