CELLS AND THE CELL THEORY 



35 



or of a material allied to starch and known as cellulose. Again 



it may be composed of lime, or made up of a hornlike substance, 



as in the case of the 



cells that secrete the 



finger nails, or the horns 



of animals. The cell 



wall is not alive, being 



simply a secretion of 



the living cytoplasm. 



The cell walls may be 



very thin, or entirely 



absent as in Figure 13. 



In other cases they may 



be very thick and form 



a tissue principally 



composed of cell wall, 



with only scattered bits 



of living protoplasm in 



the midst of a great 



mass of secreted wall substance. This is especially true in the 



case of the cartilage, as shown in Figure 4. The shape of a 



cell is usually determined by the shape of its cell wall. Figure 14 



shows a number of cells and gives an idea of the various shapes 



the cell wall may assume. 



Since the cell wall is lifeless and has only the function of sup- 

 port, the cell contents alone being alive, it follows that any 

 organism may contain both living and lifeless material. Among 

 plants the lifeless material may far surpass the living in bulk. 

 In a tree, for example, most of the trunk, roots, and branches 

 are made of the dead walls of cells which were formerly filled 

 with living protoplasm. In a large tree only a thin layer of cells 

 directly under the bark, the cells found in the leaves, buds, 

 and some cells in the roots, are actually alive. In animals a 

 much larger proportion of the body cells are alive, the bulk of 

 the muscles being living protoplasm ; but the skin, hair, cartilage, 



FlG. 13. A SINGLE-CELLED ANIMAL 



ACTINOPHRYS 

 A cell without a cell wall. 





