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 b /ing protoplasm in 

 the midst of a great 



ma-ss 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 

 ihe 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 beirg living protoplasm ; but the skin, hair, cartilage, 



FlG. 13. A SINGLE-CELLED ANIMAL 



ACTINOPHRYS 

 A cell without a cell wall. 



