CHAPTER II 



THE CELL 



Living things, or, as we say, organisms, are built of miniature 

 units known as cells, just as a house is built of stones. Many- 

 very lowly organisms, such as bacteria, consist of but a single 

 cell. All cells are delimited by a wall or membrane. The living 

 matter within them is protoplasm. A cell may be defined as a 



Fig. 4. — A typical plant cell (from the leaf of the water plant Elodea). a. The 

 wall of cellulose, the inner surface of which is lined by the protoplasmic mem- 

 brane; b, the protoplasm, termed cytoplasm when considered apart from the 

 nucleus; c, the nucleus; d, the nucleolus; e, a (green) chloroplast; /, the large, 

 central cavity or vacuole filled with an aqueous solution and traversed by proto- 

 plasmic strands, g. {Drawn by J, Plowe.) 

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protoplasmic mass enclosed by a membrane. Cells are usually 

 of microscopic size, especially when they are parts of tissue. 

 Only rarely, when existing as individual organisms, can they be 

 seen with the unaided eye, and then usually simply as a mere 

 speck. The typical plant cell possesses two coverings, a delicate, 

 living protoplasmic membrane, and a heavy wall of nonliving 

 cellulose (Fig. 4). Animal cells are usually enclosed within a 

 delicate membrane only and have no outer heavy wall (Fig. 5), 

 It was, by chance, the empty and dead cellulose housing of plant 

 cells which was first seen and called a cell. In the year 1665, the 



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