Bacteria witti 



TYPES OF PLANT CELLS 



though in many of the simplest plants and animals the body is not divided 

 into distinct chambers or cells. We speak of the individuals in these forms 

 as consisting of single cells. 



One of the simplest animals is the ameba, which lives in stagnant pools 

 and looks like an irregular lump of jelly enclosing tiny granules and bub- 

 bles. The animal responds to physical and chemical disturbances by con- 

 tracting the protoplasm, or by drawing in its pseudopodia, or "false feet". 



Variety of Cells When we look at an ordinary plant or animal, we 

 do not see the protoplasm, nor even the cells, but masses of walls of cells. 

 In the larger plants and animals the outer layers of cells are usually dead — 

 that is, they are walls without living protoplasm, just the kinds of cells that 

 Hooke saw in cork. The microscope enables us to see that some cells have 

 thicker walls or enclosing membranes than others, some hardly any (see 

 illustrations, pp. 24-25). We can see various kinds of solid bodies floating in 

 the protoplasm. There are also bubbles of clearer liquid. In some living cells 

 it is possible to see the protoplasm streaming about (see illustration, p. 26). 



Nucleus Near the center of each living cell, or at one side, we can 

 usually find a portion that seems more dense than the rest. This is called the 

 ?jucleus, which means "kernel". Since protoplasm is usually transparent, 

 it is difficult to distinguish its structure, even with the microscope. Now 

 we know that various kinds of dyes stain some materials more readily than 

 others. We can therefore use them to help distinguish the nucleus as well 

 as other structures in bits of plant and animal tissue (see illustration, p. 10). 



Multiplication of Cells Most of the plants and animals that you have 

 seen contain indefinite but very great numbers of cells. Some living things, 



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