24 ELEMENTARY BIOLOGY 



may be difificult to distinguish the parts in many plant and 

 animal cells. It has been found convenient to stain masses of 

 cells with various kinds of pigments or dyes, to make the 

 structure stand out more distinctly under the microscope. 

 When certain dyes are used, the nucleus becomes particularly 

 distinct, since it absorbs these dyes more readily than do other 

 parts of the cell. And within the nucleus we can sometimes 

 see fine little rods or strands (Fig, 3). 



37. Numbers of cells. The cells that you have seen under 

 the microscope may have suggested the question whether 

 a body has a definite number of cells. Most plants and 

 animals that you have seen probably have indefinite num- 

 bers, and these run into the countless millions. There are 

 some living things, however, that have a very definite and 

 limited number of cells. 



One of the simplest animals is the one-celled ajneba^ which lives in 

 stagnant pools and other wet situations. Under the microscope it 

 appears to be an irregular lump of jellylike matter, in which various 

 granules and bubbles can be made out. There is a nucleus, and all 

 around it movements are constantly taking place. The shape of the 

 mass of naked protoplasm is constantly changing, resulting in sluggish 

 movements of the animal. The slimy mass swallows particles that 

 may serve as food, and it crawls away from contained particles that 

 are no longer of service. The animal is sensitive to physical and 

 chemical forces in the environment, and responds to disturbances by 

 contractions of the protoplasm. 



38. Tissues. In the bodies of the plants and animals large 

 enough to be seen without a microscope, there are usually 

 many different kinds of cells, Masses of similar cells together 

 constitute what is called a tissue. Thus, in our own bodies 

 there are muscle tissue, bone tissue, brain tissue, gland tissue, 

 connective tissue, and other tissues. In the body of an ordinary 

 plant we may recognize bark cells, wood cells, pith cells, skin 

 cells, and other cells (see Figs. 4, 5), 



