PROBLEM 3. How Cells Are Arran^red in Amnials mid Plant 



Fig. 156 Which of the organs of this geranium 

 can you see? Which cannot be seen? (sullivan) 



How living things are built up. Thus 

 you see that organisms, except the very 

 simplest, are made up of parts called or- 

 gans; organs are composed of several or 

 many tissues; tissues are composed of 

 cells; and all the cells of one tissue are 

 similar to one another. 



Most organisms are so complex that 

 they have a number of organs working 

 together in an organ system. For example, 

 in your body there is the digestive sys- 

 tem consisting of the various organs 

 that have to do with digestion. Then 

 there is the skeletal system which in- 

 cludes all the bones of the body. You gans as the stomach, the mouth, and other 

 will read of other organ systems that digestive organs); it includes .yer(9?/^ we?;/- 

 are composed of a number of organs; braiie (the smooth membrane covering 

 the organs are composed of a number some internal organs and Hning body 

 of tissues, and the tissues are composed cavities); and it includes gland tissue, 

 of cells. You have been reading about Muscle tissue is of three types: volun- 

 the construction of your body but other tary muscle, involuntary muscle, and 

 complex animal organisms as well as heart muscle. In Figures 157 and 158 are 



S 131 



complex plant organisms are also com- 

 posed of cells, tissues, and organs. 



How diflFerences arise in cells. Most 

 organisms start life as a single cell. This 

 is true even of the complex animals such 

 as the vertebrates. This single cell gives 

 rise to the many millions of cells of 

 which the organism's body consists. It 

 is interesting that this cell should be able 

 to produce types of cells as different 

 from one another as are the cells of such 

 tissues as muscle, bone, and nerve tissue. 

 Many biologists are attemptingr to solve 

 this problem at the present time. What- 

 ever the explanation may be, we must 

 recognize the fact that there is differe?i- 

 tiation of the cells in an organism. The 

 cells differ from one another in appear- 

 ance and in their activities. In the more 

 complex animals the differentiation is 

 very marked. Each type of cell can usu- 

 ally be recognized either by its structure 

 or by the work it does in the animal or 

 plant. The cells are specialized in struc- 

 ture and function. 



The main kinds of animal tissues. The 

 many kinds of tissues in an animal such 

 as man can be divided into four or five 

 main tissue groups. Epithelial, muscle, 

 connective, and nerve tissues are four 

 groups. The fifth is blood. Epithelial in- 

 cludes the membrane that covers the 

 body; it also includes mucous membrane 

 (the moist membrane that lines such or- 



