PROBLEM 1 Of What Are All Living Things Composed? 



The structure of living things. All but 



the smallest plants and animals are made 

 up of distinct parts which can be seen 

 with the naked eye. Plants may have 

 roots, stems, leaves. Animals may have 

 arms, legs, a head, and many other parts 

 visible from the outside. If we wish to 

 know their internal structures we dissect 

 them (cut them up). By doing this we 

 may see the heart, the stomach, the brain, 

 the liver, and many other parts. 



Several hundred years ago the use of 

 magnifying glasses was learned by scien- 

 tists in Europe and the lejises (as they are 

 called) were improved so that they mag- 

 nified more than a hundred times. Then 

 men began to use them to discover just 

 how the parts of animals and plants are 

 constructed. They examined all kinds of 

 living objects: human skin, blood, parts 

 of insects, leaves of plants, stems, bark, 

 and so on. Robert Hooke (163 5-1 703), 

 an Englishman, was one of the first to 

 invent and to use a compound microscope. 

 He studied very thin slices of cork, which 

 is part of the bark of a species of oak 

 tree, and discovered that it was made of 

 little boxes. The walls of the boxes seen 

 by Hooke were thick. The boxes were 

 empty. He called the boxes cells. See Fig- 

 ure 138, page 106. 



What is a cell? It is interesting that 

 Hooke was the first to call attention to 

 "cells" in living things but he never really 



saw cells at all! The name "cell" has been 

 used ever since but it is now used for 

 something quite different from Hooke's 

 empty boxes. A true cell has been found 

 to be not an empty box but a tiny mass 

 of living matter. This material is difficult 

 to see because it is transparent and usually 

 almost colorless. Sometimes it can be seen 

 llo\\'ing; it is semiliquid. It was given the 

 name protoplasm. The protoplasm may 

 be surrounded by walls and it was these 

 walls that Hooke first discovered. He 

 failed to see the protoplasm itself because 

 the cork he examined was made up of cell 

 walls only. The protoplasm had disap- 

 peared. 



As more and more parts of plants and 

 animals were studied it was discovered 

 that they were all composed of little 

 masses of protoplasm and that, very 

 often, there were no thick walls; the 

 protoplasm had the thinnest of walls 

 around it or no wall at all. These dis- 

 coveries were made over a period of 

 more than 100 years. Thus only grad- 

 ually did biologists come to realize the 

 comparative unimportance of the walls 

 and the importance of the protoplasm. 

 The name "protoplasm" was chosen for 

 the living- material because the word 

 means the first or most important sub- 

 stance. 



The use of the microscope disclosed 

 two very important facts. The first was 



