io6 



Living Things Are Basically Alike unit ii 



Fk;. 138 A section through cork as seeji under 

 the vilcroscope. These are the structures which 

 Hooke named ''cells." Do these boxes seem to 

 be filled or etnpty? 



that all living things are made up of 

 cells; the second was that cells are really 

 little bits of living substance, protoplasm. 

 The microscope. Before vou go further 

 into the study of the structure of living 

 things it is wise to learn about the in- 

 strument that has made this study pos- 

 sible. The modern microscope has been 

 described briefly on pages 10-12. It 

 would be well to read those pages again. 

 If microscopes are available in your 

 school you will learn how to use them. 

 Do ExFRCiSEs I, 2, 3, and 4. 



^^'llat are the parts of a cell? There 

 are many different kinds of cells, very 

 different in shape and size. But each 

 cell is a tin\' mass of living matter called 

 protoplasm. When properly stained with 

 dyes and properly treated it can be seen 

 that each cell has three parts: the c ejl hod^ ' 

 or cytoJ^liLim (sigh'toe-plasm) which is 

 the niain part of rlie protojilasm; a small 

 ball of denser protoplasm^ catted the 

 micleus (new'klee-us) lying within the 

 cell body; and a cell inemhrane (also 

 called plasma viembrane) surrounding 

 the cell bodv\ The cytoplasm, nucleus 

 and cell membrane are all protoplasm. 

 Thus these three parrs arc all living. If 

 you do FxTRrisi .^ you will see cells 



Nucleus 



Cell body 



Cell membrane 



Fig. 139 Cells like these are jound in the lining 

 of your mouth. What are the three parts of such 

 cells? How are they different from plant cells? 

 (See text below and Figure 140.) 



from your own body which like all other 

 cells have the three parts just mentioned. 

 Plant and animal cells are fundamen- 

 tally alike; all have a cell body of cyto- 

 plasm, a nucleus, and a cell membrane. 

 There are some differences between 

 plant and animal cells, however. In most 

 plant cells the cytoplasm builds up a 

 firm cell wall of lifeless material outside 

 the membrane. This lifeless cell wall 

 usually consists of a tough, transparent 

 substance, cellulose (celPyou-lohss), or 

 an even harder woody material. Do Ex- 

 ercise 6 in order to see some plant cells. 

 By doing Exercise 7 you will see that 

 the wall of the plant cell is distinct from 

 the cell membrane. Plant cells differ 

 from animal cells in other ways, too. 

 Usually, they have one or more large 

 bubbles of liquid lying in the cytoplasm. 

 These are called vacuoles (vak'you-ohls). 

 Vacuoles are rarely found in animal 

 cells. A third difference between plant 

 and animal cells is that some of the cells 

 of green plants contain small bodies 

 called cbloroplasts (klor'oh-plasts). These 

 contain a very important green sub- 

 stance, chlorophyll, of which you will 

 read much more later. To help you with 

 this paragraph do Exercise 8. 



