46 BUILDING MATERIAL OF LIVING THINGS 



see later that they are of the utmost importance to each one of 

 us, as it is by means of the action of the sun upon them that 

 food is manufactured in the green parts of plants. 



In the elodea cells, an interesting phenomenon may be observed. 

 The protoplasm in the cell body is seen to be constantly in motion, 

 flowing slowly in the direction of the arrows shown in the diagram. 

 This streaming of protoplasm is one of the manifestations of life 

 within the cell. In many cells this movement may be observed 



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Cell wall 

 (cellulose) 



Cell membrane 



...Nucleus 

 Nucleolus 



.Chloroplasi" 



IX 



A cell of elodea, a plant. The arrows 

 show the "streaming" of the proto- 

 plasm. White spots show vacuoles. 



.Cell membrane 



.Cytoplasm 



•■Cenfrosomes 

 Nucleolus 



JM-I^ucleus 

 ^,M..PIasfld 







"Vacuole 



Diagram of a typical animal cell. In 

 the nucleus, the dark thread-like struc- 

 tures are stained chromosomes. 



and we have reason to believe that the protoplasm in most living 

 cells is in motion, thus affording a circulation of the cell contents. 

 If we now examine a specially prepared and stained cell, for ex- 

 ample, the egg cell of a worm or a frog, we shall find that the nu- 

 cleus, when stained with certain dyes, shows numerous small 

 deeply stained bodies within it. These structures are called chro- 

 mosomes (kro'm6-somz ; Gr. chroma, color; soma, body), or color- 

 bearing bodies. The number of these chromosomes in each body 

 cell of a given kind of plant or animal is always the same. The 

 chromosomes are supposed to be the bearers of the qualities which 

 can be handed down from parent to offspring; in other words, 



