492 Our Surroundings 



plainly seen. The cell wall is the very thin covering which en- 

 closes the protoplasm and separates the cell from its fellow cells. 

 The cytoplasm is the liquid nutritive part of the cell, and the 

 nucleus is the very minute, slightly denser part. The nucleus 

 is the vital part and is the source of reproduction. 



An interesting demonstration is to observe, by the aid of the 

 compound microscope, the circulation of the protoplasm in a liv- 

 ing cell of a water plant called Elodea. 



The Cell, the Unit of Structure and Function. The small- 

 est plant is a single cell containing all the parts necessary for 

 the manifestation of life. All large plants are made up of 

 many, often millions, of cells, so related to one another as to 

 form a single organism. Since all the parts of this organism 

 are composed of cells similar in structure and in function, the 

 cell may be considered the unit of both structure and function. 



Cell Division. Cells increase by division. Under normal 



conditions when a cell reaches 

 its full growth a nucleus forms 

 in each half. This is followed 

 CELLS OF ELODEA b y a division of the cell body 



The arrows indicate the movement of the into two parts and by the f Or- 

 protoplasm. . f 1t , . , 



mation of a cell wall which 



separates the two new cells. In the same way all cells divide, and 

 thus the organism grows in size. 



Comparison of the Needs of Plants and Animals. Ani- 

 mals and plants need nearly the same conditions in order to exist. 

 Like animals, plants must have food, moisture, air, and heat. 

 Plants will not survive without light. Animals can live without 

 light as long as the other conditions are present, but it should not 

 be forgotten that after a time there would be no food if there were 

 no light. So light also, in the final analysis, is necessary for the 

 maintenance of animals. 



Functions of Plants. Since plants need fopd, air, and mois- 

 ture, it follows that they must have the means of securing and 

 using these substances. Hence they perform the same functions 

 as animals to secure them, together with the additional function 

 of food-making. Under favorable conditions these processes are 



