CHAPTER VII 

 CELLS AND TISSUES 



Structure and Function of Cells 



Position of the Cell in Plant Life. — Before proceeding to the 

 study of the aduU stage of the plant more must be known about 

 the cell. If with a sharp razor a very thin section from any part 

 of a plant is made and observed with a microscope, it will appear 

 to be divided into many small divisions. A section through the 

 growing portion of a root looks like Figure 105. These little 



r divisions with what they 

 '^ contain are the cells. Cells 

 vary much in shape and are 

 so small that usually four -or 

 five hundred of them could 

 be laid side by side on a line 

 not more than an inch in 

 length. They are rarely more 

 than T^xT of an inch, and 

 sometimes less than xAo of 

 an inch in diameter. Al- 

 though cells are so exceed- 

 FiG. 105. — A small portion of a length- ingly small nevertheless, it 

 wise section through the growing region is within them that all life 

 of a root showing the cells. Very much processes take place. For 

 ^"^^'^^^^- this reason cells are often 



defined as the units of all plant and annual life. Plants need 

 phosphate, nitrates, etc., because the cells must have them. All 

 the problems of the plant relating to the soil, light, temperature, 

 etc., are problems of the cell. The plant is made up of a countless 

 number of cells and the activities of the plant are simply the sum 

 of the activities of the many cells of which the plant is composed. 

 Discovery of the Cell and Its Structures. — That plants and 

 animals are composed of cells was not revealed until the inven- 

 tion of the microscope, which, although very rude in its con- 



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