CHAPTER FOUR 



THE CELLULAR STRUCTURE OF PLANTS 



The body of every plant either is a single cell or is made up of 

 a mass of cells. Many one-celled plants are so small that they 

 can be seen only with a microscope. A large plant like a tree 

 is composed of so many cells that their number can scarcely be 

 conceived. A cubic inch of a potato, for example, contains at 

 least 600 million cells, and a cubic inch of pine wood more than a 

 billion. 



The cellular structure of plants was first noticed about the 

 middle of the seventeenth century, soon after the invention of the 

 microscope. Cells were first seen in examining thin sections of 

 cork, which is composed of layers of minute rectangular box-like 

 structures. Observations were extended to other parts of 

 plants, and to other plants, and it was finally recognized that 

 cells are the units of which all plant structures are built. 



The plant cell. Nearly two hundred years passed, however, 

 after the discovery of cells before it was known that the cell walls 

 which had been previously studied constitute merely the frame- 

 work of the plant ; that the most important part of the cell is a 

 transparent, jelly-like, living substance inclosed by the walls. 

 This hving matter is called protoplasm (Greek : protos, first, and 

 plasma, form). When we now speak of cells, we usually have 

 in mind both the protoplasm and the wall around it. A cell may 

 be defined as a unit mass of protoplasm, capable of exhibiting all 

 the phenomena usually associated with life, such as growth and 

 respiration. 



We have omitted the cell wall from the definition because some 

 cells are without walls for a part or the whole of their existence. 

 It is through the activity of the protoplasm that the walls and 

 other structures that are found in different types of cells are 

 formed ; so it seems best to define the cell in terms of the funda- 

 mental material of all cells. 



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