CHAPTER V 



SOME PROPERTIES OF CELLS AND THEIR FUNCTIONS IN 



THE ROOT 



44. Definition of cell. This is not the best place to consider 

 the nature of cells in much detail (see Chapter xvui) ; but some 

 of the facts learned in Chapter iv cannot be understood with- 

 out a few words of explanation of cell structure and functions. 



Protoplasm is the nitrogenous living substance of which 

 the most rapidly growing parts of plants are mainly composed. 

 The activities of the plant are due to the peculiar qualities and 

 powers of protoplasm. A cell is a unit of protoplasm, called 

 a protoplast. The protoplast of plants 'is usually inclosed in a 

 case or covering whose walls (cell walls) are composed of a sub- 

 stance known as cellulose. Each protoplast usually contains a 

 single denser protoplasmic structure, called the nucleus. 



In form and size cells vary greatly. Those of the root hair 

 (Fig. 23) are good examples of the slender, thread-like form; 

 those of Fig. 27 well illustrate forms commonly assumed when 

 cells are pressed upon by others on all sides, as they usually 

 are in the interior portions of the organs of higher plants. 



45. Growth and reproduction. The most remarkable property 

 of cells is their power of growth and reproduction. Growth 

 results not only from an increase in the size of cells but also 

 in their number as a result of cell division. This is the separa- 

 tion of a protoplast, generally into two independent protoplasts 

 or daughter cells, and is the fundamental cause of all growth 

 and development. The full-grown seed plant, composed of 

 millions of cells, arises from the embryo (with perhaps only a 

 few thousand), which had its beginning in a single cell. Cell 

 division is preceded by division of the nucleus (Fig. 170). 



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