THE CELL. 



73 



branch), as in common mould (Fig. 171). The variation 

 is as striking in re- 

 gard to size ; the cells 

 of Bacterium termo, 

 the common fungus 

 of putrefaction, are 

 about .00009 inch 

 long and .00005 inch 

 broad ; the Yeast- 

 plant cells are about 

 .0003 inch in diam- 

 eter; the average size 

 of cells of ordinary ^'"^ 



plants is .005 to .0005 inch ; many cells, as of bast, plant- 

 hairs, etc., are not microscopic as those mentioned above, 

 but attain a length of several inches. 



95. A cell consists of four parts, namely: (1) the cell- 

 wall (Fig. 172, w), or cell-membrane, a covering enclosing 

 the cell-contents, which are (2) the protoplasm (Gr. pro- 

 tos, first or primitive ; plasma, form), a transparent, semi- 

 fluid substance (Fig. 172, pr), con- 

 taining (3) the nucleus, a spher- 

 ical or oval body, denser than the 

 protoplasm (Fig. 172, n), and (4) 

 the cell-sap, a watery fluid occu- 

 pying cavities, called vacuoles (Fig. 

 172, v). Of these four parts the 

 protoplasm is the only one essential 

 '^''^ to the gro^Ying cell; the other 



parts may be wanting ; swarm spores have no cell-wall ; 



Fig. 171. Common Mould (partially diagrammatic). Fig. 172. A cell showing the 

 four parts ; w, cell-wall ; /r, protoplasm ; n, nucleus ; »/, nucleolus ; v, vacuoles. 



