CHAPTER I. — THE CELL. 



I29 



to markings more or less conspicuous, which may either be 

 irregular in form and distribution, or else quite regular and 

 characteristic of certain classes of cells. The markings may 

 take the form of thickenings or protuberances on the outside of 

 the cell-wall, as in the pollen grains of the Mallows, Fig. 248. 

 This cannot well occur, however, except in cells which become 

 independent at or before maturity. In those which are united 

 to form tissues they are seen as thickenings on the inner surface 

 of the wall. These may form rings, spirals or reticulations, or 

 they may be so arranged that the unthickened portions form 

 circular or oblong disc-like markings. These, in old cells,, 

 frequently become perforations. The markings of 

 cells will be more fully described when we come 

 to treat of the different kinds of tissues. 



Fig. 363. 



Fig. 364. 



Fig. 363. — Polygonal parenchyma cells from the pith of Clematis Virginiana, showing 

 the effects of mutual pressure. Magnified 125 diameters. 



Fig. 364. — Elongated cells from the stem of the common House Geranium. Magnified 

 about 250 diameters. 



The cell-wall, besides increasing in thickness, grows also in 

 surface area until it reaches maturity. Sometimes the growth 

 is nearly equal in all directions, giving rise to spherical or sphe- 

 roidal forms, or if the cells are aggregated into masses, the 

 tendency to an equiaxial growth may be modified by mutual 



