46 



GENERAL BOTANY 



cell theory. It is needless to say that this generalization was 

 the result of prolonged scientific observation and research, ex- 

 tending over many years and engaged in by a host of scientific 

 men, each of. whom added his mite to the complete conception. 



PLANT CELLS AND THEIR PARTS 



The cell structure of plants outlined above, and the general 

 structure of the plant cell, can be most easily demonstrated by 



the student in thin sections cut 

 from the softer parts of plants 

 and examined under a compound 

 microscope. 



If such sections are cut from 

 dead tissues like bottle cork or 

 pith, the honeycomblike appear- 

 ance of the sections will be very 

 obvious (Fig. 26, a). The entire 

 section will appear, like the honey- 

 comb, to be made up of boxlike 

 units termed cells, each cell unit 

 being composed of a cell wall and 

 a cell cavity inclosed by it. In 



FIG. 26. Cellular structure of pith CQrk tisgue the eellg fit closely 



without spaces between them, 

 exactly like honeycomb cells ; but 



'ell cavity 



and celery 

 a, note the intercellular spaces and 



thin cell walls ; b, cells in transverse 



section; c, same in both transverse m pith and Various Other plant 



section and long section. The cell 



cavity is shaded in c, and the thick- tissues the cells round up as they 



ened portions of the cell wall are left grow an( J leaye space s, especially 



light. The primary, or first, wall is b . J 



indicated hy a middle line. Consult at the junctions of the Cells With 



the text for the method of cell-wall ea(jh other> These sp a ce s serve in 

 thickening in such a case as this 



many instances for the passage 



of air through the plant and are called intercellular spaces. 

 If sections similar to the above are cut from living tissues, 

 as from a root (Fig. 27), the same general cellular structure 

 will be observed as that seen in sections of dead tissues. The 

 cell cavities of the living cells will, however, appear to be filled 



