THE CELL WALL 63 



investigators and for many years there was much disagreement over the ques- 

 tion. The now universally accepted principle that cells can arise only by the 

 division of pre-existing cells was first demonstrated beyond any reasonable 

 doubt by Nageli just before the middle of the Nineteenth Century. A num- 

 ber of methods are now known by which this division is accomplished but a 

 detailed discussion of them is beyond the scope of this book. In higher plants 

 cell division occurs chiefly in certain restricted regions called mcristems. The 

 production of new cells involves not only the division of pre-existing cells, but 

 the subsequent enlargement and maturation of their cell progeny. The mor- 

 phological and physiological aspects of these stages in the development of cells 

 are discussed m Chap. XXXL 



The Forms and Sizes of Cells. — All newly foi-med cells do not differ- 

 entiate morphologically in the same way. Some elongate parallel to the axis 

 of growth more than in other directions and thus produce the longer fiber 

 cells that make up much of the xylem and phloem tissues. These cells com- 

 monly develop walls that are greatly thickened. Some cells enlarge about 

 equally in all directions forming isodiametric cells, the walls of which are 

 never greatly thickened. Cells of this type are present in the pith, leaf meso- 

 phyll and in other parenchymatous tissues. According to Lewis (1935) both 

 plant and animal cells are basically tetrakaidecahedrons; i.e., 14 sided, 

 although many other geometrical shapes are found, especially in specialized 

 tj'pes of cells. An almost endless variation in cell shapes and sizes may be 

 seen in the tissues of any vascular plant, all of which may develop from 

 similarly shaped meristematic cells. Many of these are figured and described 

 in greater detail in later chapters. 



In size cells show an equally great range. Most plant cells have diameters 

 that fall somewhere between 10 /a and lOO /x. A single cubic centimeter of 

 tissue may, therefore, contain millions of cells. Some cells, however, are much 

 larger. Certain varieties of cotton, for example, produce fibers that commonly 

 attain a length of 4 cm. while the phloem fiber cells of Boehmeria nivea are 

 known to exceed 55 cm. in length. Such cells have lengths that are many 

 thousand times their diameters. 



The Cell Wall. — One of the most important features of plant cells is 

 the presence of a conspicuous cell wall that encloses the living protoplasm. 



There are great variations in the thickness of the walls of different kinds 

 of cells and even greater differences in the physical and chemical properties 

 of cell walls. In fact the walls of plant cells exhibit such great contrasts in 

 structure and chemical composition that it is difficult to single out any specific 

 cell wall as having a structure that may be considered typical of plant cell 

 walls in general. There are, however, certain features of the cell walls of the 



