Growth and Structure of the Primary Wall 



THOMAS KERR 



THE primary wall is the membrane surrounding the growing cell 

 showing reversible changes in surface area with variations in cell 

 turgor and permanent changes in surface area as a result of growth. The 

 structure of a membrane capable of undergoing such changes has not 

 been satisfactorily explained. 



All cells of higher plants Irrespective of their size, shape, or cell-wall 

 thickness have primary walls, and as long as they are in a rapid state 

 of enlargement, the cells have only primary walls. Thus the thickened 

 walls of collenchyma and the outer walls of epidermal cells which are 

 known to be cutlnlzed must be classified as primary membranes. 

 Throughout this discussion the structure of the primary wall will be 

 based chiefly on the structure of a typical parenchyma cell such as 

 one finds in the Avena coleoptlle. 



Chemically the primary walls of higher plants are known to be com- 

 posed of cellulose and pectic substance. In the cotton hair where thorough 

 investigations have been made, the cellulose of the primary wall gives 

 a typical X-ray diffraction pattern (2), has an average chain length 

 approximately 50 per cent that of the secondary wall (6), and shows 

 standard solublUtles of cellulose. Considering the wide variations In 

 chain length In different types of secondary walls it may be concluded 

 that the cellulose molecules of the primary membrane are Identical with 

 those of the secondary wall. 



For many years Investigators working on cell walls have realized that 

 the structure of the primary wall cellulose Is fundamentally different 

 from secondary wall cellulose without knowing the basis for the dif- 

 ference. The recent work of Miihlethaler (9) on the structure of the 

 primary wall under the electron microscope shows why these differences 



