THE WALLS OF PLANT CELLS 



By IRVING W. BAILEY 



DEPARTMENT OF BIOLOGY, HARVARD UNIVERSITY, CAMBRIDGE, MASS. 



In any discussioii ol' tlie cell walls of 

 plants, it is essential to ditt'erentiate between 

 two distinct cate<iories of structures. Meri- 

 stematie cells and such of their derivatives 

 as retain a capacity for <irowth and for in- 

 crease in volume are provided with a wall 

 capable of expansion and of increase in 

 surface area. This wall is also capable of 

 undergoing- various reversible changes, for 

 example, in thickness. Many tissue cells, 

 particularly those in which the mechanical 

 functions are emphasized, form a supple- 

 mentary wall that is incapable of surface 

 expansion. Such cells are unable to grow 

 or to increase in volume unless the proto- 

 plast can escape from its indurated en- 

 velope. A wall of the former category will 

 arbitrarily be designated as a prinuiry wall ; 

 the latter type of wall will be referred to 

 as a sccondiiyji wall. The secondary wall of 

 plant cells provides a more favorable 

 medium for physical, chemical, and micro- 

 scopic investigations; and, because of its 

 economic and industrial importance in tex- 

 tile fibers ami pulp and as a source of 

 cellulose and its derivatives, it has been 

 intensively studied during the past one 

 hundred years. In other words, much more 

 is known about the chemical composition, 

 the microscopic structure, and the optical 

 behavior and other physical ]u-operties of 

 secondary walls than is known about pri- 

 marj^ walls. 



Under high magnification the secondary 

 wall of plant hairs, of fibers, and of scler- 

 enchyma in general, exhibits many diverse 

 structural patterns; these range from con- 

 centric to radial patterns, or to various 

 complex combinations of such simpler pat- 

 terns. Fig. 1 illustrates a transverse sec- 

 tion of a thick secondary wall at a magnifi- 

 cation of two thousand diameters. By 

 transverse, I mean a section cut at right 

 angles to the hmg axis of the cell. The wall 



Fig. 1. Transverse .section of a tliick, unswollen 

 secondary wall, showing cloniinantly concentric 



pattern (x2000). 

 Fig. 2. Transverse section of a thick, unswollen 

 secondary wall, showing radial and ramifying pat- 

 tern (V 2000). 



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