400 THE STRUCTURE OF ECONOMIC PLANTS 



between the minute lamellae, have been described by Anderson as 

 being the result of the periodicity of the deposition of wall 

 materials. 



"A series of lamellae is deposited by the protoplast, and then an 

 interval of time intervenes before the next series of fine lamellae is 

 laid down. This interval of time makes possible in the different 

 layer divisions some changes in physical condition, and these differ- 

 ences are evidenced by slight variations in shades of color between 

 them." 



The layers of the secondary wall system are composed of unmodified 

 cellulose which may vary in its physical condition depending upon 

 its relative stage of development. 



At maturity, the fiber cell consists of its constituent part of the 

 middle lamella, the primary wall, and an extensive secondary wall. 

 The middle lamella is originally composed of pectic substances, 

 but undergoes changes as the fiber develops, the chief one being a 

 tendency to become more or less lignified. The primary wall is 

 largely cellulose, modified to some extent by pectose; and the 

 secondary wall is made up of several (usually two to five) major 

 layers of pure cellulose, which are in turn composed of thin layers 

 that have been deposited in a manner that gives a definite striated 

 appearance to the cell wall. 



Fiber Structure. — Various details with respect to the charac- 

 teristics of the fiber have been reported, including banding, which is 

 a transverse marking or thickening of the fiber wall. This may be 

 attributed to a wrinkling or folding of the cell wall; or, more 

 frequently, the bands are the remains of primary thickenings which 

 adhere to the secondary wall after retting. In addition to these 

 fragments, there also may be particles of the walls of adjacent 

 parenchymatous cells adhering to the fiber wall. Local displace- 

 ments and distortions of the fibers also occur which appear to 

 result from stresses involved in the manipulation of the fibers in 

 retting or cutting rather than to any developmental factors. The 

 minute spiral striations which can be observed on the surface of 

 the fiber are related to the manner of wall formation, since the 

 lamellae are made up of fibrils that are spirally arranged in the 

 wall. The individual fibrils are below the resolving power of the 

 microscope and the visible striations consist of groups of fibrils 

 rather than of individual ones. Another feature of the physical 

 structure of the secondary cell wall is that the direction of the spiral 



