WALL STRUCTURE IN THICK CELL WALLS 149 



swollen in cuprammonium (Schweitzer's reagent) very fine lamellation 

 may be distinguished. Unlike, however, the lamellation we have studied 

 up to now, these lamellae apparently vary little, if any, in cellulose chain 

 orientation or in content of non-cellulosic substances; in fact the 

 secondary wall of cotton hairs consists of remarkably pure cellulose. 

 The interpretation of lamellation most widely accepted is in terms of a 

 variation in porosity, a variation which recalls strongly the interpre- 

 tation now put upon the layering in the secondary walls of tracheids 

 and fibres. Observation of whole cotton hairs under the microscope, 

 particularly if the hairs are slightly swollen, reveals the presence of fine 

 striations forming a spiral round the wall which, according to the 

 variety of cotton examined and the age and length of the particular hair, 

 makes an angle with cell length somewhere in the range 25° to 45°. One 

 very striking peculiarity here, however, a characteristic which seems 

 confined to cotton hairs, is that the spiral is reversed more or less 

 regularly along the length of the hair, i.e. passing along the hair the 

 spiral may first be right-handed (the so-called Z spiral) then left-handed 

 (the so-called S spiral), again right-handed, and so on. It is somewhat 

 difficult to establish any general rule as to the orientation of the cellu- 

 lose at the points of reversal; according to published descriptions the 

 "fibrils" may turn to become parallel to the length of the hair at the 

 reversal points or, again, the fibrils of each neighbouring spiral may 

 intermingle with little sign of change in orientation. 



Corresponding to this general picture, the X-ray diagram shows the 

 diffraction arcs characteristic of cellulose, spread into arcs corresponding 

 to this spiral arrangement. It should be noted that right- and left- 

 handed spirals give the same X-ray diagram so that no peculiarities in 

 the diagram, due to the reversals of the spiral, are to be expected. 

 Normally the lateral arcs are continuous like those of the wood cells 

 illustrated in this book, and this is undoubtedly a reflection of the fact 

 that generally in a bundle of hairs the spiral angle varies rather widely. 

 When a single hair is examined in an X-ray microcamera, however, 

 the lateral arcs each break up into two spots,* as theory predicts 

 (p. 124) and the diagram then in general resembles that in Plate VI, 

 Fig. 4, except that the arcs correspond to native, and not mercerized, 

 cellulose. 



Although the X-ray diagram thus corresponds roughly to only one 



set of spirally arranged chains in the wall, it is now quite clear that 



cotton fits in with other elongated cells of the higher plant in possessing 



wall lamellae whose structure is different from that in the bulk of the 



* Private communication from Professor W. T. Astbury. 



