162 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



breadth chosen was 24 jjl and it is in these terms that the standard angle 

 is defined. 



It is abundantly clear from Fig. 56 that the length of these tracheids 



Fig. 56. The relation between cell length and spiral angle in 

 individual tracheids of Picea. 



is closely correlated with the cotangent of this standard angle. The 

 relation can obviously be fitted to an equation of the general type 



L=A-\-B cot 0i.oo» 

 where A and B are constants or, if the breadth b of the tracheids is 

 included, . L=A-\-BV{b'" cosec^ 0-1). 



The regression line drawn in Fig. 56 represents the equation, 



L=568+293cot 0,.oo, 



where L is in /ti, and a similar regression line has also been observed 

 for one other species of conifer wood. 



There can thus be no doubt but that among these tracheids, at any 

 rate, there is a rather close relationship between the length of a cell and 

 the structure of the central layer of its wall. It is natural therefore to 

 inquire whether a similar relation holds for the other layers of the 

 secondary wall. This inquiry sets us a more difficuh task, and, as yet, 



