WALL STRUCTURE IN THICK CELL WALLS 117 



The results are recorded as the mean plus or minus the standard error 

 (i.e. the standard deviation divided by the square root of the number of 

 observations). 



A series of such determinations is presented in Table VI for tracheids 

 and for a variety of fibres; these will be examined in detail later on. At 

 the moment we are interested only in the general picture of tracheid 

 structure. We note that the m.e.p. is always inclined to the length of 

 the cell through a considerable angle which varies rather widely from 

 sample to sample in a way we shall have cause to discuss later on. Now 

 whenever a whole cell is examined the effective m.e.p. is always either 

 longitudinal or transverse, never tilted in this way; in fact this difference 

 between double and single walls is one way in tracheids by which cells 

 which have been cut open may be detected. Referring back to Fig, 3 

 this must mean that the m.e.p. of the upper wall in any cell makes the 

 same angle to cell length as that of the lower wall but in the opposite 

 direction, i.e. the m.e.p. is arranged spirally round the cell. 



Such a spiral arrangement of the m.e.p. can be interpreted pro- 

 visionally in terms of the orientation of the cellulose chains. The cell 

 wall may be built up of chains all lying parallel to the m.e.p. and this 

 possibility receives strong support from other observations which can 

 be made in these cells. Striations, for instance, can often be observed 

 in the walls and, remembering the close connection found in Valonia 

 and Cladophora between striation direction and chain direction, we 

 could hazard a guess that here too the striation direction conforms to 

 the direction of the chains in the layer in which the striations are visible. 

 Now it is clear from Table VIa that the directions of the m.e.p. and the 

 striations are never far removed from each other. Equally the same 

 thing is true if we compare the run of the slit mouths of bordered pits 

 and the m.e.p. Now if there were present layers of any considerable 

 extent whose cellulose chains ran in some direction other than that of 

 the striations, then this correspondence could not hold. This therefore 

 leads immediately to the conclusion that the bulk at least of the walls 

 of these cells is built up of chains running at the angle given by the 

 m.e.p. It is, of course, not possible to be sure that other directions are 

 absolutely missing. There are some discrepancies in Table VIa between 

 striation direction and m.e.p. and, in any case, the errors involved in 

 measuring either (of the order of 1/2°) are too great to allow any 

 certainty about the exact correspondence. But it is, at any rate, certain 

 that the m.e.p. does give the position of the bulk of the chains in the 

 wall within the limits of a rather small error. The chains of cellulose in 

 conifer tracheids, therefore, and of other elongated cells in which the 



