THE PRIMARY WALL OF GROWING CELLS 



177 



an atmosphere of relative humidity 98 % gives also diffraction rings 

 typical of cellulose, presumably unmasked by the reduction of the 

 intensity of the water rings, in exactly the same disposition as later 

 found from the dried tissue (56). It is not to be denied that the removal 

 of free water consequent upon equilibration with an atmosphere of 

 98 % relative humidity may have caused some modification of structure; 

 but it is strongly to be questioned whether this comparatively small 

 degree of drying could materially affect the crystalUnity of the cellulose. 

 This correspondence of the photographs of wet and dry tissue will 

 therefore be taken here to imply that the broader details at least of 

 structure in the walls of fresh tissue can be deduced from observation 

 of dried material. 



Orientation of cellulose in primary walls 



When the cambium is stripped from a tree as described previously 

 (p. 1 54) then the resultant ribbons of tissue are far too thin to yield an 



Fig. 59. Diagrammatic representation of a composite block of flattened strips of 



cambial tissue. 



Direction A lies normal to the flat faces of the strips {i.e. along a direction radial 

 to the trunk when the strips were in position in the tree). 

 Direction B lies parallel to cell length, and direction C is mutually perpendicular 



to A and B. 



X-ray diagram. If, however, several such strips are dried on to glass 

 and subsequently piled on one another in such a way as to maintain the 

 parallel orientation of the cells from layer to layer and the resultant 

 bundle is irradiated in the direction A (Fig. 59), then this bundle yields 

 a diagram such as illustrated in Plate IX, Fig. 2. The implications of 

 such a diagram will be immediately obvious. On drying, the cells 

 collapse and become flattened in the plane of the strips (parallel to the 

 glass surface) so that in effect the block corresponds to a series of 



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