110 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



dealt with up to now have consisted of so-called native cellulose, the 

 walls in this group appear to consist of cellulose, it is true, but this has 

 been reported to be in the form known as hydrate or mercerized (46(a)). 

 If native cellulose is dispersed and coagulated from solution, or is 

 swollen in 18% caustic soda and washed, then a product is obtained 

 which gives a new diffraction pattern. This product is naturally still 

 cellulose and has the same analytical composition as the native form. 

 It is, however, more reactive chemically, with a stronger affinity for 

 dyes, etc., and the fact that its X-ray diffraction pattern is different from 



a b 



Fig. 39. Projection of the unit cell on the ac plane in {a) native cellulose (cellulose I) 

 and ib) mercerized cellulose (cellulose II). (Reproduced from Physics and Chemistry 

 of Cellulose Fibres, by P. H. Hermans, Elsevier, 1949, by permission of the author.) 



that of the native form shows that it is different crystallographically. 

 Just as the unit cell for the native form has been obtained so here, in the 

 same way, a different unit cell has been calculated and a projection of 

 this cell on the ac plane is given in Fig. 2>%b) together with a similar 

 projection of native cellulose (Fig. 39(fl)) for comparison. This is 

 usually regarded as the stable form of cellulose, the native form being 

 metastable, chiefly on account of the ease with which native cellulose 

 can be transformed into the mercerized form and the difficulty in 

 reversing the process. It is therefore of the greatest interest that the 

 alga under discussion here appears to have cellulose in the mercerized 

 condition. The wall is closely like a sheet of cellophane, though there 

 are small differences. At the time this important discovery (46(a)) was 

 made with Halicystis it was not known whether or not this alga was 

 unique in its wall composition. We now know, however, of a whole 

 range of algae with precisely the same structure (46(Z?)). 



The X-ray patterns have, as a matter of fact, at least three components. 

 Mercerized cellulose has three characteristic arcs on the equator, 

 corresponding to spacings 7-4 A., 4-4 A. and 4-0 A. (equivalent to the 

 6-1 A., 5-4 A. and 3-9 A. of native cellulose). All three arcs are 



