186 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



workers and the work of Bonner (77) may be taken as an example. 

 Bonner observed the phase difference of half coleoptiles, obtained by 

 splitting the coleoptile in two longitudinally and removing the epidermis. 

 Since the sign of the phase difference was such that the m.e.p. must he 

 transversely, he concluded that the cellulose chains in individual walls 

 must also he in this direction. Reference back to Fig. 31 wiU show that 

 this conclusion would not be justified even if single cells had been 

 examined (since both front and back walls are involved), and at the time 

 the author cast doubt on the vahdity of this interpretation by showing 

 that, if individual walls are examined, then the m.e.p. lies in a slow 

 spiral. Now, however, we know from X-ray analysis that the angle of 

 the cellulose chains to the transverse in coleoptiles between 2 and 3 cm. 

 long is not greater than 16°, the transverse orientation can be accepted 

 as an approximation. The coleoptile grows from a length of 1 cm. to a 

 length of about 3 cm. in light (or about 6 cm. in darkness, see Fig. 3, 

 p. 15), solely by the increase in length of the individual cells due to 

 absorption of water. Bonner showed that during this process the phase 

 difference of half coleoptiles remained constant with the m.e.p. trans- 

 verse, even after the cells had extended some 100%. He interpreted 

 this as implying that the constituent cellulose chains also remained 

 transversely oriented. Further investigation showed that if the halved 

 coleoptiles were stretched mechanically by only 9%, then the m.e.p. 

 became longitudinal and he took it that a reorientation had occurred 

 in the direction of stretch. He therefore concluded that no re- 

 orientation occurred during growth and that growth of a wall is some- 

 thing different from passive extension under mechanical forces. 



This orientation story, however, leaves much to be desired. The 

 maintenance of transverse m.e.p. during growth might well have meant 

 only that the angle between the chains and the transverse never exceeds 

 45°; and one can hardly imagine that mechanical extension of only 9% 

 could possibly change the orientation from almost transverse to almost 

 longitudinal. It seems much more Hkely that this latter effect is to be 

 ascribed to photoelastic phenomena, resembling the birefringence 

 induced in glass by straining, but nevertheless his general conclusion 

 still stands. If stretched walls show photoelastic effects while growing 

 walls do not, then growth can hardly be regarded as passive elongation 

 under tension. These observations, and several others Uke them, do 

 therefore indicate most clearly that the processes of growth must be 

 such as to throw little strain on the wall and therefore to lead to little 

 reorientation of the cellulose from the transverse direction. 



Further evidence can now be adduced from the studies of the algae 



