28o FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



branes behave very differently from the strong secondary walls. They 

 lack microscopic lamination and iibrilization. Since they represent the 

 intermediate membrane between neighbouring cells, they consist of 

 three lamellae, viz., the original middle lamella produced from the 

 cell plate during cell division, and the two primary walls added on 

 to it. Another important point about meristematic cell walls is that 

 no cellulose can be identified microchemically in them (Tupper-Carey 

 and Priestley, 1923). Gundermann, Wergin, and Hess (1937) 

 nevertheless detected by X-rays the fibre period of cellulose in the 

 elongating cells oi Avena coleoptiles (after removal of the epidermis). 

 As their photographs show only the interferences of the lattice planes 

 perpendicular to the chain axis, evidently the cellulose strands present 

 are either poorly crystallized or the X-ray pattern is disturbed by the 

 large amount of pectic, hemicellulosic and other non-cellulosic wall 

 substances. Thimann and Bonner (1933) found by analysis 42% 

 of cellulose in dried A.vena coleoptiles but, just as in Heyn's X-ray 

 investigations (1933, 1934), this percentage includes the epidermis 

 with thickened walls (Fig. 140b, e, p. 284). Although unthickened 

 meristem walls contain less cellulose, they certainly contain an already 

 cohesive, fine framework of cellulose strands. Seeing that the cellulose 

 is masked by other constituents of the membrane (see p. 287), it is 

 particularly fortunate that its presence can be betrayed by its bire- 

 fringence. Pectins, which accompany cellulose, have only very rarely 

 been found to show birefringence in plants (Roelofsen and Kreger^ 

 1951). 



The view I advanced (1935 b) at the International Botanical Con- 

 gress held at Amsterdam, to the effect that quite young meristematic 

 cell walls already contain a submicroscopic cellulose framework, was 

 at first disputed by Hess and his co-workers, though they overlooked 

 the birefringence of these cell walls (Hess, Trogus and Wergin, 

 1936). Later, however, they admitted that cellulose can be identified 

 by X-ray after cold water extraction, since, after the removal of water- 

 soluble intermicellar substances, collective crystallization of ex- 

 ceedingly thin strands of cellulose takes place (Hess, Kiessig, Wergin 

 and Engel, 1939). 



Birefringence enables the investigator to detect when, during the 

 formation of the young membrane after the division of the cellj 

 cellulose first makes its appearance. Becker states (1934) that the so- 



