282 



FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES 



III 



cellulose framework of a wall of this kind is illustrated in diagram 

 by Fig. 139, as derived from the birefringence and iodine dichroism 

 o^ Euphorbia latex tubes (Frey-Wyssling, 1942). 



It should be borne in mind that with tubular texture the cell wall 



is negative with respect to the cell axis. When 

 elongating tissues are stretched by mechanical 

 means, the birefringence of their cells changes 

 and becomes positive (Bonner, 1935); but if 

 they extend through growth they remain nega- 

 tively birefringent. We have to ask ourselves, 

 therefore, why the cell wall optics of artificial 

 and natural extension should be opposite. 



The electron microscopy of primary cell walls 

 has disclosed a texture of cellulose strands al- 

 most identical with the diagram of Fig. 139 

 drawn on the basis of indirect methods (Frey- 

 Wyssling, Muhlethaler and Wyckoff, 1948; 

 MiJHLETHALER, 1950a). The Strands are the same 

 as the microfibrils observed in secondary cell 

 walls (p. 105); their diameters are almost iden- 

 tical. This had not been expected, because the 

 fraction of cellulose is only a very small portion of the total amount 

 of wall substances in primary walls (Table XXV, p. 287). The microfi- 

 brils form systems which cross at different angles, but mostly so that 

 an angle smaller than 90° points in the transverse direction of the cell. 

 This causes the optical negative reaction in the polarizing microscope. 

 A new fact, however, was also found, viz., that the microfibrils are not 

 stratified in superposed planes but are interwoven, just as in a textile 

 fabric. This is the reason why primary walls do not show any lami- 

 nation and cannot be broken down into fibrils. On the other hand, 

 the question arises as to how such a woven texture can grow in area. 

 An investigation into the surface growth of these membranes has 

 therefore been started. 



Plant cytology distinguishes two different types of cell elongation, 



termed tip growth and cell extension. Tip growth is considered to consist 



in the addition of new areas to the existing wall at the distal cell end, 



such as in elongating root hairs, cotton hairs, pollen tubes, fungal 



^ k absorption coefficient, n refrative index. 



Fig. 139. Tubular texture 

 of latex tubes (from 

 Frey-Wyssling, 1942)^. 



