THE MECHANISMS OF ORIENTATION AND GROWTH 195 



have now seen, however, that this still does not take us out of the 

 morass. 



Frey-Wyssling (63(^)) has recently made use of the plasmolytic data 

 of Burstrom (63(fl)) in growing wheat roots to demonstrate an increase 

 in the extensibility of the wall during the phase of growth by vacuola- 

 tion. Taking the length of a turgid cell as (/+zl/) and of a plasmolysed 

 cell as / and calculating the tension in the wall, T, due to the internal 

 turgor pressure, he defines a value E such that 



Formally E is then analogous to Young's Modulus and Frey-Wyssling 

 does use this term. It seems, however, most improbable that the 

 tension /elongation relationship during plasmolysis is linear as the above 

 equation would suggest, and it is obviously undesirable to be precise as 

 to the definition of E. Its value does decrease most markedly during 

 the early growth phase; but rises again very steeply with no obvious 

 diminution of the rate of growth. It is therefore impossible to discern 

 in this elegant piece of work any clear correlation between growth rate 

 and tensile properties. Indeed, though a final decision must await more 

 precise data obtained in such a way as to be more readily interpreted in 

 terms of growth, it seems at the moment diflficult to avoid the con- 

 clusion that the tensile properties of the wall have very little indeed to 

 do with the regulation of growth. 



The cellulose -protein complex in growing walls 



Clearly we need to know a good deal more about the wall during 

 the growing phase, and some attention has been given to this problem. 

 Let us return for a few moments to the X-ray investigation of cambial 

 cells and oat coleoptiles briefly described above. In untreated cells it is 

 difficult to make out any arcs on the X-ray diagram corresponding to 

 the 5-4 and 6-1 A. spacings so typical of the normal cellulose diagram, 

 and even the arc corresponding to 3-9 A. is very diffuse. If, however, 

 pectin or protein or even water are removed from the wall then the 

 diagram becomes much sharper, and an arc appears at 5-5 A. which 

 probably corresponds to a fused 5-4, 6-1 A. reflection. This can mean 

 only that the ceUulose in the wall is associated closely with aU these 

 substances, and recalls in particular the chemical evidence, supported 

 by staining reactions, that proteins are present in growing walls. From 

 this it is an easy step to the conception of the wall at this stage, not as 

 an enclosing sheath which reacts passively to stimuH from within the 



13* 



