320 ]. Bonner 



absence of lAA, and analytical determinations were made by an ap- 

 propriate, sensitive, and specific method (Albersheim and Bonner, 1) 

 of the initial and final amount of pectic material in each cell wall 

 fraction. In the second type of experiment, sections were incubated 

 for 5 hrs. in C^^-labeled glucose, and the amount and specific radio- 

 activity of the pectic galacturonic acid of each fraction were deter- 

 mined. It is clear that in the presence of lAA the rate of formation 

 of cold water soluble pectin is increased twofold or more and that 

 of hot water soluble pectin 60 per cent, while the rate of synthesis of 

 residual pectin is but little influenced. 



The effect of lAA on pectic metabolism is, then, to increase rate 

 of production of the more soluble pectic molecules. One can feel in- 

 tuitively that this should in some way increase cell wall plasticity but 

 as yet no rigorous demonstration of how it does so has been achieved. 



EFFECT OF AUXIN ON INTERACTION OF CELL WALL 



COMPONENTS 



The walls of the parenchymatous cells of the Avena coleoptile 

 possess, in common with the primary walls of other cylindrical plant 

 cells, a pattern of arrangement of cellulose microfibrils known as tube 

 structure. The cellulose microfibrils are disposed in a manner pre- 

 dominantly normal to the long axis of the cell and thus in a 'barrel- 

 hoop" fashion. That this is so may be readily observed by electron 

 microscopy (Miihlethaler, 19), although it was first deduced on the 

 basis of birefringence measurements (Bonner, 5). Such measmements 

 enable one to draw conclusions as to the orientation of cellulose micro- 

 fibrils within the wall since the larger index of refraction of cellulose 

 lies parallel to the long axis of the microfibril. It is characteristic of 

 the microfibrillar network that, when stretched, the microfibrils tend 

 to align themselves in the direction of stretch. The amount of stretch- 

 ing required to elicit a given degree of reorientation is a measiue of 

 the interaction between the units of the network as has been shown 

 for model systems (Bonner, 5). It is of interest that lAA treatment ex- 

 erts a profound effect on the tendency of the cell wall microfibrils of 

 Avena coleoptiles to reorient in response to mechanical shear. This has 

 been demonstrated by the following general technique: coleoptile sec- 

 tions (from which the epidermis had been stripped) were plasmolyzed 

 in glycerine, clamped at the two ends, and stretched longitudinally 

 under a polarizing microscope. As the tissue is stretched, the initial 

 negative anisotropy (microfibrils statistically at right angles to the 

 shear axis) diminishes, becomes null (statistical isotropy), and finally 

 becomes positive (microfibrils statistically parallel to the shear axis). 

 The course of micronbrillar reorientation whh increasing stretch is 



