Mechanics of Auxin-induced Growth 



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STRETCH, 7o OF LENGTH 



Fig. 9. Reorientation of cellulose microfibrils of Avena coleoptiles (one-half coleop- 

 tiles, epidermis removed) during longitudinal stretching as a function of prior 

 (2 hrs.) lAA treatment. Path difference under polarized light measured with 

 Senarmont compensator. After Bonner (5). 



shown in Figure 9, which also contrasts the behavior of lAA-treated 

 and nontreated sections. It is clear that the reorientation of the wall 

 microfibrils in response to shear is dramatically decreased by the pres- 

 ence of lAA. This effect is not an artifact of lAA-induced growth since 

 it occurs equally strikingly in lAA-treated but nongrowing sections. 



How are we to understand the effect of lAA treatment upon the 

 response of the wall microfibrillar network to shear? Evidently the 

 microfibrils reorient in the direction of shear because they interact, 

 stick to one another here and there. In lAA-treated tissue this inter- 

 action is decreased and ability of the microfibrils to slide past one an- 

 other correspondingly increased. lAA treatment of Avena coleoptile 

 sections results in but slight effects on cellulose (Boroughs and Bon- 

 ner, 9; Ordin et al, 21, 22) . However, the facts available suggest that 

 the pectic material of the wall may constitute the glue through which 

 the microfibrils interact. 



