HANS BURSTROM 55 



too incomplete a knowledge of the physical properties of the cell wall 

 to be entitled to draw any definite conclusions, but it seems probable 

 that even if some changes occur in the wall properties, nevertheless 

 during the elongation these properties remain remarkably constant. 



Another complication in the evaluation of all data on the mechanical 

 properties of the cell walls is the fact that in several instances the elonga- 

 tion does not take place uniformly over the whole cell surface. Apical 

 growth is a well-known feature of many cells, and the differential growth 

 reported by Sinnott is probably a widespread phenomenon in ordinary 

 tissues. The consequence is that the walls of growing cells are also 

 heterogeneous with respect to their mechanical properties. In the case 

 of root-hair cells the basal parts may be elongated on an average 9 per 

 cent under a given turgor pressure whereas the apical parts will be 

 changed as much as 25 per cent. This shows that the two ends of one 

 growing cell must have different structure. Thus, average figures for 

 whole cells do not show the properties of the restricted areas where 

 growth takes place at any given moment. 



The conclusion to be drawn at present from available data is, however, 

 that we cannot abandon the old idea that the fundamental principle 

 of cell elongation is an organized, active growth of the wall. It is also 

 obvious, however, that the experimental data concerning the cell-wall 

 properties are too meager to permit more than a vague hypothesis of 

 what takes place during the cell stretching. Nevertheless, any theory 

 of the cell elongation and hormone action in growth must take into 

 account the active formation of the wall. 



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1. AuDUs, L. I., Biol. Rev., 24:51 (1949). 



2. BuRSTROM, H., Ann. Agric. Coll. Sweden, 10:1 (1942). 



3. , ibid., 209 (1942). 



4. , Fysiografiska Sdllskapets Vbrhandl. (Lund), 17:1 (1947). 



5. , Physiol. Flantarum, 1:57 (1948). 



6. DiEHL, I. M., GoRTER, C. I-, VAN Iterson, G. Ir., and Kleinhoonte, a., 



Rec. trav. bot. neerland., 36:709 (1939). 



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9. Kopp, M., Schweiz. Berichte Bot. Ges., 58:283 (1948). 



10. PoHL, R., Planta, 36:230 (1948). 



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13. Tamiya, H,, Cytologia, 8:542 (1937). 



