HANS BURSTROM 45 



to the cell elongatfbn. Of the mechanical properties of the wall the 

 reversible, elastic extensibility can be determined with a fair degree 

 of accuracy. Experiences with both roots and coleoptiles have taught 

 that the elasticity always increases at the start of the cell stretching 

 but decreases again before the cells have attained their full size. Figure 

 I shows how, in a root, the elastic tension as measured per cell increases 

 rapidly at the start to a maximum value, which is maintained during 

 the main part of the elongation. Relative to the increasing cell length 

 this means a decreasing tension of the cell walls. This increasing elasticity 

 cannot cause the elongation, but it signifies that there are changes in 

 the cell wall at a very early stage of elongation, which undoubtedly 

 are connected with the elongation process. 



These observations on the osmotic and wall properties of the cells 

 have led to the assumption that cell elongation proceeds in two phases, 

 which may be rather sharply distinguished from one another and during 

 which different conditions prevail. The first phase involves an increasing 

 elasticity of the wall, which has been explained on the basis of a loosening 

 of the joints between the micellae (7). The second phase is characterized 

 by a hardening of the wall, as evidenced by a decreasing elasticity. During 

 the second phase there is, further, a very rapid supply of nutrients to 

 the cell so that the osmotic concentration does not decrease in spite of 

 the rapid increase in cell volume. 



Similar results had earlier been obtained by Ruge (i i) with coleoptiles, 

 and he has likewise concluded that the growth proceeds in two steps, 

 the first one involving stretching of the cell without synthesis of new 

 wall materials but with an increasing extensibiUty of the walls. The 

 consequence is that during this part of the elongation the wall becomes 

 thinner in proportion to the increasing cell surface, and other observa- 

 tions (6,12) have verified that such a spreading of the wall material 

 takes place during the early part of the elongation. During the second 

 phase there is a deposition of new wall material, undoubtedly through 

 intussusception, probably also through apposition. This results in a 

 concomitant hardening of the cell wall. It seems probable that Ruge's 

 two phases for coleoptiles are identical with those shown to exist also in 

 roots, and that consequently shoots and roots behave similarly. 



This distinction between the two phases has been further supported 

 by the fact that auxins affect them in different ways (3). The first phase 

 is accelerated so that elongation starts earlier, and the second phase is 



