The Cellular Basis of Growth 41 



it grow independently of this? Burstrom ( 1957 and earlier papers ) pre- 

 sents evidence that cell elongation is not primarily a matter of water 

 uptake but is due to growth of the cell wall. He believes that this occurs 

 in two steps, the first a plastic stretching of the wall and the second the 

 production and deposition of new wall material. Auxin promotes the first 

 but probably inhibits the second. Others believe that auxin, known to 

 stimulate cell enlargement, directly increases the plasticity of the wall and 

 thus its extensibility (Heyn, 1940). This view has found recent support 

 (p. 412). 



It obviously is necessary to know just how the wall grows and particu- 

 larly whether this is by apposition of new material on its inner face or by 

 intussusception throughout. Green (1958), using techniques for measur- 

 ing radioactivity, treated elongating Nitelh cells with tritium (H H ) and 

 found by test that the inner part of the wall became radioactive. The 

 outer portion, which was not, grew thinner as the cell lengthened, thus 

 suggesting that new cell wall material was being laid down only on the 

 inside and not throughout. 



Rate of wall thickening sometimes has a direct effect on plant size. Thus 

 in a dwarf mutant of Aquilegia (Anderson and Abbe, 1933) this trait was 

 found to be due to the precocious thickening of its cell walls (p. 426). 



The important problem of the relation of deoxyribonucleic acid to 

 cell division has often been investigated. Grundmann and Marquardt 

 ( 1953) determined the content of DNA in successive phases in the mitotic 

 cycle of the nuclei of periblem cells of the root tip of Vicia. This increases 

 steadily throughout the interphase. It is reduced at telophase since it is 

 roughly proportional to nuclear volume. 



Brown and his students have used various modern techniques for a 

 study of the problems of the multiplication and growth of cells. Brown 

 and Rickless (1949), for example, cut off Cucurbita root tips of equal 

 length (1.6 mm.) and grew them in culture for 3 days, taking samples 

 every 12 hours. These tips were macerated, and in a haemacytometer the 

 total number of cells and the number of nonvacuolate cells were counted. 

 From these counts, together with a measurement of the length of each 

 root examined, it was possible to determine the rate of cell division and 

 the index of extension ( ratio of root length to number of vacuolate cells ) . 

 This method is subject to a number of errors, particularly from the as- 

 sumption that only the nonvacuolate cells were dividing. However, it 

 gave consistent results, and these were in general agreement with the 

 more laborious method of measuring cell size and volume from microtome 

 sections. The authors found that there was no division in the absence of 

 sugar in the culture medium and that the rate of division increased with 

 the addition of sugar and inorganic salts and even more with the addi- 

 tion of yeast extract. At 15° C the rate of division was higher than at 5 or 



