VI CELL ENLARGEMENT 789 



evidently change their geometry so greatly in auxins that data expressed only per 

 unit length mean little. 



The remarkable change in the relation between growth and respiration in potato 

 slices deserves special mention. When freshly cut, the respiration of this tissue is 

 inhibited by CO and HCN, the former inhibition being controlled by the CO : O2 

 ratio and reversed by light. Thus the respiration, like the growth, is mediated main- 

 ly by cytochrome oxidase. But 24 h. later (at 25°, or 4 days later if the sections are 

 kept at 14°) the respiration rate has spontaneously increased by 300-400%, and 

 now it is wholly insensitive to carbon monoxide and only slightly inhibited by 

 cyanide. This new CO-insensitive oxidase has an oxygen affinity similar to that 

 of cytochrome oxidase, and may therefore be a modified cytochrome system. Yet 

 the cell enlargement remains light-reversibly CO-sensitive, i.e. controlled by a 

 typical cytochrome oxidase, throughout (Thimann et al., 1954). The mitochon- 

 dria, in which the cytochrome oxidase is localized, do not seem to have significantly 

 changed their sensitivity to inhibitors (Hackett, 1956; Hackett and Haas, 1958). 

 The change in respiration does not take place if the tissues are kept in Nj and 

 thus it may perhaps be connected with the formation of a new protein. 



(ii) Substrates. In most cases the substrates used in the metabolism that under- 

 lies cell enlargement can only be inferred. Tubers, as was mentioned, are rich in 

 stored carbohydrates, and the often observed RQofi.o which these tissues show is 

 a good indication that carbohydrate is their main respiratory substrate. The 

 marked decrease in starch in potato disks during cell enlargement, which has been 

 observed by several workers since Reinders' experiments in 1942, fits in with this 

 finding. There is some indication that citrate promotes cell enlargement in arti- 

 choke disks (Hanson and Bonner, 1955) but no real study of the substrates for 

 growth in tubers has been made. 



The growth substrate is consumed in metabolism in two ways: (a) for energy 

 through the formation of adenosinetriphosphate (ATP) or similar materials, 

 (b) for polysaccharides to be laid down as new cell wall. During growth new wall 

 is laid down approximately proportional to the elongation, that is, the weight of 

 wall material per unit length is the same in isolated sections growing in auxin 

 solution as in comparable sections on the plant. Its composition (cf. the analyses in 

 Table i ) indicates content of several different polysaccharides and polyuronides. 



The metabolism of pea stems during growth, however, is somewhat unexpected, 

 since sugar does not appear to be the main svibstrate. Analyses show that after 

 24 h. growth the reducing sugar content has fallen from the initial 17*70 to about 

 12% (of the dry weight), but the important point is that the decrease is just as 

 great in water as in auxin, although the elongation is only a third as much 

 (Christiansen and Thimann, 1950b). The explanation of this curious fact came 

 from the observation that in presence of arsenite, fluoride or iodoacetate, in which 

 the growth and respiration were inhibited, the reducing sugar did not accumulate 

 but decreased even more. Changes in sucrose were too small to balance this. 

 Evidently, therefore, the reducing sugar is not merely a reservoir of metabolizable 

 material, but itself a product of metabolic processes. The respiratory quotient, 

 initially i.o, falls quickly to about 0.84, suggesting fat oxidation, and indeed direct 

 analysis shows that the fat content does decrease during growth, from 9% initially 



Literature j>. 8i6 



