V CELL DIVISION 775 



crown-gall bacteria produces no tumors. On the other hand, cell division must 

 first occur in order for normal cells to become tumorigenic (Braun, 1953). Con- 

 tact (3 days) with vascular tissue provided enough of a cell-division factor to 

 allow division and the bacteria alone now produced a tumor. But if NAA (i mg/1) 

 and an extract of tumors, or pure kinetin was supplied, then the tissue without 

 inoculation grew into a tumor with "striking morphological and histological 

 resemblance to crown gall" (Braun, 1956). The inference that the kinetin pro- 

 vides the same stimulus as the bacteria, and that this is synthesized by growing 

 tumors, is highly attractive. On this basis the "accoutume" tissue cultures of 

 Gautheret, which can grow without auxin, may be considered to form the neces- 

 sary traces of kinetin and to have adaptively acquired the capacity to synthesize 

 auxin, while crown-gall, in contrast, which can evidently form the necessary 

 traces of auxin may be considered to have been influenced to synthesize kinetin. 

 In a parallel way, cell-free extracts of crown-gall bacteria have caused carrot 

 tissue, treated with avixin, to produce tumors (Klein and Knupp, 1957) but 

 here the effective substance has not yet been identified. 



VI. CELL ENLARGEMENT 



(a) The elongation of tissues in an axis 



Since the minimum working definition of growth (see section II) is Irreversible 

 Increase in Volume, it follows that it is cell enlargement which fulfills the primary 

 requirements of^ growth. Usually enlargement occurs directly after cell division, so 

 that the small cells of a rapidly-dividing meristem either elongate in the direction 

 of the axis (in a shoot or root), widen perpendicular to it (in the cambium), or 

 enlarge isodiametrically (in pith, in tubers or in fruits). 



When an axis elongates in this way, there is no special reason to believe that all 

 cells respond at the same rate to the natural supply of auxins and other factors, 

 nor that they inherently would elongate to the same extent. The firmness and con- 

 tinuity of the cell-wall holds them together and thus imposes a uniformity in the 

 tissue which is very likely not present in the individual cells. This situation leads 

 to the development of tissue tensions, or mechanical strains, in growing organs, 

 which have been known since the time of Hofmeister in the i88o's. Hollow stalks 

 of dandelions, daffodils or onions show these very strongly; if they are slit length- 

 wise when growing, the two halves roll up, because of the tension in the epidermis 

 or outer cell layers. The opposite reaction is shown by pithy organs like the petioles 

 of rhubarb in which, when the outer layers are removed, the central tissues swell 

 and elongate, showing that they had been under compression. 



These tensions became prominent again when it was found that auxins cause 

 an inward curvature in slit stems {cf. section Illb). In water alone, outward cur- 

 vature occurs due to the tissue tension; in auxin solution, these same outer layers 

 are caused to elongate more than the inner ones and hence an inward curvature 

 results. Peeling off the epidermis, by removing one of the most responsive layers, 

 greatly reduces the curvatures. Extensive analyses (Thimann and Schneider, 1938; 

 Jost, 1938; Diehl et al., 1939; Went, 1939; Schneider, 1942) show that the 

 curvature is not due to differential auxin uptake on the two sides, but mainly to 



Literature p. 8i 6 



