774 PLANT GROWTH lO 



(doubtless in presence of traces of natural auxin) or at very low levels of lAA, 

 produces large numbers of buds, and at higher concentrations the combination 

 causes massive cell division to form unorganized callus (Strong, 1958). 



In these experiments a clear distinction can be drawn between nuclear division 

 and cell division (mitosis and cytokinesis). Tobacco pith treated with auxin alone 

 exhibits a few mitoses (Naylor et al., 1954) which may perhaps be due to the pres- 

 ence of small amounts of natural kinins; these mitoses often do not lead to cell 

 division, and as a result binucleate and polyploid cells were found. On the other 

 hand, kinetin alone, in a wide range of concentrations, produced no mitoses 

 (Das et al., 1956). The combination (kinetin 0.05-1.6 mg/1, lAA 0.2-2.0 rng/l) 

 causes onset of cell division after 1-2 days, followed by a pause which is ascribed 

 to the fact that all the cells ready to divide had been stimulated into doing so, 

 while others had to "get ready." After another day or two, divisions occurred at 

 a more or less constant rate, 2-3 % of the cells being observed in mitosis at any 

 one time. Practically all these mitoses were followed by cell division. Data not yet 

 presented in detail show that either auxin or kinetin can cause an increase in DNA 

 in the nuclei, but it is the subsequent chromosome apparatus and cytokinesis 

 which requires both factors. 



Skoog and Tsui had earlier shown (1951) that adenine exerts the same effects 

 on bud formation, but kinetin is active in concentrations about 1000 times as low 

 as those of adenine. Several synthetic analogs, including especially 6-benzylamino- 

 purine, are almost as active as kinetin. Perhaps, therefore, the observed small 

 activity of adenine and other purines may be due to their conversion in vivo to a 

 kinin. As stated in section II, the class of kinins, which includes kinetin, are con- 

 sidered to be primarily cell-division-promoting substances. Whether their action 

 is limited to division is, however, not yet clear. Leaf sections floated on kinetin 

 solution are caused to grow in thickness, as was long ago noted with adenine, but 

 the important point here is that the growth results only from cell enlargement 

 (Kuraishi and Okumura, 1956). A more complex process is that in which kinetin 

 causes the elongation of buds that were inhibited by auxin (see section Vlld). 

 This involves both division and enlargement. It is possible that kinetin, in con- 

 junction with auxin, stimulates the necessary cell division, and that the subsequent 

 enlargement of the newly divided cells is dependent upon auxin; however, it is 

 also possible, and perhaps equally probable, that both substances act on some 

 process in the cell at the molecular level (such as the DNA synthesis mentioned 

 above) and that the subsecjuent effects may or may not go beyond cell division, 

 depending on what other factors are limiting. 



Interaction between auxin and kinetin may explain the production of tumors 

 by crown-gall bacteria. On the one hand, auxin is essential, since many years ago 

 inactive strains of bacteria were made tumorigenic by supplying lAA (see review 

 of Riker et al., 1946) and, more recently, ionizing radiations have been shown to 

 reduce growth of crown galls while added auxin reinstates it again (Klein and 

 Vogel, 1956). For example, 1500 r of X-rays reduced the tumor diameter to 8% of 

 the control, while in presence of lAA it was 80% of the control. Neutrons and gam- 

 ma rays had similar effects. Ionizing radiations are known to inactivate lAA both 

 in vivo and in vitro. Isolated tobacco pith tissue, without auxin, inoculated with 



