144 Alma Howard and S. R. Pelc 



These observations are interpreted to mean that DNA is 

 synthesized by those cells which are preparing for a further 

 division, but is not synthesized by cells which will differentiate 

 without further division. A cell which synthesizes DNA 

 transmits this to its daughter nuclei, which then retain it. 



The fact that most cells in the non-mitotic upper parts of 

 the root do not show autoradiograph after 48 hours in ^-P 

 means that the exchange rate of phosphorus between DNA 

 in cell nuclei and other phosphorus compounds in the cell is 

 very low indeed. The autoradiographs used in this work are 

 of such a sensitivity that a nucleus containing as few as 50 

 atoms of tracer which disintegrated during exposure would be 

 counted as positive. 



II. Nuclear Uptake of ^^S 



The methods of culture of roots, and the fixation, hydrolysis 

 and autoradiographing of cells, were the same as those 

 described for the previous experiments. ^^S was given as 

 sodium sulphate, at 1 fiC. per cc, without carrier, for 20 hours. 

 Autoradiographs appeared over a proportion of non-dividing 

 nuclei (Fig. 7) and over dividing cells. In the latter, the auto- 

 radiograph was associated with the chromosomes (Fig. 8). 



The compound in which ^^S exists in these cells is not known 

 to us. The treatment of the tissue before autoradiography 

 can be expected to have removed all the inorganic and acid- 

 soluble sulphur compounds, including the histones. Since 

 the autoradiograph is seen over some nuclei only, and not 

 over all (Fig. 7), it is unlikely to be due to ^^S which has been 

 exchanged for stable sulphur in compounds present in all 

 cells. For the same reason, it is unlikely to be due to proteins 

 which have been precipitated on to nuclear structures. A 

 reasonable possibility thus exists that the autoradiographs 

 are due to ^^S in the form of nucleoprotein which has been 

 synthesized during the 20 hours of treatment. The likelihood 

 that this is true is increased by the fact that the percentage 

 of cells of different classes which show nuclear autoradiograph 

 after 20 hours in ^^S is similar to the percentage which show 



