140 Alma Howard and S. R. Pelc 



are nucleic acid, phosphoprotein, and some phospholipid. 

 The one most probably present in quantity is DNA. Squashes 

 were subjected to a solution of deoxyribonuclease.* These 

 squashes showed no nuclear autoradiograph, while an auto- 

 radiograph could be obtained over the nuclei of cells subjected 

 in the same way to a boiled solution of the enzyme. We 

 therefore conclude that the fixation, hydrolysis and washing 

 of the tissue has removed from the cell nuclei all phosphorus 

 compounds present in sufficient amounts to give an autoradio- 

 graph except the DNA, and that the presence of an autoradio- 

 graph over a nucleus means that ^^P is present in the nucleus 

 in the form of DNA. This could occur either throufifh the 

 synthesis of new DNA since the beginning of ^^P treatment, 

 or through the exchange of stable for radioactive P. As is 

 discussed below, the turnover of P in non-dividing nuclei is 

 very low indeed, and we therefore consider that a nucleus 

 which shows an autoradiograph has synthesized DNA during 

 the period of treatment. 



Observations 



^^P was given to roots over time periods of 2, 4, 6, 9, 12, 16, 

 20, 24, 36 and 48 hours. Squashes were then examined and 

 the percentage of dividing and non-dividing meristematic 

 cells which show autoradiograph was determined. The 

 results are expressed in Fig. 2. The number of non-dividing 

 nuclei which show autoradiograph rises from 20 per cent at 

 2 hours to above 90 per cent at 48 hours. Dividing cells 

 show no autoradiograph at 2 or 4 hours. At 6 and 9 hours 

 the figure is about 10 per cent, and this value rises steeply 

 to 100 per cent at about 16 hours. Thus there is a lag of 

 about 6 hours between the appearance of positive non-dividing 

 and positive dividing cells (Fig. 3). If the dividing cell class 

 is broken down into stages of division, we see a few positive 

 prophases appearing first, and at later times, metaphases and 

 finally anaphases and telophases (Fig. 4) which are positive. 



♦We are indebted to Dr. Barbara Holmes for a preparation of the enzyme. 



