Winifred E. Brenchley 241 



a few series, however, the water culture solutions were sampled at 

 intervals and the nitrogen present estimated as potassium nitrate. These 

 figures indicate the amount of KNO3 that had disappeared from the 

 solutions in a given time and enable some idea to be formed of the 

 amount of nitrate taken up and utilised by the plant in the production 

 of varying weights of dry matter. The great objection to this method of 

 estimating the nitrogen absorption is that the amount of nitrogen that 

 is lost by decomposition or denitrification is probably a variable un- 

 known quantity for which it is difficult to devise a method of determina- 

 tion. Tests made at various times with solutions allowed to remain in 

 the bottles for some days or weeks without any plants growing in them 

 seem to show a slight loss of nitrate, though this is not very considerable 

 even after the lapse of some time. The figures obtained in the present 

 experiments are probably sufficiently accurate to give a true indication 

 of the trend of affairs even though the actual quantitative measure they 

 represent cannot be fully accepted. 



Analyses of the solutions for KNO3 were made at various times in 

 the unchanged series L and N and the changed series K and M. When 

 the solutions were unchanged the KNO3 available throughout the life 

 of the plant was only -6 gm. per plant and by the time the experiments 

 were finished very little or none of this remained in the solutions. The 

 results obtained in these cases are summarised in Table I, in which 

 the figures apply to the unit of ten plants, grown in six litres of 

 solution. 



Table I (Solutions not changed). 



The ratio column of Table I shows clearly that as the pea plant gets 

 older the increase in dry matter produced becomes less dependent upon 

 the amount of nitrate absorbed by the roots, and that even when very 

 little intake is possible owing to exhaustion of the food supplies some 



