O. Biddulph 273 



The ultimate fate of downward moving phosphorus in the phloem, 

 if it is not used metabolically, is undoubtedly to move up again through 

 the xylem. The rapidly moving transpiration stream serves as a vehicle 

 for transport of those ions which reach it by diffusion across adjoining 

 tissue (6, 7). Biddulph and Markle (■?) have shown a slight relative 



Age of plants 



12 days 



8 days 



5th. alt. leaf 

 4th. alt. leaf 

 3rd. olt. leaf 

 2nd. alt. leaf 

 1st. alt. leaf 

 Stem 



Opposite leaves 

 Roots 



10,000 10,000 



P 32 in Counts/min./ q. fresh tissue 



Figure 7. Movement of radiophosphorus into parts 

 of bean plants. The plants to the left of the vertical line 

 received P 32 between the fourth and eighth days of 

 growth; they were then harvested. The plants to the 

 right of the line received P 32 between the fourth and 

 eighth days, but none from the eighth to the twelfth day; 

 they were harvested on the twelfth day. The occurrence 

 of P 32 in the third, fourth, and fifth leaves is at the ex- 

 pense of that formerly present in the older organs. 



