O. Biddulph 265 



Transpiration delivers to xylem extremities dissolved materials in 

 proportion to the amount of water lost from the tissue. It should be 

 remembered that the capture and metabolic use of elements from the 

 transpiration stream may continually alter the mineral composition 

 within the transpiration stream as it moves upward and, hence, the 

 mineral composition of solutions delivered to the extremities of the 

 xylem system may differ from that which begins its movement in the 



root region. 



A characteristic distribution of radiophosphorus (P 32 ) to leaves of 

 all ages on bean plants is shown in Figure 2. It can readily be seen that 

 uniformity of delivery is more characteristic of older leaves than of 

 younger ones. We have been unable to assign relative weights to the 

 two factors, transpiration and metabolic use, in the determination of 

 the quantity of P 32 which will move to a given tissue, but it can be seen 

 that the highest relative concentration invariably occurs in the region 

 of highest growth rate. A carefully controlled microclimate around 

 various meristematic tissues might aid in the further solution of this 

 problem. 



DEPOSITION OF MINERALS IN LEAVES 



So far we have assumed that no chemical or physical reactions occur 

 in the xylem system which might precipitate or absorb mineral com- 

 ponents of the transpiration stream. I wish to discuss this possibility 

 and to present positive evidence bearing on this point. Precipitation 

 reactions in the xylem would alter the composition of the mineral solu- 

 tion available at the xylem extremities. Nutritional unbalance may then 

 result. This condition could be further accentuated by a more rapid re- 

 moval of water than of minerals (all or certain ones) from the trans- 

 piration stream. 



Olsen (8) has presented some evidence that there may be a precipita- 

 tion of ferric phosphate in the veins of leaves of the corn plant under 

 certain growth conditions. This seems entirely possible. The phenome- 

 non occurs on root surfaces where the epidermal cells begin the initial 

 accumulation of minerals from the nutrient medium. Here, actively 

 metabolizing cells are acquiring materials from a solution surrounding 

 them. These materials cross the cortex and are "secreted" into the xylem 



