264 Mineral Nutrition of Plants 



transfer across the cortex to the cells surrounding the xylem. Here, in 

 a process "akin to secretion," according to Hoagland (4), the minerals 

 enter the xylem system wherein they ascend to the aerial extremities of 

 the plant. This process of secretion, if it may be so called, is one of the 

 least known of all processes in the realm of mineral translocation. Its 

 elucidation undoubtedly must await a fuller understanding of the 

 absorption process. Since so little is known of this "secretion" process 

 as it applies to translocation, it will not be discussed here. 



The delivery of mineral elements to the leaves of mature plants most 

 certainly occurs primarily through the xylem tissues. Stout and Hoag- 

 land (9) have contributed the most direct evidence bearing on this 

 point. In their experiments the phloem was isolated from the xylem by 

 the insertion of waxed paper between the tissues but the paper left the 

 longitudinal continuity of both elements intact. Radioactive nutrient 

 elements were then placed in the root environment and the path of 

 these elements was followed by the radiation emitted as the nutrients 

 ascended the stem. At the place where xylem and phloem had been 

 separated a preference of paths was shown: namely, most, if not all, 

 movement in an upward direction occurred in the xylem. This experi- 

 ment seems direct and conclusive for conditions where at least moderate 

 transpiration exists. 



The fate of minerals as they are swept upward by a rapidly moving 

 transpiration stream might be varied according to the mineral element, 

 the plant, and the conditions existing within the plant at the moment. 

 There are several possibilities, enumerated as follows: 1) a portion of 

 the material will be captured by the cells adjacent to the xylem, in 

 particular the cambium and young phloem; 2) a portion may move 

 laterally via rays and the like to actively metabolizing cells; 3) a por- 

 tion may be deposited in the leaves having moved there via the transpi- 

 ration stream; or 4) a portion may move directly to the apical primordia 

 and adjacent regions of active metabolism. 



In a general sense there are two basic phenomena which influence 

 the direction of movement of minerals within a plant. These are 

 metabolic use and transpiration. The intensity of these factors in any 

 tissue will determine the net movement to the tissue. The metabolic 

 use of an element establishes gradients responsible for a continued flow. 



