274 Mineral Nutrition of Plants 



gain in P 3 ~ within the xylem in ascending the stem from base to tip in 

 cases where the radioisotope entered the stem from an injected leaf. 



The status of re-export of mineral nutrients from leaves via the 

 phloem system is that of an equalizing mechanism which comes into 

 play when the transpirational factor in mineral distribution results in 

 an "overshooting" of the requirement for metabolic use within the 

 the particular tissue or organ, or when an impaired flow from the 

 roots renders the meristems dependent on the existing supply within 

 the plant organs. Movement of minerals in the phloem system makes 

 it possible for the meristems to continue to receive and use minerals 

 from other parts of the plant, withdrawing them where solubility and 

 mobility permit. Elements like phosphorus, whose deficiency symptoms 

 occur first in the lower leaves, appear to be freely mobile in the phloem 

 and, consequently, the older tissues may continue to furnish the ele- 

 ment to the meristem until they become depleted to a level below that 

 required for normal metabolic functions. Conversely, the meristems 

 are among the first tissues to suffer deficiencies when calcium, boron, 

 iron, and perhaps others are involved. This means that withdrawal 

 from older mature tissues does not constitute an important source of 

 these elements to the meristems. In other words, these elements are 

 not mobilized and translocated fast enough to avert a disaster in the 

 meristems. 



The study of translocation of minerals in plants is indeed in its 

 infancy. It is an important study and should merit careful attention, 

 for, frequently, it would appear that symptoms of malnutrition are as 

 often due to failures in the distribution system as to failures in the 

 initial absorption. Chlorosis is certainly in this category and in eco- 

 nomic importance it ranks high among those factors responsible for 

 decreased yield of economic plants. Radioactive and separated stable 

 isotopes furnish tools which are extremely versatile and offer unique 

 advantages in the study of translocation problems. Likewise, the bio- 

 chemical processes making up the over-all metabolic patterns can be 

 studied piecemeal by means of these tools until they yield the secrets 

 of the forces which obtain in the orderly sequence of events constitut- 

 ing the life process. 



