SPECIFIC ROLES OF MINERAL ELEMENTS IN PLANTS 421 



root nodule development in legumes, while a deficiency of this element has 

 been reported to have a retarding effect on cell division and fruiting in some 

 species. 



Phosphorus. — A very large proportion (often over 50 per cent) of the 

 phosphorus in a mature plant is located in the fruits and the seeds. Phos- 

 phorus apparently is readily redistributed in plants from one organ to another. 

 Young growing tissues, therefore, may gain in phosphorus content at the 

 expense of older tissues. Such redistributions are more likely to occur when 

 the supply of phosphorus in the soil is somewhat deficient. 



Unlike nitrogen and sulfur, phosphorus is not reduced in plants but is 

 linked into organic combinations in a highly oxidized form. Phosphorus 

 enters into the composition of lecithin and other phospholipids and of the 

 nucleic acids. The nucleoproteins are synthesized by chemical combination 

 of nucleic acids with proteins (Chap. XXVI). Deficiency of phosphorus in- 

 terferes with the synthesis of these compounds and hence may interrupt nor- 

 mal cell division in meristematic tissues. Phosphates act as the co-enzyme of 

 zymase (Chap. XXX) and seem to exert an accelerating effect on other 

 oxidizing and reducing enzymes. There is also some evidence that phosphorus 

 is necessary for hydrolytic transformations of carbohydrates in plants, par- 

 ticularly the change from starch to sugars. 



In general phosphorus is most abundant in young, meristematic cells, and 

 is utilized in considerable quantities in such growing regions in the forma- 

 tion of nucleoproteins and other phosphorus-containing compounds. Phos- 

 phorus is also used in considerable quantities during the period of maturation 

 of fruits and seeds. The phosphorus requirements of annual plants are there- 

 fore relatively heavy during the first few weeks after germination, and again 

 near the end of their life cycle. 



Addition of phosphates to the soil often favors root development. Hence 

 soil treatment wnth phosphates is a standard agricultural practice in growing 

 root crops such as turnips and mangolds. 



Calcium. — A large proportion of the calcium in most plants is located in 

 the leaves. Calcium is relatively immobile in plants, little redistribution of 

 this element occurring from tissues in which it has once been utilized to other 

 parts of the plant. 



Calcium apparently plays a manifold role in plant metabolism. It is a 

 structural component of plant cell walls in the form of the calcium pectate 

 of the middle lamella. Calcium ions have pronounced effects upon the per- 

 meability of the cytoplasmic membranes and upon the hydration of colloids. 

 Deficiency of calcium results in a stunting of root development of many 

 species. There is also some evidence that an insufficient supply of calcium 



