NUTRITIVE FUNCTIONS OF SALTS 105 



potassium causes senescence; but it is more likely that the plant 

 removes the potassium from older regions and its absence from 

 older regions is a result of senescence rather than a cause. Potas- 

 sium salts are slightly radioactive, and their effect upon the gen- 

 eral health and nutrition of the plant has been attributed by Sto- 

 klasa (1920) to this property. 



Sulphur.— This element reaches the plant as soluble sulphates 

 and is necessary in fairly large quantities, as it enters into the com- 

 position of proteins. Although the seed contains enough to last 

 the plant for a long time, its absence is ultimately felt. A de- 

 ficiency of sulphur results in a retardation of cell division similar 

 to that produced by an absence of potassium. Fruiting is also 

 hindered or suppressed. Legumes are especially sensitive to its 

 absence, possibly because it is necessary for the development of 

 the tubercles. Most soils contain sufficient sulphur and consider- 

 able is returned to the earth in rain, but large areas of the West 

 must be fertilized with sulphur before alfalfa or other legume 

 crops can be grown. It is thought by many to be important also 

 in the respiratory processes of the cell. 



Boron. — Boron is among the latest elements to be proved in- 

 dispensable to plants. While there are some who do not wish to 

 admit it to the exalted position of an essential element and would 

 explain its favorable action as due to its alkaline nature (Morris, 

 1931) or to the fact that it injures competing organisms of the 

 soil (Kellerman), most workers are now agreed that the boron is 

 really used by the plant. Another cause of disagreement is the 

 fact that it is toxic if more than 5-20 parts per million are present 

 in the culture solution. Soy beans were improved by 2.5 mg. of 

 boron per liter in the culture solution, but when as much as a pound 

 per acre was added to plants in the field, the leaves were dis- 

 tinctly injured. Likewise 50 p.p.m. retarded wheat 40%, and 100 

 p.p.m. stopped growth almost completely; 3-20 p.p.m. applied 

 as sodium or potassium borate stimulated growth up to the flower- 

 ing stage. There is also a great difference in the requirements of 

 various plants. Legumes and members of the Solanaceae (toma- 

 toes, potatoes, and tobacco) can use somewhat larger amounts 

 than cereals. Boron seems to be related to the utilization of cal- 

 cium. In its absence, the cells cease to divide normally, the roots 

 develop abnormally or slightly with a poor root cap, and the 

 nodules of legumes do not form properly, owing to a poor develop- 



