INTRODUCTION 51 



Potassium is essential to the formation of starch, 

 for photo-synthesiS; or the transfer of carbohydrates 

 to the different parts of the plant, cannot take place 

 without it, and there is a cessation of sugar- and 

 starch-formation. The absence of potassium com- 

 pounds in the soil or nutrient salts also tends to 

 produce undersized fruits or seeds. In drought or 

 other abnormal seasons plants flag or wilt more 

 readily where there is a deficiency of potassium, and 

 they are less able to resist the attacks of parasitic 

 fungi, rusts, etc. 



To some extent a deficiency of potassium may be 

 repaired by the supply of sodium. 



Calcium is beneficial in leaf and seed production. 

 It may counteract the effect of saline salts. The 

 root system is impoverished when there is a deficiency 

 of calcium compounds in the soil. Calcium, like 

 potassium, is chiefly found in the leaf. 



Magnesium is useful in seed production. Chloro- 

 phyll is, according to Willstatter, a compound of 

 magnesium. Magnesium is also instrumental in the 

 formation of oil in seeds, etc. 



Iron, as has been mentioned, is essential to chloro- 

 phyll formation, and without a trace of ferric salts it 

 is not formed. A trace of manganese is found in 

 some plants. 



Chlorine is not essential to plant growth, but is 

 generally present in the form of chlorides in rain- 

 water. Similarly, iodides and fluorides are bene- 

 ficial. 



