11 



overcoming these harmful effects of magnesium salts. On this 

 account, a large amount of experimental study has been given to 

 the question of the calcium-magnesium ratio in plants. Numer- 

 ous analyses of plant ashes have established the fact that there is a 

 fairly definite ratio of this kind, which ratio, however, varies with 

 the species of plant and is not correlated with the ratio of these 

 elements present in the soil on which the plant grows, as was for- 

 merly believed. Cereal plants, as a rule, contain approximately 

 twice as much lime as magnesia; while leafy plants (tobacco, 

 cabbage, etc.) usually contain about four times as much calcium 

 oxide as magnesium oxide. 



Iron is essential to chlorophyll-formation. It is not a con- 

 stituent of the chlorophyll molecule, as is magnesium; but in the 

 absence of iron from the culture solution, a plant fails to produce 

 chlorophyll and a green plant which is deprived of a supply of iron 

 rapidly becomes etiolated. The way in which iron is related to 

 chlorophyll-formation is not known. 



Iron is taken from the soil by plants in the smallest propor- 

 tions of any of the essential elements. Only soluble ferric com- 

 pounds seem to serve as a suitable source of supply of the element; 

 ferrous compounds being usually highly toxic to plants. 



Sulfur is an essential element of plant food. The amounts 

 required by plants were supposed, until recently, to be relatively 

 small. This was due to the fact that earlier studies took account 

 only of the sulfur which, on analysis, appeared as sulfates in the 

 ash. Improved methods of analysis, which insure that the sulfur 

 which is present in the plant tissue in organic combinations is 

 oxidized under such conditions that it is not lost by volatilization 

 during the combustion of the material, have shown that the total 

 sulfur which is present in many plants approaches the quantity of 

 phosphorus which is present in the same tissue. Furthermore, 

 recent field and pot experiments have shown that at least a con- 

 siderable part of the beneficial effects of many fertilizers, which 

 has previously been attributed to the calcium, potassium, or 

 phosphorus which they contain, is actually due to the sulfur 

 present as sulfates in the fertilizers used. 



Sulfur occurs in the organic compounds of plants, associated 

 with phosphorus. It seems probable that its physiological uses 

 are similar to those of the latter element; but there is as yet no 

 experimental evidence to establish its exact role in the economy 



