OUTLINES OF THE PHYSIOLOGY OF NUTRITION 205 



leaves are green is that a certain amount of iron is 

 contained in the seed. The disease caused by the 

 want of iron is called chlorosis, which means pallor, 

 and is very easily cured. If a few drops of a solution 

 of an iron salt be added to the water, the pale leaves 

 soon begin to turn green, and the same result is 

 produced if the surface of the leaf be painted with 

 the dilute iron compound. It is very important to 

 add no more iron than is necessary. In quantities 

 sufficient to colour the water it is poisonous to the 

 plant. The way in which iron is necessary is thus 

 made quite clear. Without iron no chlorophyll can 

 be formed. 



The necessity for the six elements which are 

 known to enter into the composition of the tissues is 

 self-evident, and is easily proved experimentally. 

 Suppose, for example, that we wish to determine 

 whether phosphorus is essential. All we have to do 

 is to leave out the calcium phosphate and substitute 

 for it the same quantity of calcium sulphate. The 

 growth of the seedling soon ceases as soon, in fact, 

 as the phosphorus in the seed has been used up. In 

 like manner we can prove the necessity for the other 

 tissue-forming elements. 



The presence of potassium, calcium, and magnesium 

 is also essential. If we substitute sodium nitrate for 

 potassium nitrate, the seedling stops growing; and 

 the same is the case if we replace the calcium and 

 magnesium compounds by those of any other metals. 

 These elements are in some way or other necessary 

 for the process of assimilation ; in their absence the 



