56 BOTANY. [CHAP. n. 



from the sulphates of the soil, such as calcic sulphate 

 (Ca SOJ ; phosphorus from phosphates, etc. 



The elements mentioned as forming the organic 

 matters of the plant, as protoplasm, starch, cellulose, 

 etc., actually build up the substance of the plant, 

 whereas those forming the inorganic parts, or ash, do 

 not necessarily enter into the composition of the tissues, 

 nevertheless in many instances their presence has been 

 proved to be indispensable in connection with certain 

 chemical changes resulting in the formation of certain 

 substances upon which the life of the plant depends. 

 Thus, iron obtained in the form of a chloride or sulphide 

 is necessary for the chemical production of the green 

 colour of chlorophyll, if iron is intentionally withheld 

 from plant food used in experimental culture, the parts 

 that would be normally green become yellowish only, 

 even if exposed to light, whereas if very minute traces 

 of iron are added to the food of such a plant, the chloro- 

 phyll is very soon formed. Iron does not enter into 

 the composition of chlorophyll, but its presence in very 

 small proportions is necessary to set up the chemical 

 changes that result in the formation of this substance. 

 The very general occurrence of chlorophyll-bearing 

 plants on all parts of the earth shows the wide range of 

 iron in a soluble form present in the soil. The formation 

 of starch depends on the presence of potassium. It 

 must be understood that potassium is not the only 

 factor necessary for the formation of starch ; but if this 

 substance is absent, even if all other conditions are 

 favourable, as in the case of iron and chlorophyll so also 

 with starch which contains no potassium, the latter 

 being necessary for promoting the chemical changes 



