226 PART III. VEGETABLE PHYSIOLOGY. 



behind as ash. If the combustible portion be analyzed, it will 

 be found to contain the elements Carbon, Oxygen, Hydrogen 

 and Nitrogen, and an analysis of the ash will show it to consist 

 of Potassium, Phosphorus, Sulphur, Calcium, Magnesium, and 

 usually, also, other elements. Among these constituents we must 

 distinguish those which are essential, those which are of minor 

 importance, and those which are merely accidental. Those found 

 to be essential to the lives of all plants are Carbon, Oxygen, 

 Hydrogen, Nitrogen, Sulphur, Potassium and Phosphorus. The 

 first three are the constituents of all carbohydrates, as starch, 

 cellulose, sugar, etc. ; in addition to these, protoplasm contains, 

 as essential constituents, nitrogen and sulphur. Potassium and 

 phosphorus, though not properly constituents of protoplasm, are 

 always found in relation to it, and closely associated with the 

 activities of the plant. The former appears to be essential to the 

 formation of starch, and to be concerned, also, in its transfer 

 from one part of the plant to another, while the latter, though its 

 functions are not well understood, enters as an essential constit- 

 uent into some of the important organic compounds of the plant, 

 as, for example, nuclein and chlorophyll. In the form of the 

 phosphates, it also promotes the process of metabolism in the 

 cell, and probably by rendering albuminoid matters more soluble, 

 aids the transfer of these important substances. 



Additional elements essential to the lives of some plants are 

 Iron, and, perhaps, Chlorine. Iron is found in minute quantities 

 in nearly all plants, but is really essential only to the chlorophyll- 

 bearing ones. Plants deprived of iron do not develop chloro- 

 phyll, but appear as if bleached. It is still somewhat uncertain 

 whether chlorine is really essential to any plant, but some exper- 

 iments appear to prove that it is necessary to Buckwheat and a 

 few other plants. 



The elements of minor importance to the plant are Calcium, 

 Magnesium, Sodium and Silicon. It has been found by experi- 

 ment that plants cease to thrive after a time when completely 

 deprived of calcium salts. The reason of this is not fully known. 

 Both calcium and magnesium are of value to the plant, however, 

 from the fact that their phosphates, nitrates and sulphates are 

 important sources of sulphur, nitrogen and phosphorus. They 

 are probably useful, also, in fixing and rendering harmless certain 



