6g6 CHEMICAL PROCESSES IN THE PLANT. 



It is self-evident that we have only to consider as elementary food-substances 

 those which are indispensably necessary for the process of nutrition ; while those the 

 presence of which in the plant is proved by analysis, but which may also be absent 

 without its nutrition being impaired, may be considered as accidental admixtures. 



Of the first importance among the indispensable food-materials are the ele- 

 ments of the combustible substance which are present in all plants without 

 exception, viz. Carbon, Hydrogen, Oxygen, Nitrogen, and Sulphur ; because they are 

 included in the chemical formula of cellulose or of the albuminoids which con- 

 stitute protoplasm, and because therefore without these substances the plant-cell 

 itself could not exist. It may be inferred also from the invariable presence of 

 Potassium, Calcium, Magnesium, Iron, and Phosphorus in plants, that they are 

 indispensable constituents of their food, and still more from the fact established 

 by actual experiment, that the nutrition and growth of all plants hitherto ex- 

 amined for this purpose is impossible or abnormal if any one of these elements is 

 wanting. In the case of Sodium, Manganese, and Silicon this has not yet been 

 proved ; it would appear rather that they may be dispensed with in the chemical 

 process of nutrition. That Chlorine is necessary for the perfect nutrition of 

 Polygonum Fagopyrum has been shown by Nobbed Whether Iodine and Bromine 

 play the part of true food-materials in the marine plants in which they are found 

 has not yet been ascertained ; and these two elements may, from the mode of their 

 occurrence, be for the present neglected as unimportant in this respect. 



In the more general considerations as to the nutrition of plants we have there- 

 fore chiefly to do with the following elements : — 



Carbon, Hydrogen, Oxygen, Nitrogen, Sulphur; 

 Potassium, Calcium, Magnesium, Iron ; 

 Phosphorus, Chlorine; 

 to which are to be added, under certain circumstances, Sodium and Silicon. 



The physiological importance of these elementary substances is however very 

 different. Those placed in the first line compose, as already mentioned, the greater 

 part of the substance of the plant ; they mainly form the organised and organisable 

 part of the plant and of every individual cell ; their importance therefore lies in the 

 fact that they furnish the chief materials for the construction of the plant. The 

 constituents of the ash, on the other hand, are of less importance in this respect, 

 if only in consequence of their much smaller quantity; they appear to promote 

 chemical decompositions and combinations in plants, in consequence of which the 

 far more abundant combustible principles are constructed out of the first-named five 

 elements. 



Carbon is a necessary constituent of every organic compound in varying proportion; 

 usually about one-half the weight of the entire dried substance of the plant consists 

 of this element. If the large quantity of vegetable matter which is annually produced 

 is taken into account, the fact becomes the more remarkable that this enormous 

 quantity of carbon is derived from the carbon dioxide of the atmosphere of which 

 it forms on the average only about 0*04 per cent. It is only the cells which contain 

 chlorophyll — and these only under the influence of sunlight — that have the power of 



Landwirthschaftliche Versuchsstationen, vol. YII, 1865. 



