696 
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. VII, 1865. 
