66 NUTRIENT SALTS. 
NUTRIENT SALTS. 
If wood, leaves, seeds, or any other parts of plants are subjected to a high 
temperature with free access of air, the first changes that occur are in the com- 
pounds of nitrogen and of carbon contained in the heated matter. They turn 
black, are charred and burnt, and ultimately the products of combustion pass into 
the atmosphere in gaseous condition. The incombustible part which remains 
behind is called the “ash.” The quantity of this ash, as well as its composition, 
varies very much in different species of plants, and even in different parts of the 
same plant. Generally the weight of ash is only one or two per cent of the entire 
weight of the plant in a dry state before burning. The greatest relative proportion 
of ash is that which is obtained from the combustion of those hydrophytes which 
live in the sea; and next in quantity is the ash of the family of Oraches which 
abound on salt-steppes. On the other hand, the smallest quantity is that afforded by 
fungi and mosses, by Sphagnum in particular, and with these must be mentioned 
the tropical orchids living on the barks of trees. Seeds and wood yield relatively 
much less ash than leaves. But, as above remarked, some ash is formed upon the 
combustion of any part of a plant or even of a single cell, and this residue of ash 
sometimes allows of our recognizing exactly the size, form, and outline of the cells. 
The universal distribution of ash-forming constituents permits us to conclude with 
certainty that they do not exist fortuitously in plants, but are essential to them. 
That these constituents are indispensable may also be proved directly. If an 
attempt is made to nourish a plant on filtered air and distilled water exclusively, the 
plant soon dies; but if a small quantity of the constituents of its ash are added to 
the distilled water in which the roots are immersed, the plant grows visibly in the 
solution, and develops leaves and flowers and even seeds capable of germination. 
Experiments of this kind with cultures have been the means of almost com- 
pletely establishing the division between those constituents which are indispensable 
for all plants, and those which are only necessary under certain conditions and to 
particular species, or, still less, only beneficial. Those elements must be regarded 
as essential, which are used by plants for the process of construction, and enter 
into the composition of the protoplasm or of the cell-membrane—such, for instance 
as are essential constituents of proteid substances, or are in some way necessary 
to the formation of these products. Amongst these must be included sulphur, 
phosphorus, potassium, calcium, and magnesium. Some plants, especially those 
that live in the sea, require sodium, iodine and chlorine, and, for green plants, iron 
is necessary. Silicon is also very important for most plants in helping them to 
flourish in the wild state. Most of these elements are taken into a plant, in the 
covrse of nutrition, in a condition of extreme oxidation, that is to say in combina- 
tion with a quantity of oxygen; in fact, as a general rule, they are absorbed in 
the form of salts, and we may for the sake of brevity include all the mineral food- 
stuffs under the name of nutrient salts or food-salts. 
