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 

 course 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. 



