The names of those who have worked in this department are very numerous. Among them may be mentioned Knop, Sachs, Stohmann, Nobbe, Rautenberg, Kühn, Lucanus, W. Wolff, Hampe, Beyer, E. Wolff, P. Wagner, Bretschneider and Lehmann. The results obtained by these and other experimenters have demonstrated the following facts.
The substances which have been found in the ash of plants are: potash, soda, lime, magnesia, oxide of iron, oxide of manganese, phosphoric acid, sulphuric acid, silica, carbonic acid, chlorine, lithia, rubidia, alumina, oxide of copper, bromine, iodine, and occasionally even other substances. Of these, however, only six are probably absolutely necessary for plant-growth—viz., potash, lime, magnesia, oxide of iron, phosphoric acid, and sulphuric acid. Three other substances seem also to be almost invariably present, and may possibly be essential—in very minute quantities at any rate—viz., chlorine, soda, and silica. With regard to alumina and oxide of copper, these constituents must be regarded as accidental; while iodine and bromine only occur in the ash of marine plants.
Method of Absorption of Plant-food.
A department of vegetable physiology which has had much work devoted to it is the method in which plant-roots absorb their food. The plant's nourishment is absorbed in solution by means of the roots. Its absorption takes place, according to Fischer and Dutrochet, who have investigated the subject at great length, by the process known as endosmosis. It has also been established by numerous experiments, that different plants require different constituents in different proportions.
Water as a Carrier of Plant-food.
The function performed by water, as the carrier of plant-food, and the motion of the sap of the plant, are questions which have also received much attention. The motion of the plant's sap seems to have attracted a great deal of attention at a very early stage of the study of plant physiology. As far back as 1679, Marriotte studied it. Among other old experimenters were Hales, Guettard, Sénébier, Saint-Martin, de Candolle, and Miguel. In more recent times, it has been investigated by Schübler, Lawes and Gilbert, Knop, Sachs, Unger, and Hosäus. Some idea of the enormous amount of water transpired by plant-leaves may be gained by the statement that from 233 lb. to 912 lb. of water are transpired for every pound of plant-tissue formed.[32]
Agronomy.
When we come to deal with questions relating to the chemistry of the soil, we find that so much investigation has been devoted to this one branch of agricultural chemistry as to constitute it a special branch by itself—known in France under the name of agronomie—and being taught in the large agricultural colleges by special professors of the subject. The value of studying the properties of soils was recognised at an early period. This study was for long largely confined to their physical, or, what are popularly known as their mechanical properties. Thus Sir Humphry Davy ascertained many important facts with regard to the heat and water absorbing and retaining properties of soils.
Retention by Soil of Plant-food.
It was not till a later period that the power soils possess of fixing from their watery solutions various plant-foods, both organic and inorganic, was discovered. The earliest recognition of this most important property of soils was made by Gazzeri, who, in 1819, called attention to the fact that the dark fluid portion of farmyard manure was purified on passing through clay. He concluded that soils, more especially clayey soils, possessed the property of being able to fix from their watery solutions the necessary plant-food constituents, and fix them beyond risk of loss, only affording a gradual supply to the plant as required.