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, Senebier, Saint-Martin, de Candolle, and Miguel. In more recent times, it has been investigated by Schuebler, Lawes and Gilbert, Knop, Sachs, Unger, and Hosaeus. 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._