and it is from experiments carried out in
water-culture that much of our present knowledge, in regard to the
relation of the ash constituents to the plant, is due.

The names of those who have worked in this department are very numerous.
Among them may be mentioned Knop, Sachs, Stohmann, Nobbe, Rautenberg,
Kuehn, 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, 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. S