erimented
with. From further experiments, carried out by Dr Paul Wagner and
Wolff, _glycin_, _tyrosin_, and _kreatin_ are able to be assimilated by
the plant.

_Plants able to absorb certain Forms of Organic Nitrogen._

We may conclude, then, from these interesting experiments, that plants
are able to absorb certain organic forms of nitrogen. That they do so in
nature to any extent is extremely improbable, such organic forms of
nitrogen being rarely present in the soil, or if present, being
converted into ammonia or nitrate salts before assimilation.

_Nature of Humus in the Soil._

While on the subject of organic nitrogen, reference may be briefly made
to that substance known as _humus_,--the name applied to the organic
portion of soils,--a substance which figures so largely in early
theories of plant-nutrition. The most elaborate investigation of the
composition of humus has been carried out by Mulder. According to
Mulder, it is composed of a number of organic bodies, and he has
identified the following substances--ulmin, humin, ulmic, humic, geic
acids, &c. These bodies are composed of carbon, hydrogen, and oxygen,
which are invariably associated with nitrogen. Detmer and Simon have
further investigated the subject. The true function of humus, it would
seem, in addition to its numerous mechanical properties, is to furnish,
by its decomposition, carbonic acid and nitrogen--in the form of
ammonia and nitric acid--to the soil; the former acting as a solvent of
the mineral food, the latter as the source of the plant's nitrogen. The
old theory, therefore, that the presence of humus in a soil is a
condition of fertility, is not so far removed from the truth. Where
there is an abundance of humus in the soil there is likely also to be an
abundance of nitrogen.

_Relation of Ammonia to the Plant._

It seems to be beyond doubt that nitrogen is directly absorbed by plants
in the form of ammonia. Liebig, as we have seen, concluded that this was
the great source of nitrogen for the plant, and that the ammonia
compounds present in the air were an all-sufficient supply. Subsequent
research, while confirming his belief so far as regards the capability
of plants to assimilate nitrogen in the form of ammonia, has proved that
the amount of ammonia present in the air is very minute, and utterly
inadequate to supply the plant with the whole of its nitrogen.
Investigations have been made on this subject by Graeger, Fresenius,