268 
REVIEW OF LEIBIG’S VEGETABLE PHYSIOLOGY. 
ORIGINAL CORRESPONDENCE. 
Translated for the Gardeners' Chronicle , 
from the German of Dr. Hugo Mohl. 
REVIEW OF LEIBIG’S VEGETABLE PHYSIOLOGY. 
(Continued from Page 250.) 
Another important point, says Professor Mohl, 
(p. 37,) which Leibig’s theory does not explain, 
is, that the saline bases absorbed by plants are not 
only absorbed m the shape of carbonates, (which 
are easily decomposed by the mere vegetable 
acids,) but often also in the shape of phosphate^, 
sulphates, &rc. According to all experience, these 
salts are not less essential to vegetation than those 
bases combined with organic acids. Silica, also, 
is an ingredient equally essential to;the growth of 
most, if not all plants. Which part these substan¬ 
ces take in the-vital process, is (says Mohl) almost 
unknown, unless, indeed, we may presume that 
the sulphates yield plants the sulphur required for 
some of their organs. Of the phosphates we know 
still less; we are ignorant why they chiefly occur 
in young plants r and in their seed; and we are 
perfectly ignorant of the quantity required for vege¬ 
table growth—for analysis shows that the amount 
of phosphates varies considerably even in the same 
organ in plants grown on different soils, as is best 
seen in the different sorts of grain. 
In that chapter which is devoted to the Culture 
of Plants, Professor Leibig puts forth a regular 
theory of vegetable nutrition—as far, namely, (ob¬ 
serves Dr. Mohl,) as Leibig’s unconnected way of 
writing admits of any systematic arrangement. 
He again starts from the assumption, that humus 
can not be absorbed and used as food by plants; 
for two reasons—one chemical, and the other 
physiological. Be, in the first instance, denies 
that the humus of vegetable mould possesses the 
properties ascribed to it by chemists, it being abso¬ 
lutely insoluble in water, and not combining with 
earth into soluble salts. The latter, he says, may 
be seen in calcareous caves, whose stalactites, in¬ 
stead of consisting of humate of lime, do not con¬ 
tain a trace of vegetable matter. Dr. Mohl says 
that he does not intend to settle these opinions, 
for he has no doubt that chemists will take them 
up in due time. He merely throws out the fol¬ 
lowing remarks: “ It can not be positively assert¬ 
ed that the humates contained in vegetable mould 
are insoluble in water, because -water will dissolve 
out of the soil a certain amount of an organic, 
brownish substance—an experiment which can be 
made with any garden soil; still coal of humus 
seems to possess the property of subtracting these 
substances from a solution passing or filtering 
through soil, otherwise (as Leibig has- stated) all 
our springs would contain brown water. It ap¬ 
pears, moreover, that besides coal of humus, the 
inorganic substances of the soil themselves possess 
(although in a lesser degree) the property of with¬ 
drawing from water the substances dissolved in 
it—-a circumstance to which the greater purity of 
springs coming from a great depth may be ascri¬ 
bed. Still this withdrawal of organic substances 
is obviously only a partial one, for our spring-wa¬ 
ter is never free from organic substances—a fact 
borne out by analysis, as well as by the putres¬ 
cence to which spring-water is subject. This per¬ 
fectly agrees with the new experiments of Saus- 
sure, who found in all waters an azotised substance 
soluble in water.” All these facts, therefore, give 
quite another result from that which Leibig has 
arrived at, viz., that the water which filters through 
vegetable mould will always supply plants with 
some portion of organic matter. ’How far this 
substance influences the- nutritive process of plants, 
was the proper question for Leibig to examine; 
but which he has neglected to enter into. Saus- 
sure has, however, made some experiments on the 
absorption of humate of potash, and the extract of 
vegetable mould by the rootsof plants (Bibl. Univ., 
vol. xxxvi., p. 340.) Although these experiments 
do not strictly prove that this is really the case; 
under all circumstances; still they go so far as to 
prove that it will be the case as long as the roots 
are healthy, and do not, by their putrescence, yield 
humous substances- to the water. Leibig, in a 
subsequent refutation of these experiments, ascribes 
their telling against him to-an- error of calculation !. 
In another part of this chapter (p. 109) Leibig 
says that humus, if not properly exposed to the 
influence of atmospheric air, will form with water 
a solution of a brown color ; but that no plants can 
grow in such soil, for the humus will consume all. 
the oxygen contained in the air. It is (says Dr s . 
Mohl) quite inconceivable how a man can write 
such things, when the inspection of any peat moss 
will show the fallacy of such assertions. 
The second argument which Leibig brings forth. 
to support his favorite assertion of plants not feed¬ 
ing on humus, is based on physiological grounds. 
He first announces as a general principle, that, un¬ 
der the appellation of food, such substances only 
can be included, as being derived from external; 
sources, can maintain all the vital functions, and 
which the organs of a plant can use for the forma¬ 
tion of the substances peculiar to them. This 
ambiguous definition includes (says Dr. Mohl), 
some anomalies, and does not apply to several re¬ 
puted kinds of food. Starch, for instance, is cer¬ 
tainly one of the substances on which man feeds; 
yet he could not live on it alone. In farther ex¬ 
planation of his views, Leibig adduces the exam¬ 
ple of a grain of wheat, which contains the neces¬ 
sary ingredients of the germ, and the first fibres 
of roots; and he adds that we have to suppose 
that these ingredients are mixed just in the pro¬ 
portion that is required for the development of 
those organs. If one of the ingredients, say starch 
or gluten, were superabundant, they would not 
serve either for the formation of leaves or other¬ 
wise. Carbon, also, as well as ammonia and wa¬ 
ter, are always combined in plants with an azotised 
matter; and it is for this reason that substances, 
containing no nitrogen, like gum, sugar, &c., and 
consequently no humic acid, (which stands next to 
them in a chemical point of view,) are not used 
as food by plants, but would rather impede the 
vital process, and kill the plant, (p. 116.) In 
analysing this string of assertions, Dr. Mohl ob¬ 
serves, in the first instance, that the absorption of 
azotised compounds as food by plants is a fact 
doubted by no one ; but he doubts whether the 
