REVIEW OF LEIEIG’s VEGETABLE PHYSIOLOGY. 
269 
quantitative proportion between the absorption 
and digestion of azotised and unazotised substan¬ 
ces, assumed by Leibig, be based on fact. Leibig 
says that this proportion must exist in the grains 
of wheat. But Hermstadt’s fine experiments on 
the different sorts of grain show quite the contra¬ 
ry ; for one sort of wheat contains 41 per cent, of 
starch to 34 per cent, of gluten, while another sort 
shows a proportion of G5 to 9. But this varying 
proportion of azotised and unazotised substances 
also occurs in the organs of vegetation themselves, 
as may be seen in the roots of beet cultivated on 
a soil rich in vegetable mould, or in one manured 
with animal matter. Chemistry, therefore, does 
not support Leibig’s assertions. Indeed, he him¬ 
self, in some degree, contradicts his own state¬ 
ments, when he says (p. 119) that if plants obtain 
a greater proportion of carbon than of nitrogen, 
then the carbon will not be used for forming glu¬ 
ten, or albumen, or wood, nor for any constituent 
part of an organ, but will be secreted in the form 
of sugar, starch, oil, wax, resin, mannite, gum, &c. 
The assumption (continues Dr. Mohl) that the 
organs of plants consist of gluten, albumen, and 
wood, and that other constituents, like sugar, 
starch, &c., are mere secretions, is decidedly 
wrong in an anatomical point of view, for the solid 
substance of all organs consists of woody fibre 
alone, and all the other ingredients are merely 
preserved in the cellular substance, &;c. The 
same objection may be raised against the opinion, 
that starch, gum, &c., preserved in the cells, are 
mere excrements, and not constituent parts of the 
organ. On the other hand, Dr. Mohl considers 
Leibig’s opinion, that a greater amount of nitrogen 
is required for forming woody fibre, than sugar, 
gum, starch, &c., as perfectly true. This is cor¬ 
roborated by the experiments of Payen, who found 
in all young organs, while in a stale of vigorous 
development, an abundance of nitrogenous juices, 
which leads to the conclusion that nitrogenous 
substances are essential to the development of new 
elementary organs, a position fully borne out by 
recent microscopical experiments on the formation 
of the cellular tissue in plants. If this is the case, 
we may also assume that the formation of sub¬ 
stances nearly related to woody fibre, such as su¬ 
gar, gum, and starch, requires also a certain 
amount of nitrogen; and that a less amount, al¬ 
though it may suffice for the abundant formation 
of those substances, will only produce a small 
amount of woody fibre, is an opinion adopted by 
Leibig. It may be also assumed, that if such an 
amount of nitrogen as is sufficient for the forma¬ 
tion of a moderate quantity of woody fibre, is to 
be divided between the formation of gum and 
other similar substances, (composed of carbonic 
acid and water,) and that of woody fibre—the 
same amount of nitrogen might suffice for the 
formation of a greater amount of woody fibre, and 
therefore, for the more vigorous growth of the 
plant; in those cases where the plant has already 
absorbed part of its food in the form of substances, 
which (being composed of carbonic acid and water) 
have also attained the first degree of vegetable as¬ 
similation. 
Leibig believes, that, in this case, the function 
of the leaves will be impaired. This, however, it 
is impossible to decide, “ as we have not even an 
idea of the variations which the assimilative pro¬ 
cess may undergo, according to the difference of 
the substances on which plants feed. It is also, 
in most cases, less essential to know whether 
plants absorb only organic substances and water, 
than whether it is necessarily in conjunction with 
inorganic matter, that such absorption takes place; 
in which case, the digestion (Verarbeitung) of in¬ 
organic substances would not be suspended; but, 
in conjunction with this assimilative process, an¬ 
other and different one would make its appear¬ 
ance.” To decide on the existence of these pro¬ 
cesses, it would be necessary to know the meta¬ 
morphosis which food undergoes in plants. But 
this is not the case, as we neither are aware of 
the manner in which gum or sugar are formed, 
nor of the further changes of these substances into 
woody fibre. Chemistry, it is true, can change 
wood and starch into sugar, but only by the ap¬ 
plication of strong chemical agencies, which we 
know are not made use of by plants. To imitate 
the real process is beyond the reach of chemistry; 
still, it is to this very process that plants owe their 
growth. Under such circumstances, all that we 
can do is to make plants absorb organic substan¬ 
ces, and to observe the phenomena which their 
growth exhibits subsequently. It has been shown 
already that certain plants live only when they 
absorb organic substances; our cultivated plants 
prove the same thing, as well as the detailed ex¬ 
periments of Davy and Saussure, which have not 
been hitherto disproved. The theory of Leibig, 
therefore, is untenable, and is equally unsupported 
by experiment, or by exact reasoning on the nu¬ 
tritive processes of plants. 
Leibig has himself felt that arguments founded 
on fact are required for the support of his theory, 
and he has pitched upon Mount Vesuvius to sup¬ 
ply it. He dwells, therefore, (page 131) on the 
luxuriant crops in the environs of this mountain, 
in a “ soil, which, according to its origin, does not 
contain the least trace of organic matter, and still 
is considered as the very type of fertility.” We 
possess, however, no chemical analysis of this soil, 
derived as it is from volcanic cinders, neither does 
Leibig say anything on the subject, but merely 
appeals to its general volcanic origin. But it 
would be a surprising thing (says Dr. Mohl,) if a 
soil, on which, for many centuries past, most ex¬ 
tensive farming operations (Dreischwirthschaft ?) 
have been carried on, should be destitute of hu¬ 
mus. This could be only the case, if, after every 
few years, it was again covered so deeply with 
the ashes of constantly new eruptions, that the 
ancient soil and all its organic matter should be 
placed beyond the reach of new crops. This, how¬ 
ever, we know is not the case, and even such 
eruptions as do take place at intervals could not 
effect this. The heaviest fall of ashes (since the 
destruction of Pompeii) was that of 1822, which 
amounted on the slope of the mountain to 3 inch¬ 
es, and in the plain from 15 to 18 inches; but this 
was (according to Humboldt) the treble of any 
previous fall of ashes. We know, moreover, that 
even the slightest fall of volcanic cinders kills 
