44 ? 
PLANTS. 
simple water and air, are produced those 
wonderful diversities of peculiar juices and 
fruits, which the vegetable world affords; 
and the immense variety of tastes, smells, 
&c. In the same vegetable what a variety is 
found ! The hark is different in taste from 
•the wood, the peculiar juices have something 
different from them both, and the pith of 
some plants affords a matter which could not 
have been expected from their exterior qua- 
lities. The root is often different from the 
stem, and the fruit from both, in all their sen- 
sible qualities. 
In whatever way the nourishment of vege- 
tables is received, it may fairly be said to 
consist principally of water. We are inclined 
to believe, however, that calcareous earth, 
in small portions, may enter into the compo- 
sition of a.t least many vegetables; since ani- 
mals which exist entirely on vegetable food 
are found to have in their solid parts, the 
bones for instance, a considerable portion of 
this substance ; though it must be confessed 
that chemical analysis, as far as it has hitherto 
gone, does not warrant us in supposing cal- 
careous earth to be an essential constituent 
of all vegetable matter. It may be said fur- 
ther, that on some occasions the addition of 
other matters, as of different kindsof manure, 
tulds greatly to the growth of vegetables ; but 
in whatever degree a rich soil or dung may 
add to the luxuriance of growth, other facts 
•seem to prove that it is not essential to vege- 
tation. It is well known that many herbs 
flourish in pure water ; and that pear, plum, 
and cherry-trees, planted in pure moss, have 
.arrived at such perfection as to produce good 
fruit. 
Different theories have been advanced fo 
account for the operation of manures in pro- 
moting the growth of vegetables, none of 
which seem altogether satisfactory. The 
common opinion is, that the substances em- 
ployed as manures contain those principles 
which constitute the food of plants, and 
which are absorbed by their roots. This hy- 
pothesis is doubtless true to a certain extent, 
especially when it is considered that carbon 
forms a great part of many manures. An- 
other opinion is, that manures act by bring- 
ing soils to such a consistence as is favourable 
to the growth of the root* of vegetables, and 
to the affording of them water in a proper 
quantity. A third opinion is, that manures 
act as stimuli on the roots of vegetables, and 
thus excite them to more vigorous action. 
Some authors think that manures act as sol- 
vents on matters previously contained in the 
soil, and thus lit them for entering the roots 
of plants; and others, that they act chemi- 
cally, by forming combinations which are 
favourable to vegetation. Which of these 
hypotheses is best founded, it is difficult to 
determine; but it does not seem unlikely 
that they may all have some foundation in 
fact. 
When we attempt to discover the compo- 
nent principles of the objects around us, and 
the sources whence they were supported, we 
are lost in the greatness and diversity of the 
scenes presented to us. We see animals 
nourished by vegetables, vegetables appa- 
rently by the remains of animals, and fossils 
composed of the relics of both of these king- 
doms. It seems certain, however, that ve- 
getables preceded animals. A seed of moss 
Judging in a crevice of a bare rock is nou- 
rished by tiff atmosphere, and the moisture 
afforded by the rains and dews. It comes 
to perfection, and sheds its seeds in the 
mouldering remains of its own substance. 
Its offspring do the same, till a crust of vege- 
table mould is formed sufficiently" thick for 
the support of grass and other vegetables of 
the same growth, 't he same process going 
forward, shrubs, and lastly the largest trees, 
may find a firm support on the once barren 
rock, and brave (he efforts of the tempest. 
From the advantages derived from a change* 
of crops in agriculture, it has been supposed 
that different vegetables derive different 
kinds of nourishment from the same soil, se- 
lecting what is best adapted to their own sup- 
port, and leaving a supply of nourishment of 
another kind for vegetables of another genus. 
Was this, however, the case, vegetables 
would not so much impede each other’s 
growth when placed near together. And in 
the operation of grafting, we have a clear 
proof that the juices received by the root of 
one species of tree may, by the organization 
of the inserted twig, be subservient to the 
growth of leaves, flowers, and fruit, of a dif- 
ferent kind. The advantage derived from a 
change of crops may be better explained on 
other principles: some plants extend their 
roots horizontally on the surface of the soil, 
others strike them downwards to a consider- 
able depth. Some plants are found to bind 
or harden the soil, others to loosen it. Thus, 
for example, wheat and rye-grass render a 
soil stiff; while pulse, clover, and turnips, 
pulverize it. By varying the crops, there- 
fore, the soil is preserved in a middle state, 
between too much stiffness and too much fri- 
ability". Nor is this the only good effect 
arising from this difference of roots. From 
this circumstance some vegetables draw their 
nourishment from the surface of the earth, 
while others derive it partly from a greater 
depth ; so that by a change of crops, a larger 
portion of the soil is made to contribute to 
the nourishment of plants than could have 
been effected by the cultivation of any" single 
species. One other advantage to be derived 
from a change of crops is this: Some plants 
extract almost the whole of their nourish- 
ment from the soil ; and this is particularly 
the case with those which are most valuable, 
and which contain the greatest quantity of 
solid matter. By the repetition of such crops, 
however, the soil is found to become too 
much exhausted. There are other plants 
which derive a large proportion of their nou- 
rishment from the air; by such therefore the 
soil will be much less exhausted, and under 
a crop of them will be in some measure at 
rest. The good effects of a change of crops 
may therefore be sufficiently explained, with- 
out supposing that each particular species of 
vegetables is nourished by a different kind of 
food. This opinion is also necessarily attend- 
ed with two great difficulties ; one is, that 
there exists in every soil as many" distinct 
kinds of nourishment as there are species of 
plants capable of growing in that soil ; the 
other, that plants are endued with the faculty 
of selecting, from all these kinds, their own 
proper nourishment. The former of these 
suppositions is too absurd to merit the least 
attention ; and the latter lias been disproved 
by actual experiment; since plants are not 
able to prevent their roots from absorbing 
such matters as prove poisonous to them. 
Other writers, however, have been more mo- 
derate; and though they have rejected the 
idea of specific nourishment in general, have 
nevertheless imagined that the hypothesis 
might be well founded with respect to parti- 
cular species of vegetables. I bis they infer 
from the existence of specific manures, as 
soot for saintfoin, ashes for white clover, and 
some others. It does not seem possible, 
however, to draw a line of distinction ; and 
if we reject the idea of a specific nourish- 
ment in general, we cannot admit it in parti- 
cular instances. 
In order to discover whether plants have 
an actual power of distinguishing matters pre- 
sented to their roots, a gentleman of science 
made, among others, the following experi- 
ment: 
A vigorous plant of mint was placed in a 
two-ounce phial, tilled with filtrated well-wa- 
ter, to which were added four drops of a mode- 
rately strong solution of sulphate of iron. On 
examining the plant the following day, no 
other effect was observed, than that the very 
tips of the radicles were withered and black, 
l our more drops of the solution were now 
added. On the third day the appearances 
were the same ; and no new change taking 
place on the fourth, twelve more drops of the 
solution were added. On the fifth day" the 
roots appeared of a yellowish-green colour, 
and the top drooped very much. The larger 
leaves were pretty much withered and black- 
ened. The absorption of the water appeared 
to be in some measure impeded, but not en- 
tirely prevented. On the sixth day the 
whole plant w 7 as withering very fast; the 
roots became of a dark olive-green colour, 
and the larger leaves were become very 
black, especially the footstalks and the pro- 
jecting fibres. On the seventh day the black- 
ness had made still further progress, and the 
plant was dead. A sufficient proof that spine 
of the iron was absorbed by the plant, may 
be drawn from the following circumstance : 
Its leaves, when macerated in distilled water, 
produced a black colour with galls. The 
leaves of a plant of mint, which had been 
nourished by water alone, when tried by the 
same test, produced no colour whatever. 
Tills experiment proves two points ; that 
plants have not the power of rejecting even 
injurious matters when presented to their 
roots, and that other matters besides water 
and air are capable of being absorbed by 
them. 
The benefit produced by the common cus- 
tom of letting lands lie tallow", has not yet 
been satisfactorily explained. Something 
may no doubt be attributed to the destruction 
of weeds, but more probably to. some change 
produced in the soil by its being exposed to 
tlie action of the sun and air. The manage- 
ment of nitre-beds may" tend to throw some 
light on this subject. These are composed 
of calcareous earth and dung cemented to- 
gether. After being exposed for some months 
to the air, they are found to contain a quan- 
tity of nitric acid, which, uniting to the calca- 
reous earth, forms a kind of salt, which is 
extracted by lixiviation. Now calcareous 
earth and dung are two of the most powerful 
kinds of manure, and it does not seem im- 
probable that their fertilizing powers may be 
in some manner connected with their pro 
perty of affording nitrous acid. 
