_electrum_, from whence the word electricity is derived, as that
substance is peculiarly, and was once supposed to be exclusively,
electric. It is found either deeply buried in the bowels of the earth,
or floating on the sea, and is supposed to be a resinous body which has
been acted on by sulphuric acid, as its analysis shows it to consist of
ah oil and an acid. The oil is called _oil of amber_, the acid the
_succinic_.
EMILY.
That oil I have sometimes used in painting, as it is reckoned to change
less than the other kinds of oils.
MRS. B.
The last class of vegetable substances that have changed their nature
are _fossil-wood_, _peat_, and _turf_. These are composed of wood and
roots of shrubs, that are partly decomposed by being exposed to moisture
under ground, and yet, in some measure, preserve their form and organic
appearance. The peat, or black earth of the moors, retains but few
vestiges of the roots to which it owes its richness and combustibility,
these substances being in the course of time reduced to the state of
vegetable earth. But in turf the roots of plants are still discernible,
and it equally answers the purpose of fuel. It is the combustible used
by the poor in heathy countries, which supply it abundantly.
It is too late this morning to enter upon the history of vegetation. We
shall reserve this subject, therefore, for our next interview, when I
expect that it will furnish us with ample matter for another
conversation.
CONVERSATION XXII.
HISTORY OF VEGETATION.
MRS. B.
The VEGETABLE KINGDOM may be considered as the link which unites the
mineral and animal creation into one common chain of beings; for it is
through the means of vegetation alone that mineral substances are
introduced into the animal system, since, generally speaking, it is from
vegetables that all animals ultimately derive their sustenance.
CAROLINE.
I do not understand that; the human species subsists as much on animal
as on vegetable food, and there are some carnivorous animals that will
eat only animal food.
MRS. B.
That is true; but you do not consider that those that live on animal
food, derive their sustenance equally, though not so immediately, from
vegetables. The meat that we eat is formed from the herbs of the field,
and the prey of carnivorous animals proceeds, either directly or
indirectly, from the same source. It is, therefore, through this channel
that the simple elements become a part of the animal frame. We should in
vain attempt to derive nourishment from carbon, hydrogen, and oxygen,
either in their separate state, or combined in the mineral kingdom; for
it is only by being united in the form of vegetable combination, that
they become capable of conveying nourishment.
EMILY.
Vegetation, then, seems to be the method which Nature employs to prepare
the food of animals?
MRS. B.
That is certainly its principal object. The vegetable creation does not
exhibit more wisdom in that admirable system of organisation, by which
it is enabled to answer its own immediate ends of preservation,
nutrition, and propagation, than in its grand and ultimate object of
forming those arrangements and combinations of principles, which are so
well adapted for the nourishment of animals.
EMILY.
But I am very curious to know whence vegetables obtain those principles
which form their immediate materials?
MRS. B.
This is a point on which we are yet so much in the dark, that I cannot
hope fully to satisfy your curiosity; but what little I know on this
subject, I will endeavour to explain to you.
The soil, which, at first view, appears to be the aliment of vegetables,
is found, on a closer investigation, to be little more than the channel
through which they receive their nourishment; so that it is very
possible to rear plants without any earth or soil.
CAROLINE.
Of that we have an instance in the hyacinth and other bulbous roots,
which will grow and blossom beautifully in glasses of water. But I
confess I should think it would be difficult to rear trees in a similar
manner.
MRS. B.
No doubt it would, as it is the burying of the roots in the earth that
supports the stem of the tree. But this office, besides that of
affording a vehicle for food, is far the most important part which the
earthy portion of the soil performs in the process of vegetation; for we
can discover, by analysis, but an extremely small proportion of earth in
vegetable compounds.
CAROLINE.
But if earths do not afford nourishment, why is it necessary to be so
attentive to the preparation of the soil?
MRS. B.
In order to impart to it those qualities which render it a proper
vehicle for the food of the plant. Water is the chief nourishment of
vegetables; if, therefore, the soil be too sandy, it will not retain a
quantity of water sufficient to supply the roots of the plants. If, on
the contrary, it abound too much with clay, the water will lodge in such
quantities as to threaten a decomposition of the roots. Calcareous soils
are, upon the whole, the most favourable to the growth of plants: soils
are, therefore, usually improved by chalk, which, you may recollect, is
a carbonat of lime. Different vegetables, however, require different
kinds of soils. Thus rice demands a moist retentive soil; potatoes a
soft sandy soil; wheat a firm and rich soil. Forest trees grow better in
fine sand than in a stiff clay; and a light ferruginous soil is best
suited to fruit-trees.
CAROLINE.
But pray what is the use of manuring the soil?
MRS. B.
Manure consists of all kinds of substances, whether of vegetable or
animal origin, which have undergone the putrid fermentation, and are
consequently decomposed, or nearly so, into their elementary principles.
And it is requisite that these vegetable matters should be in a state of
decay, or approaching decomposition. The addition of calcareous earth,
in the state of chalk or lime, is beneficial to such soils, as it
accelerates the dissolution of vegetable bodies. Now, I ask you, what is
the utility of supplying the soil with these decomposed substances?
CAROLINE.
It is, I suppose, in order to furnish vegetables with the principles
which enter into their composition. For manures not only contain carbon,
hydrogen, and oxygen, but by their decomposition supply the soil with
these principles in their elementary form.
MRS. B.
Undoubtedly; and it is for this reason that the finest crops are
produced in fields that were formerly covered with woods, because their
soil is composed of a rich mould, a kind of vegetable earth, which
abounds in those principles.
EMILY.
This accounts for the plentifulness of the crops produced in America,
where the country was but a few years since covered with wood.
CAROLINE.
But how is it that animal substances are reckoned to produce the best
manure? Does it not appear much more natural that the decomposed
elements of vegetables should be the most appropriate to the formation
of new vegetables?
MRS. B.
The addition of a much greater proportion of nitrogen, which constitutes
the chief difference between animal and vegetable matter, renders the
composition of the former more complicated, and consequently more
favourable to decomposition. The use of animal substances is chiefly to
give the first impulse to the fermentation of the vegetable ingredients
that enter into the composition of manures. The manure of a farm-yard is
of that description; but there is scarcely any substance susceptible of
undergoing the putrid fermentation that will not make good manure. The
heat produced by the fermentation of manure is another circumstance
which is extremely favourable to vegetation; yet this heat would be too
great if the manure was laid on the ground during the height of
fermentation; it is used in this state only for hot-beds, to produce
melons, cucumbers, and such vegetables as require a very high
temperature.
CAROLINE.
A difficulty has just occurred to me which I do not know how to remove.
Since all organised bodies are, in the common course of nature,
ultimately reduced to their elementary state, they must necessarily in
that state enrich the soil, and afford food for vegetation. How is it,
then, that agriculture, which cannot increase the quantity of those
elements that are required to manure the earth, can increase its produce
so wonderfully as is found to be the case in all cultivated countries?
MRS. B.
It is by suffering none of these decaying bodies to be dissipated, but
in applying them duly to the soil. It is by a judicious preparation of
the soil, which consists in fitting it either for the general purposes
of vegetation, or for that of the particular seed which is to be sown.
Thus, if the soil be too wet, it may be drained; if too loose and sandy,
it may be rendered more consistent and retentive of water by the
addition of clay or loam; it may be enriched by chalk, or any kind of
calcareous earth. On soils thus improved, manures will act with double
efficacy, and if attention be paid to spread them on the ground at a
proper season of the year, to mix them with the soil so that they may be
generally diffused through it, to destroy the weeds which might
appropriate these nutritive principles to their own use, to remove the
stones which would impede the growth of the plant, &c. we may obtain a
produce an hundred fold more abundant than the earth would spontaneously
supply.
EMILY.
We have a very striking instance of this in the scanty produce of
uncultivated commons, compared to the rich crops of meadows which are
occasionally manured.
CAROLINE.
But, Mrs. B., though experience daily proves the advantage of
cultivation, there is still a difficulty which I cannot get over.
A certain quantity of elementary principles exist in nature, which it is
not in the power of man either to augment or diminish. Of these
principles you have taught us that both the animal and vegetable
creation are composed. Now the more of them is taken up by the vegetable
kingdom, the less, it would seem, will remain for animals; and,
therefore, the more populous the earth becomes, the less it will
produce.
MRS. B.
Your reasoning is very plausible; but experience every where contradicts
the inference you would draw from it; for we find that the animal and
vegetable kingdoms, instead of thriving, as you would suppose, at each
other’s expense, always increase and multiply together. For you should
recollect that animals can derive the elements of which they are formed
only through the medium of vegetables. And you must allow that your
conclusion would be valid only if every particle of the several
principles that could possibly be spared from other purposes were
employed in the animal and vegetable creations. Now we have reason to
believe that a much greater proportion of these principles than is
required for such purposes remains either in an elementary state, or
engaged in a less useful mode of combination in the mineral kingdom.
Possessed of such immense resources as the atmosphere and the waters
afford us, for oxygen, hydrogen, and carbon, so far from being in danger
of working up all our simple materials, we cannot suppose that we shall
ever bring agriculture to such a degree of perfection as to require the
whole of what these resources could supply.
Nature, however, in thus furnishing us with an inexhaustible stock of
raw materials, leaves it in some measure to the ingenuity of man to
appropriate them to its own purposes. But, like a kind parent, she
stimulates him to exertion, by setting the example and pointing out the
way. For it is on the operations of nature that all the improvements of
art are founded. The art of agriculture consists, therefore, in
discovering the readiest method of obtaining the several principles,
either from their grand sources, air and water, or from the
decomposition of organised bodies; and in appropriating them in the best
manner to the purposes of vegetation.
EMILY.
But, among the sources of nutritive principles, I am surprised that you
do not mention the earth itself, as it contains abundance of coals,
which are chiefly composed of carbon.
MRS. B.
Though coals abound in carbon, they cannot, on account of their hardness
and impermeable texture, be immediately subservient to the purposes of
vegetation.
EMILY.
No; but by their combustion carbonic acid is produced; and this entering
into various combinations on the surface of the earth, may, perhaps,
assist in promoting vegetation.
MRS. B.
Probably it may in some degree; but at any rate the quantity of
nourishment which vegetables may derive from that source can be but very
trifling, and must entirely depend on local circumstances.
CAROLINE.
Perhaps the smoky atmosphere of London is the cause of vegetation being
so forward and so rich in its vicinity?
MRS. B.
I rather believe that this circumstance proceeds from the very ample
supply of manure, assisted, perhaps, by the warmth and shelter which the
town affords. Far from attributing any good to the smoky atmosphere of
London, I confess I like to anticipate the time when we shall have made
such progress in the art of managing combustion, that every particle of
carbon will be consumed, and the smoke destroyed at the moment of its
production. We may then expect to have the satisfaction of seeing the
atmosphere of London as clear as that of the country. --But to return to
our subject: I hope that you are now convinced that we shall not easily
experience a deficiency of nutritive elements to fertilise the earth,
and that, provided we are but industrious in applying them to the best
advantage by improving the art of agriculture, no limits can be assigned
to the fruits that we may expect to reap from our labours.
CAROLINE.
Yes; I am perfectly satisfied in that respect, and I can assure you that
I feel already much more interested in the progress and improvement of
agriculture.
EMILY.
I have frequently thought that the culture of the land was not
considered as a concern of sufficient importance. Manufactures always
take the lead; and health and innocence are frequently sacrificed to the
prospect of a more profitable employment. It has often grieved me to see
the poor manufacturers crowded together in close rooms, and confined for
the whole day to the most uniform and sedentary employment, instead of
being engaged in that innocent and salutary kind of labour, which Nature
seems to have assigned to man for the immediate acquirement of comfort,
and for the preservation of his existence. I am sure that you agree with
me in thinking so, Mrs. B.?
MRS. B.
I am entirely of your opinion, my dear, in regard to the importance of
agriculture; but as the conveniences of life, which we are all enjoying,
are not derived merely from the soil, I am far from wishing to
depreciate manufactures. Besides, as the labour of one man is sufficient
to produce food for several, those whose industry is not required in
tillage must do something in return for the food that is provided for
them. They exchange, consequently, the accommodations for the
necessaries of life. Thus the carpenter and the weaver lodge and clothe
the peasant, who supplies them with their daily bread. The greater stock
of provisions, therefore, which the husbandman produces, the greater is
the quantity of accommodation which the artificer prepares. Such are the
happy effects which naturally result from civilised society. It would be
wiser, therefore, to endeavour to improve the situation of those who are
engaged in manufactures, than to indulge in vain declamations on the
hardships to which they are too frequently exposed.
But we must not yet take our leave of the subject of agriculture; we
have prepared the soil, it remains for us now to sow the seed. In this
operation we must be careful not to bury it too deep in the ground, as
the access of air is absolutely necessary to its germination; the earth
must, therefore, lie loose and light over it, in order that the air may
penetrate. Hence the use of ploughing and digging, harrowing and raking,
&c. A certain degree of heat and moisture, such as usually takes place
in the spring, is likewise necessary.
CAROLINE.
One would imagine you were going to describe the decomposition of an old
plant, rather than the formation of a new one; for you have enumerated
all the requisites of fermentation.
MRS. B.
Do you forget, my dear, that the young plant derives its existence from
the destruction of the seed, and that it is actually by the saccharine
fermentation that the latter is decomposed?
CAROLINE.
True; I wonder that I did not recollect that. The temperature and
moisture required for the germination of the seed is then employed in
producing the saccharine fermentation within it?
MRS. B.
Certainly. But, in order to understand the nature of germination, you
should be acquainted with the different parts of which the seed is
composed. The external covering or envelope contains, besides the germ
of the future plant, the substance which is to constitute its first
nourishment; this substance, which is called the _parenchyma_, consists
of fecula, mucilage, and oil, as we formerly observed.
The seed is generally divided into two compartments, called _lobes_, or
_cotyledons_, as is exemplified by this bean (PLATE XV. Fig. 1.)--the
dark-coloured kind of string which divides the lobes is called the
_radicle_, as it forms the root of the plant, and it is from a
contiguous substance, called _plumula_, which is enclosed within the
lobes, that the stem arises. The figure and size of the seed depend very
much upon the cotyledons; these vary in number in different seeds; some
have only one, as wheat, oats, barley, and all the grasses; some have
three, others six. But most seeds, as, for instance, all the varieties
of beans, have two cotyledons. When the seed is buried in the earth, at
any temperature above 40 degrees, it imbibes water, which softens and
swells the lobes; it then absorbs oxygen, which combines with some of
its carbon, and is returned in the form of carbonic acid. This loss of
carbon increases the comparative proportion of hydrogen and oxygen in
the seed, and excites the saccharine fermentation, by which the
parenchymatous matter is converted into a kind of sweet emulsion. In
this form it is carried into the radicle by vessels appropriated to that
purpose; and in the mean time, the fermentation having caused the seed
to burst, the cotyledons are rent asunder, the radicle strikes into the
ground and becomes the root of the plant, and hence the fermented liquid
is conveyed to the plumula, whose vessels have been previously distended
by the heat of the fermentation. The plumula being thus swelled, as it
were, by the emulsive fluid, raises itself and springs up to the surface
of the earth, bearing with it the cotyledons, which, as soon as they
come in contact with the air, spread themselves, and are transformed
into leaves. --If we go into the garden, we shall probably find some
seeds in the state which I have described--
[Illustration: Plate XV. Vol. II. p. 250
Germination.
Fig. 1 & 2.
A.B Cotyledons.
C Envelope.
D Radicle.
Fig. 3.
A.B Cotyledons.
C Plumula.
D Radicle.
Fig. 4.
A.B. Cotyledons.
C Plumula.
D Radicle.
Fig. 5. Apparatus to illustrate the mechanism of breathing.
A.A Glass Bell.
B Bladder representing the lungs.
C Bladder representing the Diaphragm.]
EMILY.
Here are some lupines that are just making their appearance above
ground.
MRS. B.
We shall take up several of them to observe their different degrees of
progress in vegetation. Here is one that has but recently burst its
envelope--do you see the little radicle striking downwards? (PLATE XV.
Fig. 2.) In this the plumula is not yet visible. But here is another in
a greater state of forwardness--the plumula, or stem, has risen out of
the ground, and the cotyledons are converted into seed leaves. (PLATE
XV. Fig. 3.)
CAROLINE.
These leaves are very thick and clumsy, and unlike the other leaves,
which I perceive are just beginning to appear.
MRS. B.
It is because they retain the remains of the parenchyma, with which they
still continue to nourish the young plant, as it has not yet sufficient
roots and strength to provide for its sustenance from the soil. --But,
in this third lupine (PLATE XV. Fig. 4.), the radicle had sunk deep
into the earth, and sent out several shoots, each of which is furnished
with a mouth to suck up nourishment from the soil; the function of the
original leaves, therefore, being no longer required, they are gradually
decaying, and the plumula is become a regular stem, shooting out small
branches, and spreading its foliage.
EMILY.
There seems to be a very striking analogy between a seed and an egg;
both require an elevation of temperature to be brought to life; both at
first supply with aliment the organised being which they produce; and as
soon as this has attained sufficient strength to procure its own
nourishment, the egg-shell breaks, whilst in the plant the seed-leaves
fall off.
MRS. B.
There is certainly some resemblance between these processes; and when
you become acquainted with animal chemistry, you will frequently be
struck with its analogy to that of the vegetable kingdom.
As soon as the young plant feeds from the soil, it requires the
assistance of leaves, which are the organs by which it throws off its
super-abundant fluid; this secretion is much more plentiful in the
vegetable than in the animal creation, and the great extent of surface
of the foliage of plants is admirably calculated for carrying it on in
sufficient quantities. This transpired fluid consists of little more
than water. The sap, by this process, is converted into a liquid of
greater consistence, which is fit to be assimilated to its several
parts.
EMILY.
Vegetation, then, must be essentially injured by destroying the leaves
of the plant?
MRS. B.
Undoubtedly; it not only diminishes the transpiration, but also the
absorption by the roots; for the quantity of sap absorbed is always in
proportion to the quantity of fluid thrown off by transpiration. You
see, therefore, the necessity that a young plant should unfold its
leaves as soon as it begins to derive its nourishment from the soil;
and, accordingly, you will find that those lupines which have dropped
their seed-leaves, and are no longer fed by the parenchyma, have spread
their foliage, in order to perform the office just described.
But I should inform you that this function of transpiration seems to be
confined to the upper surface of the leaves, whilst, on the contrary,
the lower surface, which is more rough and uneven, and furnished with a
kind of hair or down, is destined to absorb moisture, or such other
ingredients as the plant derives from the atmosphere.
As soon as a young plant makes its appearance above ground, light, as
well as air, becomes necessary to its preservation. Light is essential
to the development of the colours, and to the thriving of the plant. You
may have often observed what a predilection vegetables have for the
light. If you make any plants grow in a room, they all spread their
leaves, and extend their branches towards the windows.
CAROLINE.
And many plants close up their flowers as soon as it is dark.
EMILY.
But may not this be owing to the cold and dampness of the evening air?
MRS. B.
That does not appear to be the case; for in a course of curious
experiments, made by Mr. Senebier, of Geneva, on plants which he reared
by lamp-light, he found that the flowers closed their petals whenever
the lamps were extinguished.
EMILY.
But pray, why is air essential to vegetation, plants do not breathe it
like animals?
MRS. B.
At least not in the same manner; but they certainly derive some
principles from the atmosphere, and yield others to it. Indeed, it is
chiefly owing to the action of the atmosphere and the vegetable kingdom
on each other, that the air continues always fit for respiration. But
you will understand this better when I have explained the effect of
water on plants.