Conversations on Chemistry Part 98

And how admirable the design of Providence, who makes every different part of the creation thus contribute to the support and renovation of each other!

But the intercourse of the vegetable and animal kingdoms through the medium of the atmosphere extends still further. Animals, in breathing, not only consume the oxygen of the air, but load it with carbonic acid, which, if acc.u.mulated in the atmosphere, would, in a short time, render it totally unfit for respiration. Here the vegetable kingdom again interferes; it attracts and decomposes the carbonic acid, retains the carbon for its own purposes, and returns the oxygen for ours.

CAROLINE.

How interesting this is! I do not know a more beautiful ill.u.s.tration of the wisdom which is displayed in the laws of nature.

MRS. B.

Faint and imperfect as are the ideas which our limited perceptions enable us to form of divine wisdom, still they cannot fail to inspire us with awe and admiration. What, then, would be our feelings, were the complete system of nature at once displayed before us! So magnificent a scene would probably be too great for our limited and imperfect comprehension, and it is no doubt among the wise dispensations of Providence, to veil the splendour of a glory with which we should be overpowered. But it is well suited to the nature of a rational being to explore, step by step, the works of the creation, to endeavour to connect them into harmonious systems; and, in a word, to trace in the chain of beings, the kindred ties and benevolent design which unites its various links, and secure its preservation.

CAROLINE.

But of what nature are the organs of plants which are endued with such wonderful powers?

MRS. B.

They are so minute that their structure, as well as the mode in which they perform their functions, generally elude our examination; but we may consider them as so many vessels or apparatus appropriated to perform, with the a.s.sistance of the principle of life, certain chemical processes, by means of which these vegetable compounds are generated. We may, however, trace the tannin, resins, gum, mucilage, and some other vegetable materials, in the organised arrangement of plants, in which they form the bark, the wood, the leaves, flowers, and seeds.

The _bark_ is composed of the _epidermis_, the _parenchyma_, and the _cortical layers_.

The epidermis is the external covering of the plant. It is a thin transparent membrane, consisting of a number of slender fibres, crossing each other, and forming a kind of net-work. When of a white glossy nature, as in several species of trees, in the stems of corn and of seeds, it is composed of a thin coating of siliceous earth, which accounts for the strength and hardness of those long and slender stems.

Sir H. Davy was led to the discovery of the siliceous nature of the epidermis of such plants, by observing the singular phenomenon of sparks of fire emitted by the collision of ratan canes with which two boys were fighting in a dark room. On a.n.a.lysing the epidermis of the cane, he found it to be almost entirely siliceous.

CAROLINE.

With iron then, a cane, I suppose, will strike fire very easily?

MRS. B.

I understand that it will. --In ever-greens the epidermis is mostly resinous, and in some few plants is formed of wax. The resin, from its want of affinity for water, tends to preserve the plant from the destructive effects of violent rains, severe climates, or inclement seasons, to which this species of vegetables is peculiarly exposed.

EMILY.

Resin must preserve wood just like a varnish, as it is the essential ingredient of varnishes?

MRS. B.

Yes; and by this means it prevents likewise all unnecessary expenditure of moisture.

The parenchyma is immediately beneath the epidermis; it is that green rind which appears when you strip a branch of any tree or shrub of its external coat of bark. The parenchyma is not confined to the stem or branches, but extends over every part of the plant. It forms the green matter of the leaves, and is composed of tubes filled with a peculiar juice.

The cortical layers are immediately in contact with the wood; they abound with tannin and gallic acid, and consist of small vessels through which the sap descends after being elaborated in the leaves. The cortical layers are annually renewed, the old bark being converted into wood.

EMILY.

But through what vessels does the sap ascend?

MRS. B.

That function is performed by the tubes of the alburnum, or wood, which is immediately beneath the cortical layers. The wood is composed of woody fibre, mucilage, and resin. The fibres are disposed in two ways; some of them longitudinally, and these form what is called the silver grain of the wood. The others, which are concentric, are called the spurious grain. These last are disposed in layers, from the number of which the age of the tree may be computed, a new one being produced annually by the conversion of the bark into wood. The oldest, and consequently most internal part of the alburnum, is called heart-wood; it appears to be dead, at least no vital functions are discernible in it. It is through the tubes of the living alburnum that the sap rises.

These, therefore, spread into the leaves, and there communicate with the extremities of the vessels of the cortical layers, into which they pour their contents.

CAROLINE.

Of what use, then, are the tubes of the parenchyma, since neither the ascending nor descending sap pa.s.ses through them?

MRS. B.

They are supposed to perform the important function of secreting from the sap the peculiar juices from which the plant more immediately derives its nourishment. These juices are very conspicuous, as the vessels which contain them are much larger than those through which the sap circulates. The peculiar juices of plants differ much in their nature, not only in different species of vegetables, but frequently in different parts of the same individual plant: they are sometimes saccharine, as in the sugar-cane, sometimes resinous, as in firs and evergreens, sometimes of a milky appearance, as in the laurel.

EMILY.

I have often observed, that in breaking a young shoot, or in bruising a leaf of laurel, a milky juice will ooze out in great abundance.

MRS. B.

And it is by making incisions in the bark that pitch, tar, and turpentine are obtained from fir-trees. The durability of this species of wood is chiefly owing to the resinous nature of its peculiar juices.

The volatile oils have, in a great measure, the same preservative effects, as they defend the parts, with which they are connected, from the attack of insects. This tribe seems to have as great an aversion to perfumes, as the human species have delight in them. They scarcely ever attack any odoriferous parts of plants, and it is not uncommon to see every leaf of a tree destroyed by a blight, whilst the blossoms remain untouched. Cedar, sandal, and all aromatic woods, are on this account of great durability.

EMILY.

But the wood of the oak, which is so much esteemed for its durability, has, I believe, no smell. Does it derive this quality from its hardness alone?

MRS. B.

Not entirely; for the chesnut, though considerably harder and firmer than the oak, is not so lasting. The durability of the oak is, I believe, in a great measure owing to its having very little heart-wood, the alburnum preserving its vital functions longer than in other trees.

CAROLINE.

If incisions are made into the alburnum and cortical layers, may not the ascending and descending sap be procured in the same manner as the peculiar juice is from the vessels of the parenchyma?

MRS. B.

Yes; but in order to obtain specimens of these fluids, in any quant.i.ty, the experiment must be made in the spring, when the sap circulates with the greatest energy. For this purpose a small bent gla.s.s tube should be introduced into the incision, through which the sap may flow without mixing with any of the other juices of the tree. From the bark the sap will flow much more plentifully than from the wood, as the ascending sap is much more liquid, more abundant, and more rapid in its motion than that which descends; for the latter having been deprived by the operation of the leaves of a considerable part of its moisture, contains a much greater proportion of solid matter, which r.e.t.a.r.ds its motion. It does not appear that there is any excess of descending sap, as none ever exudes from the roots of plants; this process, therefore, seems to be carried on only in proportion to the wants of the plant, and the sap descends no further, and in no greater quant.i.ty, than is required to nourish the several organs. Therefore, though the sap rises and descends in the plant, it does not appear to undergo a real circulation.

The last of the organs of plants is the _flower_, or _blossom_, which produces the _fruits_ and _seed_. These may be considered as the ultimate purpose of nature in the vegetable creation. From fruits and seeds animals derive both a plentiful source of immediate nourishment, and an ample provision for the reproduction of the same means of subsistence.

The seed which forms the final product of mature plants, we have already examined as const.i.tuting the first rudiments of future vegetation.

These are the princ.i.p.al organs of vegetation, by means of which the several chemical processes which are carried on during the life of the plant are performed.

EMILY.

But how are the several principles which enter into the composition of vegetables so combined by the organs of the plant as to be converted into vegetable matter?

MRS. B.

By chemical processes, no doubt; but the apparatus in which they are performed is so extremely minute as completely to elude our examination.