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In the middle of these, or round a great number of ligneous fibres, are some vessels, which are not so narrow, composed of a silvery elastic blade, formed spirewise, like a spring. These are arteries, and seldom contain any thing but air.

These four orders of vessels, which are dispersed through all the parts of the vegetable, in proportion to the nature and function of each, compose, at least in trees and shrubs, three principal and concentric beds, the bark, the wood, and the pith. The bark or rind, which is the outer covering, is smooth, even, and shining in some, and rough, chanelled and hairy in others: it is formed of the widest fibres, that are the least pressed together, and which admit within them the most air. The wood, which is placed under the rind, has, on the contrary, narrower and more contracted tubes. Its utricles are less replenished or dilated; and this only has arteries. The pith, which is situate in the heart of the plant, is little more than a collection of utricles, which are greater and more capacious than those of the bark and wood.

From the structure of plants we proceed to their nutrition, by their roots and leaves. The saline, unctuous, and subtile slime, which the water separates from the coarse earth, and keeps in a dissolved state, is the principal nutriment of plants. The different species of manure only contribute to the fertilizing of land, in proportion as they introduce into it a greater quantity of a spongy powder or active salt'.

After having been admitted into the body of the root by the extremity of the fibres, the nutritious juice rises into the ligneous fibres from the trunk or stalk, and passes into the utricles that adhere

'See No. XXIV. On the Food of Plants.

to them. It is there prepared and digested. It afterward enters into the proper vessels, under the form of a coloured fluid more or less thick, which we may conjecture to be with respect to the plant, what the chyle or blood is to the animal. Being filtered by the finer or more winding tubes, it is at last conveyed to all the parts, to which it unites, and increases their bulk.

The quantity of nutriment which a plant derives from the earth is in proportion to the number and size of its leaves; the smaller and fewer in number the leaves are, the less it draws.

The nutrition of vegetables is likewise effected immediately by their leaves. They not only serve for raising the sap, preparing it, and discharging its superfluity, but they are a kind of roots that pump from the air the juices they transmit to the neighbouring parts. The dew, which arises from the ground, is the principal foundation of this aërial nourishment. The leaves present to it their inferior surface, which is always furnished with an infinite number of small pipes that are always ready to absorb it. And that the leaves may receive no prejudice in the exercise of this function, they are dispersed with such art on the stalk and branches, that those which immediately precede, do not cover such as succeed them. Sometimes they are placed alternately on two opposite and parallel lines. Sometimes they are distributed by pairs, that cross each other at right angles. Sometimes they are ranged on the angles of polygons circumscribed on the branches, and so disposed, that the angles of the inferior polygon correspond with the sides of the superior. And sometimes the leaves ascend the whole length of the stalk and branches, in one or more parallel spiral lines.

By a mechanism, which is doubtless very sim

ple, the root of the plant forces itself into the earth, the branches shoot out on each side, the leaves expose their superior surface to the open air, and their inferior surface to the earth, or the inner part of the plant. If a seed be sown the contrary way, the radicle and the little stalk will each bend backward; the former, in order to penetrate into the earth, and the latter to gain the air. a young stalk be kept inclined, its extremity, notwithstanding, will grow upright. Bend the branches of all sorts of plants; cause the inferior surfaces of their leaves to turn toward the sky; you will soon perceive that all these leaves will resume their former position.

If

Many great naturalists have supposed a circulation of the sap in plants; but Dr. Hales has demonstrated, that the sap does not circulate, but that it ascends and descends. In order to understand the motion of the sap, according to his principles, it is to be considered, that during the heat of a summer's day, all plants perspire freely from the pores of their leaves and bark. At that time, their juices are highly rarefied. The diameters of the trachea, or air-vessels, are enlarged, so as to press upon and straiten the vessels that carry the sap; in consequence of which, their juices, not being able to escape by the roots, are pressed upward where there is the least resistance, and perspire off the excrementitious parts by the leaves and branches, in the form of vapour. When the solar heat declines, the trachea are contracted, the sap vessels are enlarged, and the sap sinks down in the manner of the spirits of a thermometer. In consequence of this change, the capillary vessels of the leaves and top branches become empty. Being surrounded with the humid vapours of the evening, they fill themselves, from the known laws of attraction, and send down the new acquired juices to be mixed with those that are

more elaborated. As soon as the sun has altered the temperature of the air, the trachea become again distended, and the sap-vessels are straitened. The same cause always produces the same effect; and this alternate ascent and descent through the same system of vessels, continues as long as the plant survives. The irregular motion of the stem and branches is another cause that contributes to the ascent of the sap. Whenever these parts are agitated by the air, they are made to assume a variety of angles, whereby the sap-vessels are suddenly straitened. The contained juices, consequently receive reiterated impulses, similar to what happens to the blood of animals from the contraction of the heart. These observations

convey a general idea of the motion of the sap, which varies according to the temperature of the weather, which is seldom the same in any succeeding moment; and, therefore, the sap must sometimes move more quickly, and sometimes slow: it may rise and fall many times in the day, pushed forward by sudden heats, and falling by sudden cold. Thus the juices are blended, and the secretions forwarded.

An ingenious female naturalist, Mrs. Agnes Ibbetson, has published an interesting series of Essays on Vegetable Physiology in the Philosophical Magazine.' In a few particulars she assigns different functions to certain portions of plants, to those pointed out in these Papers; though in the main she agrees. In her comparison between animal and vegetable life, she proves, that the vegetable frame is a mere muscular creature, having life only, but no sensation; possessing alone the property of the muscles, irritability. The comparison proves also, that as the only part in the human body that possesses selfmotion, independent of all other parts, is the

muscle; so the only part in the plant that possesses the same vis insita, is the spiral wire, which therefore operates as the muscle of the vegetable. (See the Philosophical Magazine for August, 1815.)

No. XXIX.

THE PROCESS OF NATURE IN THE VEGETATION OF PLANTS.

Nor only through the lenient air this change,
Delicious, breathes; the penetrative sun,
This force deep darting to the dark retreat
Of vegetation, sets the streaming power
At large, to wander o'er the verdant earth,
In various hues.

THOMSON.

Hos natura modos primum dedit: his genus omne
Sylvarum fruticumque viret.

Thus Nature did ordain,

VIRGIL.

For trees, and shrubs, and all the sylvan reign.

DRYDEN.

THE theory of the ascent and descent of the sap in plants, with which I concluded my former paper, is beautifully adopted by Thomson, in a noble apostrophe to the God of Seasons:

Hail, Source of Being! Universal Soul

Of heaven and earth! Essential Presence hail!
To Thee I bend the knee; to Thee my thoughts,
Continual, climb; who, with a master-hand,
Hast the great whole into perfection touched.
By Thee the various vegetative tribes,
Wrapt in a filmy net, and clad with leaves,
Draw the live ether, and imbibe the dew:

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